Essential idea: every living organism inherits a blueprint for life from its parents.
3.1 Genes
UNDERSTANDINGS:
U3.1.1 A gene is heritable factor that consists of a length of DNA and influences a specific characteristic.
'Heritable' means passed on from parent to offspring and 'characteristic' refers to genetic traits such as your hair colour or your blood type.
Chromosomes are bundles of long strands of DNA (which contains genes). If you could unwind a chromosome, it would be like unravelling a ball of string. In eukaryotes that reproduce sexually, chromosomes always come is pairs (except in gametes). Humans have 46 chromosomes in 23 pairs. The DNA in eukaryotes is associated with proteins which help to keep the DNA organised.
U3.1.2 A gene occupies a specific position on a chromosome.
The term used to describe the location of a gene on a chromosome is 'locus'.
The chromosomal locus of a gene might be written "6p21.3". Here the 6 is the chromosome number. The letter p indicates that the position is on the chromosome's short arm; q indicates the long arm. The numbers following the letter represent the position on the arm: band 2, sub-band 1, sub-sub-band 3. The bands are visible under a microscope when the chromosome is suitably stained. Each of the bands is numbered, beginning with 1 for the band nearest the centromere. Sub-bands and sub-sub-bands are visible at higher resolution.
U3.1.3 The various specific forms of a gene are alleles.
U3.1.4 Alleles differ from one another by one or a few bases.
U3.1.5 New alleles are formed by mutation.
An allele is one of two or more alternative forms of a gene that arise by mutation and are found at the same place (same locus) on a chromosome. Alleles differ from other alleles by one or a few bases. Organisms have two alleles for each trait. When the alleles of a pair are heterozygous, one is dominant and the other is recessive. The dominant allele is expressed and the recessive allele is masked.
The genes which determine eye colour for example have more than one form. Some people have genes which give them brown eyes, others have genes for blue or green eyes. Also, in some people, earlobes are attached and in others, they are not. The gene for this trait comes in two possible forms: one allele for attached earlobes and one allele for non-attached earlobes.
Genetic conditions are caused by DNA mutations that cause a change in one of the genes affecting the way the body works or develops. These gene variants can then be passed from generation to generation.
U3.1.6 The genome is the whole of the genetic information of an organism.
U3.1.7 The entire base sequence of human genes was sequenced in the Human Genome Project.
A genome is an organism’s complete set of DNA, including all of its genes. Each genome contains all of the information needed to build and maintain that organism. In humans, a copy of the entire genome—more than 3 billion DNA base pairs—is contained in all cell's nucleus.
The Human Genome Project was an international research effort to determine the sequence of the human genome and identify the genes that it contains. The Project was coordinated by the National Institutes of Health and the U.S. Department of Energy. Additional contributors included universities across the United States and international partners in the United Kingdom, France, Germany, Japan, and China. The Human Genome Project formally began in 1990 and was completed in 2003, 2 years ahead of its original schedule.
The work of the Human Genome Project has allowed researchers to begin to understand the blueprint for building a person. As researchers learn more about the functions of genes and proteins, this knowledge will have a major impact in the fields of medicine, biotechnology, and the life sciences.
APPLICATION:
A3.1.1 The causes of sickle cell anaemia, including a base substitution mutation, a change to the base sequence of mRNA transcribed from it and a change to the sequence of a polypeptide in haemoglobin.
Base substitution mutation:
The consequence of changing one base could mean that a different amino acid is placed in the growing polypeptide chain during translation. This may have little or no effect on the organism or it may have a major influence on the organism's physical characteristics (phenotypes).
In humans, a mutation is sometimes found in the gene which produces haemoglobin for red blood cells. This mutation gives a different shape to the haemoglobin molecule. The difference leads to red blood cells which are 'sickle'-shaped (or cresent-shaped) and not biconcave like usual. The mutated red blood cell with the characteristic curved shape made its discoverers think of a sickle (a curved knife used to cut tall plants). The condition which results from this mutation is therefore called sickle cell anaemia.
The kind of mutation that causes sickle cell anaemia is called a base substitution mutation. In this case, one base is substituted for another so that the codon GAG becomes GTG. So, during translation instead of adding glutamic acid, which is the intended amino acid, valine is added. Since valine has a different shape and different properties from glutamic acid, the shape of the resulting polypeptide chain is modified. Thus, the haemoglobin molecule has a different shape as does the red blood cell.
A3.1.2 Comparison of the number of genes in humans with other species.
SKILL:
S3.1.1 Use of a database to determine differences in the base sequence of a gene in two species.
UNDERSTANDINGS:
U3.1.1 A gene is heritable factor that consists of a length of DNA and influences a specific characteristic.
'Heritable' means passed on from parent to offspring and 'characteristic' refers to genetic traits such as your hair colour or your blood type.
Chromosomes are bundles of long strands of DNA (which contains genes). If you could unwind a chromosome, it would be like unravelling a ball of string. In eukaryotes that reproduce sexually, chromosomes always come is pairs (except in gametes). Humans have 46 chromosomes in 23 pairs. The DNA in eukaryotes is associated with proteins which help to keep the DNA organised.
U3.1.2 A gene occupies a specific position on a chromosome.
The term used to describe the location of a gene on a chromosome is 'locus'.
The chromosomal locus of a gene might be written "6p21.3". Here the 6 is the chromosome number. The letter p indicates that the position is on the chromosome's short arm; q indicates the long arm. The numbers following the letter represent the position on the arm: band 2, sub-band 1, sub-sub-band 3. The bands are visible under a microscope when the chromosome is suitably stained. Each of the bands is numbered, beginning with 1 for the band nearest the centromere. Sub-bands and sub-sub-bands are visible at higher resolution.
U3.1.3 The various specific forms of a gene are alleles.
U3.1.4 Alleles differ from one another by one or a few bases.
U3.1.5 New alleles are formed by mutation.
An allele is one of two or more alternative forms of a gene that arise by mutation and are found at the same place (same locus) on a chromosome. Alleles differ from other alleles by one or a few bases. Organisms have two alleles for each trait. When the alleles of a pair are heterozygous, one is dominant and the other is recessive. The dominant allele is expressed and the recessive allele is masked.
The genes which determine eye colour for example have more than one form. Some people have genes which give them brown eyes, others have genes for blue or green eyes. Also, in some people, earlobes are attached and in others, they are not. The gene for this trait comes in two possible forms: one allele for attached earlobes and one allele for non-attached earlobes.
Genetic conditions are caused by DNA mutations that cause a change in one of the genes affecting the way the body works or develops. These gene variants can then be passed from generation to generation.
U3.1.6 The genome is the whole of the genetic information of an organism.
U3.1.7 The entire base sequence of human genes was sequenced in the Human Genome Project.
A genome is an organism’s complete set of DNA, including all of its genes. Each genome contains all of the information needed to build and maintain that organism. In humans, a copy of the entire genome—more than 3 billion DNA base pairs—is contained in all cell's nucleus.
The Human Genome Project was an international research effort to determine the sequence of the human genome and identify the genes that it contains. The Project was coordinated by the National Institutes of Health and the U.S. Department of Energy. Additional contributors included universities across the United States and international partners in the United Kingdom, France, Germany, Japan, and China. The Human Genome Project formally began in 1990 and was completed in 2003, 2 years ahead of its original schedule.
The work of the Human Genome Project has allowed researchers to begin to understand the blueprint for building a person. As researchers learn more about the functions of genes and proteins, this knowledge will have a major impact in the fields of medicine, biotechnology, and the life sciences.
APPLICATION:
A3.1.1 The causes of sickle cell anaemia, including a base substitution mutation, a change to the base sequence of mRNA transcribed from it and a change to the sequence of a polypeptide in haemoglobin.
Base substitution mutation:
The consequence of changing one base could mean that a different amino acid is placed in the growing polypeptide chain during translation. This may have little or no effect on the organism or it may have a major influence on the organism's physical characteristics (phenotypes).
In humans, a mutation is sometimes found in the gene which produces haemoglobin for red blood cells. This mutation gives a different shape to the haemoglobin molecule. The difference leads to red blood cells which are 'sickle'-shaped (or cresent-shaped) and not biconcave like usual. The mutated red blood cell with the characteristic curved shape made its discoverers think of a sickle (a curved knife used to cut tall plants). The condition which results from this mutation is therefore called sickle cell anaemia.
The kind of mutation that causes sickle cell anaemia is called a base substitution mutation. In this case, one base is substituted for another so that the codon GAG becomes GTG. So, during translation instead of adding glutamic acid, which is the intended amino acid, valine is added. Since valine has a different shape and different properties from glutamic acid, the shape of the resulting polypeptide chain is modified. Thus, the haemoglobin molecule has a different shape as does the red blood cell.
A3.1.2 Comparison of the number of genes in humans with other species.
SKILL:
S3.1.1 Use of a database to determine differences in the base sequence of a gene in two species.
ESSENTIAL IDEA: CHROMOSOMES CARRY GENES IN A LINEAR SEQUENCE THAT IS SHARED BY MEMBERS OF A SPECIES.
3.2 Chromosomes
UNDERSTANDINGS:
U3.2.1 Prokaryotes have one chromosome consisting of a circular DNA molecule.
U3.1.2 Some prokaryotes also have plasmids but eukaryotes do not.
Prokaryotic cell structure is simpler than that of a eukaryote. A eukaryotic cell has multiple chromosomes that are contained in a membrane-bounded nucleus and, usually, a variety of other membrane-bounded organelles, whereas prokaryotes lack such structures. In prokaryotes, also known as bacterium, there is a single, circular chromosome, which is sometimes called a genophore or nucleiod to distinguish it from eukaryotic chromosomes. It appears as a snarl of fibers visible under an electron microscope. In comparison with a eukaryote there is very little protein associated with a prokaryotic chromosome.
In addition to this single major chromosome, a prokaryotic cell may also include much smaller rings of DNA called plasmids. Plamids often provide the bacterial cell with genes it needs to survive in particular adverse environments, for example, in the presence of antibiotics or in the absence of certain nutrients.
U3.2.3 Eukaryote chromosomes are linear DNA molecules associated with histone proteins.
The DNA molecules of eukaryotic cells are paired with a type of protein called histone. There are several histones and each helps in DNA packaging. Packaging is essential because the nucleus is microscopic but a single human molecule of DNA may be 4cm long.
When looking at an unfolded DNA with an electron microscope, you see what look like beads on a string. Each of the beads is a nucleosome. A nucleosome consists of two molecules of each of four different histones. The DNA wraps twice around these eight protein molecules (octomeric disc). The DNA is attached to the histones because DNA is negatively charged and histones are positively charged. Between the nucleosomes is a single string of DNA. There is often a fifth type of histone attached to the linking string of DNA near each nucleosome. This fifth histone leads to further wrapping (packaging) of the DNA molecule and eventually to the highly condensed or supercoiled chromosomes.
U3.2.4 In a eukaryote species there are different chromosomes that carry different genes.
Though similar in basic appearance, different chromosomes vary slightly in size and shape. In addition, when chromosomes are stained with fluorescent dyes they develop distinctive patterns of bright and dark bands. These subtle differences enable cell biologists to distinguish different chromosomes from one another, much as field biologists learn to distinguish members of a pod of whales by the marks and scars on their fins.
The largest chromosome of an organism is generally referred to as chromosome 1, the next largest as chromosome 2, and so on. Different chromosomes contain different genes. That is, each chromosome contains a specific chunk of the genome. For example, in humans the gene for alpha globin, a part of the hemoglobin protein that carries oxygen in red blood cells, is found on chromosome 16. The gene for beta globin, the other part of the hemoglobin protein, is found on chromosome 11.
U3.2.5 Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes.
All genes have more than one version, so when chromosomes are inherited from parent cells, the version of the gene, or allele, is not always the same. Although homologous chromosomes have the same genes, they do not have to have the same allele for each gene.
If each chromosome of the homologous chromosomes pair has the same allele for a particular gene, it is said to be homozygous. If the alleles are different, then the cell is referred to as heterozygous. The way in which the alleles interact determines the characteristics of the offspring. Alleles can be dominant or recessive and, if different alleles are present, the dominant allele will determine the characteristic of the offspring.
We use the letter n to denote the number of unique chromosomes in an organism. In eukaryotes, there are n pairs of chromosomes. With two of each, that makes a total of 2n per cell. Thus, the shorthand way to write the chromosome number for a haploid cell is n and diploid is 2n.
For example, the chromosome that you get from your father might have the allele for brown eyes, whereas the homologous chromosome you get from your mother might have the allele for blue eyes. Both chromosomes carry the eye colour gene, but the specific eye colour alleles are different.
U3.2.6 Diploid nuclei have pairs of homologous chromosomes.
U3.2.7 Haploid nuclei have one chromosome of each pair.
In a diploid human cell, the 46 chromosomes are grouped into 23 pairs of chromosomes called homologous chromosomes. Homologous means similar in shape and size and it means that the two chromosomes carry the same genes. The reason there are two of each is that one came from the father and the other from the mother.
Haploid cells on the other hand contain half the number of chromosomes as a diploid cell, giving them only one chromosome from each homologous pair.
U3.2.8 The number of chromosomes is a characteristic feature of members of a species.
This specification point is self-explanatory - DNA occurs in the form of chromosomes and the chromosomes vary in number depending on the species.
U3.2.9 A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.
U3.2.10 Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.
The sex chromosomes are the X and Y chromosomes and they are the ones that determine what sex you are. Any chromosome which is not a sex chromosome is called an autsome. Human have 22 pairs of autosomes and one pair of sex chromosomes.
If a trait or gene is described as autosomal, its locus is on one of the 22 pairs of autosomes. A trait or gene which is said to be sex-linked therefore has its locus on a sex chromosome. Where a gene is located determines whether or not the trait it controls is more common in males or females. When a trait is more common in one sex than the other, the chances are good that the trait is sex-linked and that the locus of the gene is on either the X or Y chromosome or both. If there is no pattern to the frequency of a trait between males and females, it is most likely an autosomal trait.
APPLICATION:
A3.2.1 Cairn's technique for measuring the length of DNA molecules by autoradiography.
A3.2.2 Comparison of genome size in T2 phase, Escherichia coli, Drosophila melanogaster, Homo sapiens and Paris japonica.
A3.2.3 Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaria, Oryza sativa and Parascaris equorum.
A3.2.4 Use of karyograms to deduce sex and diagnose Down Syndrome in humans.
SKILL:
S3.2.1 Use of databases to identify the locus of a human gene and its polypeptide product.
NOTE: 'karyograms' and 'karyotypes' are NOT the same thing. Karyotype is a property of a cell - the number and type of chromosomes present in the nucleus, not a photograph or diagram of them.
UNDERSTANDINGS:
U3.2.1 Prokaryotes have one chromosome consisting of a circular DNA molecule.
U3.1.2 Some prokaryotes also have plasmids but eukaryotes do not.
Prokaryotic cell structure is simpler than that of a eukaryote. A eukaryotic cell has multiple chromosomes that are contained in a membrane-bounded nucleus and, usually, a variety of other membrane-bounded organelles, whereas prokaryotes lack such structures. In prokaryotes, also known as bacterium, there is a single, circular chromosome, which is sometimes called a genophore or nucleiod to distinguish it from eukaryotic chromosomes. It appears as a snarl of fibers visible under an electron microscope. In comparison with a eukaryote there is very little protein associated with a prokaryotic chromosome.
In addition to this single major chromosome, a prokaryotic cell may also include much smaller rings of DNA called plasmids. Plamids often provide the bacterial cell with genes it needs to survive in particular adverse environments, for example, in the presence of antibiotics or in the absence of certain nutrients.
U3.2.3 Eukaryote chromosomes are linear DNA molecules associated with histone proteins.
The DNA molecules of eukaryotic cells are paired with a type of protein called histone. There are several histones and each helps in DNA packaging. Packaging is essential because the nucleus is microscopic but a single human molecule of DNA may be 4cm long.
When looking at an unfolded DNA with an electron microscope, you see what look like beads on a string. Each of the beads is a nucleosome. A nucleosome consists of two molecules of each of four different histones. The DNA wraps twice around these eight protein molecules (octomeric disc). The DNA is attached to the histones because DNA is negatively charged and histones are positively charged. Between the nucleosomes is a single string of DNA. There is often a fifth type of histone attached to the linking string of DNA near each nucleosome. This fifth histone leads to further wrapping (packaging) of the DNA molecule and eventually to the highly condensed or supercoiled chromosomes.
U3.2.4 In a eukaryote species there are different chromosomes that carry different genes.
Though similar in basic appearance, different chromosomes vary slightly in size and shape. In addition, when chromosomes are stained with fluorescent dyes they develop distinctive patterns of bright and dark bands. These subtle differences enable cell biologists to distinguish different chromosomes from one another, much as field biologists learn to distinguish members of a pod of whales by the marks and scars on their fins.
The largest chromosome of an organism is generally referred to as chromosome 1, the next largest as chromosome 2, and so on. Different chromosomes contain different genes. That is, each chromosome contains a specific chunk of the genome. For example, in humans the gene for alpha globin, a part of the hemoglobin protein that carries oxygen in red blood cells, is found on chromosome 16. The gene for beta globin, the other part of the hemoglobin protein, is found on chromosome 11.
U3.2.5 Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes.
All genes have more than one version, so when chromosomes are inherited from parent cells, the version of the gene, or allele, is not always the same. Although homologous chromosomes have the same genes, they do not have to have the same allele for each gene.
If each chromosome of the homologous chromosomes pair has the same allele for a particular gene, it is said to be homozygous. If the alleles are different, then the cell is referred to as heterozygous. The way in which the alleles interact determines the characteristics of the offspring. Alleles can be dominant or recessive and, if different alleles are present, the dominant allele will determine the characteristic of the offspring.
We use the letter n to denote the number of unique chromosomes in an organism. In eukaryotes, there are n pairs of chromosomes. With two of each, that makes a total of 2n per cell. Thus, the shorthand way to write the chromosome number for a haploid cell is n and diploid is 2n.
For example, the chromosome that you get from your father might have the allele for brown eyes, whereas the homologous chromosome you get from your mother might have the allele for blue eyes. Both chromosomes carry the eye colour gene, but the specific eye colour alleles are different.
U3.2.6 Diploid nuclei have pairs of homologous chromosomes.
U3.2.7 Haploid nuclei have one chromosome of each pair.
In a diploid human cell, the 46 chromosomes are grouped into 23 pairs of chromosomes called homologous chromosomes. Homologous means similar in shape and size and it means that the two chromosomes carry the same genes. The reason there are two of each is that one came from the father and the other from the mother.
Haploid cells on the other hand contain half the number of chromosomes as a diploid cell, giving them only one chromosome from each homologous pair.
U3.2.8 The number of chromosomes is a characteristic feature of members of a species.
This specification point is self-explanatory - DNA occurs in the form of chromosomes and the chromosomes vary in number depending on the species.
U3.2.9 A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.
U3.2.10 Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.
The sex chromosomes are the X and Y chromosomes and they are the ones that determine what sex you are. Any chromosome which is not a sex chromosome is called an autsome. Human have 22 pairs of autosomes and one pair of sex chromosomes.
If a trait or gene is described as autosomal, its locus is on one of the 22 pairs of autosomes. A trait or gene which is said to be sex-linked therefore has its locus on a sex chromosome. Where a gene is located determines whether or not the trait it controls is more common in males or females. When a trait is more common in one sex than the other, the chances are good that the trait is sex-linked and that the locus of the gene is on either the X or Y chromosome or both. If there is no pattern to the frequency of a trait between males and females, it is most likely an autosomal trait.
APPLICATION:
A3.2.1 Cairn's technique for measuring the length of DNA molecules by autoradiography.
A3.2.2 Comparison of genome size in T2 phase, Escherichia coli, Drosophila melanogaster, Homo sapiens and Paris japonica.
A3.2.3 Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaria, Oryza sativa and Parascaris equorum.
A3.2.4 Use of karyograms to deduce sex and diagnose Down Syndrome in humans.
SKILL:
S3.2.1 Use of databases to identify the locus of a human gene and its polypeptide product.
NOTE: 'karyograms' and 'karyotypes' are NOT the same thing. Karyotype is a property of a cell - the number and type of chromosomes present in the nucleus, not a photograph or diagram of them.
essential idea: alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes.
3.3 Meiosis
UNDERSTANDINGS:
U3.3.1 One diploid nucleus divides by meiosis to produce four haploid nuclei.
U3.3.2 The halving of the chromosome number allows a sexual life cycle with fusion of gametes.
U3.3.3 DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.
U3.3.4 The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.
U3.3.5 Orientation of pairs of homologous chromosomes prior to separation is random.
U3.3.6 Separation of pairs od homologous chromosomes in the first division of meiosis halves the chromosome number.
U3.3.7 Crossing over and random orientation promotes genetic variation.
U3.3.8 Fusion of gametes from different parents promotes genetic variation.
Meiosis is a form of cell division which results in gametes. Although meiosis has some similarities to mitosis, it is important to understand that there are some fundamental differences.
One characteristic which makes meiosis unique is that each new cell that results from the process has only half the number of chromosomes that a typical cell in that organism has. For instance, humans have 46 chromosomes in their cells, but in the sperm and egg cells, there are only 23 chromosomes in each cell. Cells which contain half the chromosome number are called haploid cells. Diploid cells contain the full chromosome number.
This type of cell division is called a reduction division because the number of chromosomes has been reduced. This reduction is necessary in gamete production because during sexual reproduction, each parent contributes (roughly) 50% of the genetic information. The cells formed from the cell division are referred to as daughter cells. Meiosis generates four haploid daughter cells and each cell has a unique mix of half of the genetic information of the parent cell.
Steps of meiosis:
Synthesis phase (or S phase) of Interphase
1. Chromosomes in a diploid cell replicate so that the end product of meiosis can be four haploid cells.
Prophase I
1. Chromosomes become visible as the DNA becomes more compact i.e. the chromosomes condense.
2. Homologous chromosomes are attracted to (associate with) each other and pair up and become sister chromatids.
3. Crossing over (recomination) takes places between non-sister chromatids - chiasmata is the location at which crossing over takes place on chromosomes.
4. Nuclear membrane disintergrates.
Metaphase I
1. The bivalents (pairs of homologous chromosomes i.e. two sister chromatids) line up at/across the cell's equator.
2. Spindle fibres made from microtubules form.
3. Random orientation takes place.
Anaphase I
1. Spindle fibres from the poles at each end of the cell attach to one sister chromatid and pull them (contract) to opposite poles.
2. Non-disjunction (when pairs of chromosomes do not separate) can occur here and will affect the chromosome number of all four gametes.
Telephase I
1. Spindles and spindle fibres disintergrate.
2. Usually, the chromosomes uncoil and new nuclear membranes form.
Now meiosis II takes place in order to separate the sister chromatids.
Prophase II
1. DNA condenses into visible chromosomes again.
2. No crossing over takes place.
3. Nuclear membranes disintergrate.
Metaphase II
1. Sister chromatids line up at the equator of each cell.
2. Random orientation takes place again.
Anaphase II
1. Spindle fibres form and break the centromeres of each sister chromatid, releasing each as individual chromosomes.
2. Spindle fibres contract and pull chromosomes to opposite poles of the cells.
3. Because of random orientation, the chromosomes can be pulled towards either of the newly forming daughter cells.
4. Non-disjunction can occur here as well.
Telephase II
1. Chromosomes unwind their strands of DNA.
2. Nuclear envelopes form around each of the four haploid cells, preparing them for cytokinesis (cytoplasmic division of each of the two cells).
MEIOSIS = DIPLOID -> DIPLOID -> HAPLOID
APPLICATION:
A3.3.1 Non-disjunction can cause Down syndrome and other chromosome abnormalities.
Non-disjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. There are three forms of nondisjunction: failure of a pair of homologous chromosomes to separate in meiosis I, failure of sister chromatids to separate during meiosis II, and failure of sister chromatids to separate during mitosis. Nondisjunction results in daughter cells with abnormal chromosome numbers (aneuploidy).
A3.3.2 Studies showing age of parents influences chances of non-disjunction.
The frequency of having a child with Down syndrome, for example, rises with maternal age due to a peculiarity of meiosis in female mammals. Meiosis is originated in the foetal ovary, arresting at metaphase I with the homologous chromosomes aligned for segregation. Cells remain in this state until the time of ovulation, often decades later in humans. The longer cells remain in the arrested state, the greater the chance that there will be a nondisjunction event when meiosis resumes.
A3.3.3 Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks.
Chorionic villus sampling
Chorionic villus sampling (CVS) is a prenatal test that is used to detect birth defects, genetic diseases and other problems during pregnancy. During the test, a small sample of cells (called chorionic villi) is taken from the placenta where it attaches to the wall of the uterus. Chorionic villi are tiny parts of the placenta that are formed from the fertilized egg, so they have the same genes as the baby.
You may be offered CVS if you have certain risk factors for having a baby with a birth defect or genetic disease, so that problems can be found early in pregnancy. One of the main risks associated with CVS is miscarriage, which is the loss of the pregnancy in the first 23 weeks. This is estimated to occur in 1-2% of cases.
There are also some other risks, such as infection or needing to have the procedure again because it wasn't possible to test the first sample removed. The risk of CVS causing complications is higher if it is carried out before the 10th week of pregnancy (it takes places between 11th and 14th week), which is why the test is only carried out after this point.
Risks of CVS:
· Miscarriage - the risk of miscarriage appears to be slightly higher when the tissue sample is taken through the cervix (transcervical) rather than the abdominal wall (transabdominal). The risk of miscarriage also increases if the baby is smaller than normal for his or her gestational age.
· Rh sensitization - chorionic villus sampling might cause some of the baby's blood cells to enter your bloodstream. If you have Rh negative blood and you haven't developed antibodies to Rh positive blood, you'll be given a drug called Rh immunoglobulin after the test to prevent you from producing antibodies against your baby's blood cells. A blood test can detect if you've begun to produce antibodies.
· Infection - rarely, chorionic villus sampling might trigger a uterine infection.
Amniocentesis
Amniocentesis is a procedure used to obtain a small sample of the amniotic fluid (which contains cells from the foetus) that surrounds the foetus during pregnancy.
An amniocentesis is generally offered to women between the 15th and 20th weeks of pregnancy who are at increased risk for chromosome abnormalities. This includes women who are over 35 years of age at delivery, or those who have had an abnormal maternal serum (blood) screening test indicating an increased risk for a chromosomal abnormality or neural tube defect.
Amniocentesis helps confirm a tentative diagnosis of an abnormality previously found with other testing. It may also find that a foetus does not have the abnormality that was suspected.
Risks of amniocentesis:
· Miscarriage - second-trimester amniocentesis carries a slight risk of miscarriage
· Needle injury
· Leaking amniotic fluid
· Infection
· Infection transmission
SKILL
S3.3.1 Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells.
ESSential idea: the inheritance of genes follows patterns.
3.2 Inheritance
UNDERSTANDINGS:
U3.4.1 Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed.
In 1865, an Auustrian monk named Gregor Mendel published results of his experiments on how garden pea plants passed on their characteristics. At the time, the term 'gene' did not exist (he used the term 'factor' instead) and the role that DNA played would not be discovered for nearly another century.
Mendel used artificial pollination in a series of experiments in which he carefully chose the pollen of various plants to fertilise other plants. He used a small brush to place the polln on the reproductive parts of the flowers, thus replacing the insects which do it naturally. This technique takes away the role of chance because the experimenter knows exactly which plants are fertilised by which pollen. In one cross, he wanted to see what would happen if he bred tall plants with short plants. The result was that he got all tall plants. But then when he crossed the resulting plants with each other, some of the offspring in the new generation were short.
U3.4.2 Gametes are haploid so contain only one allele of each gene.
Since gametes (sex cells) contain only half the number of a diploid cell i.e. 23 chromosomes, they only contain one allele of a certain gene (the other alleles are on the other homologous chromosome).
U3.4.3 The two alleles of each gene separate into different haploid daughter nuclei during meiosis.
Meiosis process explained above.
U3.4.4 Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles.
Fertilisation:
1. Many sperm cells are needed so that one can achieve fertilisation.
2. Sperm cells begin pushing their way through surrounding follicle cells.
3. First sperm to reach zona pellucida uses enzymes of acrosome to penetrate the egg cell wall.
4. Fusion of membranes results in cortical reaction.
5. Haploid nucleus of sperm enters egg - this restores the diploid number.
The alleles present in the egg and sperm can either be the same as each other or different. It is by chance the offspring develops any specific characteristic.
U3.4.5 Dominant alleles mask the effects of recessive alleles but co-dominant alleles have joint effects.
A dominant allele is an allele that has the same effect on the phenotype whether it is paired with the same allele or a different one. Dominant alleles are always expressed in the phenotype.
A recessive allele is an allele that has no effect on the phenotype unless present in a homozygous state i.e. BOTH alleles are recessive.
A co-dominant allele is a pair of alleles that both affect the phenotype when present in a heterozygous state. For example, a parent with curly hair and a parent with straight hair can have children with different degrees of hair curliness as both allele influence hair condition when both are present in the genotype.
U3.4.6 Many genetic diseases in humans are due to recessive alleles of autosomal genes, although some genetic diseases are due to dominant or co-dominant alleles.
Huntington's disease:
Huntington's disease is caused by a dominant allele. This genetic condition causes severely debilitating nerve damage but the symptoms do not show until the individual is around 40 years old. As a result, someone who has the gene for Huntington's disease does not know it for certain until they have started a career and possibly a family.
The symptoms include difficulty walking, speaking and holding objects. Within a few years, the person loses complete control of his or her muscles and dies an early death. Since it is dominant, all it takes is one 'H' allele in the person's genetic makeup to cause the condition.
Cystic Fibrosis:
Cystic fibrosis is characterized by the buildup of thick, sticky mucus that can damage many of the body's organs. The disorder's most common signs and symptoms include progressive damage to the respiratory system and chronic digestive system problems. The features of the disorder and their severity vary among affected individuals.
Mucus is a slippery substance that lubricates and protects the linings of the airways, digestive system, reproductive system, and other organs and tissues. In people with cystic fibrosis, the body produces mucus that is abnormally thick and sticky. This abnormal mucus can clog the airways, leading to severe problems with breathing and bacterial infections in the lungs. These infections cause chronic coughing, wheezing, and inflammation. Over time, mucus buildup and infections result in permanent lung damage, including the formation of scar tissue (fibrosis) and cysts in the lungs.
CF is a recessive disorder, which means that both parents must pass on the defective gene for any of their children to get the disease. If a child inherits only one copy of the faulty gene, he or she will be a carrier. Carriers don't actually have the disease, but they can pass it on to their children.
Co-dominance in the shape of red blood cells:
The prefix Hb refers to the gene that codes for haemoglobin. The superscript letter is for the typical shape of haemoglobin, A for normal and S for the shape that causes sickle cells. The genotypes and phenotypes are as follows:
Hb^AHb^A genotype generates haemoglobin that results in the phenotype with biconcave red blood cells (normal).
Hb^SHb^S genotype generates haemoglobin that results in the phenotype with curved red blood cells (severe sickle cell anaemia).
Hb^AHb^S genotype generate some of each type of haemoglobin because the alleles show co-dominance (the person who is said to have the sickle cell trait has fewer sickle-shaped cells so the anaemia is much less severe. This person also has some resistance to malaria).
U3.4.7 Some genetic diseases are sex-linked. The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes.
Any genetic trait whose allele has its locus on the X or Y chromosome is said to be sex-linked. Often genetic traits which show sex linkage affect one gender more than the other. Two examples of genetic traits which have this particularity are colour blindness and haemophilia.
Colour blindness is the inability to distinguish between certain colours, usually green and red. To people who are colour blind, the two colours look the same; they would not see the difference between a red and a green apple for example.
Haemophilia is a disorder in which blood does not clot properly. For most people, a small cut or scrape on their skin stops bleeding after a few minutes and eventually a scab forms. This process is called clotting. Individuals with haemophilia risk bleeding to death from what most would consider a minor injury such as a bruise, which would rapture tiny blood vessels. Such bleeding can also occur in internal organs. Medical treatments like specialised injections help to give people who suffer from haemophilia a better quality life.
Since the alleles for both colour blindness and haemophilia are found only on the X chromosome and not on the Y chromosome, the Y chromosome has no other allele to mask the 'bad' allele and thus these diseases are significantly more common in males than in females. It is extremely rare for a girl to have such sex-linked conditions.
U3.4.8 Many genetic diseases have been identified in humans but most are very rare.
Rare genetic disorders here.
U3.4.9 Radiation and mutagenic chemicals increase the mutation rate and can cause genetic diseases and cancer.
Ionising radiation, the type of radiation released by radioactive materials, contributes to DNA mutation. Like UVB (ultraviolet B), ionising radiation causes direct DNA damage that lead to mutations. Exposure to ionising radiation leads to double-stranded breaks in DNA, so both strands of the DNA molecule are broken at the same locus/location. The cell however can repair this type of breakage by reattaching the DNA strands together. On the other hand, sometimes double-stranded breaks occur along the entire length of the DNA. Mutations occur if the repair mechanisms in the cell re-attach the wrong section of DNA back together, so that a part of the DNA strand goes missing. This may lead to the deletion of important genes, or a change in the locus of a gene within the DNA. These types of mutations are linked to the development of a number of cancers, including leukaemia. In summary, by directly or indirectly ionising, radiation can cause changes in a cell's ability to conduct repair and reproduction.
Mutagenic agents can be classified into three categories: physical (e.g. gamma rays), chemical (e.g. ethyl methane sulphonate) and transposable elements (such as transposons, retrotransposons, T-DNA, retroviruses). At present, limited data are available on the scope of genetic effects induced at the molecular level in plants and on the specificity and relative efficiency of these different categories of agents. These effects involve DNA damage, which results in base pair changes (single/simple nucleotide polymorphisms, SNPs), small insertions and deletions (indels) and chromosomal rearrangements.
APPLICATION:
A3.4.1 Inheritance of ABO blood groups.
ABO Blood Group System
1. If you have type A antigens on the surface of your red blood cells, you also have anti-B antibodies in your plasma.
2. If you have type B antigens on the surface of your red blood cells, you also have anti-A antibodies in your plasma.
3. If you have type A and type B antigens on the surface of your red blood cells, you do not have antibodies to A or B antigens in your plasma.
4. If you have neither type A nor type B antigens on the surface of your red blood cells, you have anti-A and anti-B antibodies in your plasma.
UNDERSTANDINGS:
U3.4.1 Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed.
In 1865, an Auustrian monk named Gregor Mendel published results of his experiments on how garden pea plants passed on their characteristics. At the time, the term 'gene' did not exist (he used the term 'factor' instead) and the role that DNA played would not be discovered for nearly another century.
Mendel used artificial pollination in a series of experiments in which he carefully chose the pollen of various plants to fertilise other plants. He used a small brush to place the polln on the reproductive parts of the flowers, thus replacing the insects which do it naturally. This technique takes away the role of chance because the experimenter knows exactly which plants are fertilised by which pollen. In one cross, he wanted to see what would happen if he bred tall plants with short plants. The result was that he got all tall plants. But then when he crossed the resulting plants with each other, some of the offspring in the new generation were short.
U3.4.2 Gametes are haploid so contain only one allele of each gene.
Since gametes (sex cells) contain only half the number of a diploid cell i.e. 23 chromosomes, they only contain one allele of a certain gene (the other alleles are on the other homologous chromosome).
U3.4.3 The two alleles of each gene separate into different haploid daughter nuclei during meiosis.
Meiosis process explained above.
U3.4.4 Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles.
Fertilisation:
1. Many sperm cells are needed so that one can achieve fertilisation.
2. Sperm cells begin pushing their way through surrounding follicle cells.
3. First sperm to reach zona pellucida uses enzymes of acrosome to penetrate the egg cell wall.
4. Fusion of membranes results in cortical reaction.
5. Haploid nucleus of sperm enters egg - this restores the diploid number.
The alleles present in the egg and sperm can either be the same as each other or different. It is by chance the offspring develops any specific characteristic.
U3.4.5 Dominant alleles mask the effects of recessive alleles but co-dominant alleles have joint effects.
A dominant allele is an allele that has the same effect on the phenotype whether it is paired with the same allele or a different one. Dominant alleles are always expressed in the phenotype.
A recessive allele is an allele that has no effect on the phenotype unless present in a homozygous state i.e. BOTH alleles are recessive.
A co-dominant allele is a pair of alleles that both affect the phenotype when present in a heterozygous state. For example, a parent with curly hair and a parent with straight hair can have children with different degrees of hair curliness as both allele influence hair condition when both are present in the genotype.
U3.4.6 Many genetic diseases in humans are due to recessive alleles of autosomal genes, although some genetic diseases are due to dominant or co-dominant alleles.
Huntington's disease:
Huntington's disease is caused by a dominant allele. This genetic condition causes severely debilitating nerve damage but the symptoms do not show until the individual is around 40 years old. As a result, someone who has the gene for Huntington's disease does not know it for certain until they have started a career and possibly a family.
The symptoms include difficulty walking, speaking and holding objects. Within a few years, the person loses complete control of his or her muscles and dies an early death. Since it is dominant, all it takes is one 'H' allele in the person's genetic makeup to cause the condition.
Cystic Fibrosis:
Cystic fibrosis is characterized by the buildup of thick, sticky mucus that can damage many of the body's organs. The disorder's most common signs and symptoms include progressive damage to the respiratory system and chronic digestive system problems. The features of the disorder and their severity vary among affected individuals.
Mucus is a slippery substance that lubricates and protects the linings of the airways, digestive system, reproductive system, and other organs and tissues. In people with cystic fibrosis, the body produces mucus that is abnormally thick and sticky. This abnormal mucus can clog the airways, leading to severe problems with breathing and bacterial infections in the lungs. These infections cause chronic coughing, wheezing, and inflammation. Over time, mucus buildup and infections result in permanent lung damage, including the formation of scar tissue (fibrosis) and cysts in the lungs.
CF is a recessive disorder, which means that both parents must pass on the defective gene for any of their children to get the disease. If a child inherits only one copy of the faulty gene, he or she will be a carrier. Carriers don't actually have the disease, but they can pass it on to their children.
Co-dominance in the shape of red blood cells:
The prefix Hb refers to the gene that codes for haemoglobin. The superscript letter is for the typical shape of haemoglobin, A for normal and S for the shape that causes sickle cells. The genotypes and phenotypes are as follows:
Hb^AHb^A genotype generates haemoglobin that results in the phenotype with biconcave red blood cells (normal).
Hb^SHb^S genotype generates haemoglobin that results in the phenotype with curved red blood cells (severe sickle cell anaemia).
Hb^AHb^S genotype generate some of each type of haemoglobin because the alleles show co-dominance (the person who is said to have the sickle cell trait has fewer sickle-shaped cells so the anaemia is much less severe. This person also has some resistance to malaria).
U3.4.7 Some genetic diseases are sex-linked. The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes.
Any genetic trait whose allele has its locus on the X or Y chromosome is said to be sex-linked. Often genetic traits which show sex linkage affect one gender more than the other. Two examples of genetic traits which have this particularity are colour blindness and haemophilia.
Colour blindness is the inability to distinguish between certain colours, usually green and red. To people who are colour blind, the two colours look the same; they would not see the difference between a red and a green apple for example.
Haemophilia is a disorder in which blood does not clot properly. For most people, a small cut or scrape on their skin stops bleeding after a few minutes and eventually a scab forms. This process is called clotting. Individuals with haemophilia risk bleeding to death from what most would consider a minor injury such as a bruise, which would rapture tiny blood vessels. Such bleeding can also occur in internal organs. Medical treatments like specialised injections help to give people who suffer from haemophilia a better quality life.
Since the alleles for both colour blindness and haemophilia are found only on the X chromosome and not on the Y chromosome, the Y chromosome has no other allele to mask the 'bad' allele and thus these diseases are significantly more common in males than in females. It is extremely rare for a girl to have such sex-linked conditions.
U3.4.8 Many genetic diseases have been identified in humans but most are very rare.
Rare genetic disorders here.
U3.4.9 Radiation and mutagenic chemicals increase the mutation rate and can cause genetic diseases and cancer.
Ionising radiation, the type of radiation released by radioactive materials, contributes to DNA mutation. Like UVB (ultraviolet B), ionising radiation causes direct DNA damage that lead to mutations. Exposure to ionising radiation leads to double-stranded breaks in DNA, so both strands of the DNA molecule are broken at the same locus/location. The cell however can repair this type of breakage by reattaching the DNA strands together. On the other hand, sometimes double-stranded breaks occur along the entire length of the DNA. Mutations occur if the repair mechanisms in the cell re-attach the wrong section of DNA back together, so that a part of the DNA strand goes missing. This may lead to the deletion of important genes, or a change in the locus of a gene within the DNA. These types of mutations are linked to the development of a number of cancers, including leukaemia. In summary, by directly or indirectly ionising, radiation can cause changes in a cell's ability to conduct repair and reproduction.
Mutagenic agents can be classified into three categories: physical (e.g. gamma rays), chemical (e.g. ethyl methane sulphonate) and transposable elements (such as transposons, retrotransposons, T-DNA, retroviruses). At present, limited data are available on the scope of genetic effects induced at the molecular level in plants and on the specificity and relative efficiency of these different categories of agents. These effects involve DNA damage, which results in base pair changes (single/simple nucleotide polymorphisms, SNPs), small insertions and deletions (indels) and chromosomal rearrangements.
APPLICATION:
A3.4.1 Inheritance of ABO blood groups.
ABO Blood Group System
1. If you have type A antigens on the surface of your red blood cells, you also have anti-B antibodies in your plasma.
2. If you have type B antigens on the surface of your red blood cells, you also have anti-A antibodies in your plasma.
3. If you have type A and type B antigens on the surface of your red blood cells, you do not have antibodies to A or B antigens in your plasma.
4. If you have neither type A nor type B antigens on the surface of your red blood cells, you have anti-A and anti-B antibodies in your plasma.
A3.4.2 Red-green colour blindness and haemophilia as examples of sex-linked inheritance.
See U3.4.7.
A3.4.3 Inheritance of cystic fibrosis and Huntington's disease.
See U3.4.6.
A3.4.4 Consequences of radiation after nuclear bombing of Hiroshima and accident at Chernobyl.
SKILL:
S3.4.1 Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
This Punnett grid is dihybrid however monohybrid is much easier - only two alleles from each parent are displayed.
S3.4.2 Comparison of predicted and actual outcomes of genetic crosses using real data.
S3.4.3 Analysis of pedigree charts to deduce the pattern of inheritance of genetic diseases.
See U3.4.7.
A3.4.3 Inheritance of cystic fibrosis and Huntington's disease.
See U3.4.6.
A3.4.4 Consequences of radiation after nuclear bombing of Hiroshima and accident at Chernobyl.
SKILL:
S3.4.1 Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
This Punnett grid is dihybrid however monohybrid is much easier - only two alleles from each parent are displayed.
S3.4.2 Comparison of predicted and actual outcomes of genetic crosses using real data.
S3.4.3 Analysis of pedigree charts to deduce the pattern of inheritance of genetic diseases.
essential idea: biologists have developed techniques for artificial manipulation of dna, cells and organisms.
3.5 Genetic modification and biotechnology
UNDERSTANDINGS:
U3.5.1 Gel electrophoresis is used to separate proteins or fragments of DNA according to size.
U3.5.2 PCR can be used to amplify small amounts of DNA.
PCR is repeated cycling of three steps:
U3.5.3 DNA profiling involves comparison of DNA.
UNDERSTANDINGS:
U3.5.1 Gel electrophoresis is used to separate proteins or fragments of DNA according to size.
U3.5.2 PCR can be used to amplify small amounts of DNA.
PCR is repeated cycling of three steps:
- Denature DNA
The DNA is heated to 95° C. This breaks the weak hydrogen bonds that hold DNA strands together in a helix, allowing the strands to separate creating single stranded DNA. - Primer Annealing
The mixture is cooled to anywhere from 45-72° C. This allows the primers to bind (anneal) to their complementary sequence in the template DNA. - Extension
The reaction is then heated to 72° C, the optimal temperature for DNA polymerase to act. DNA polymerase extends the primers, adding nucleotides onto the primer in a sequential manner, using the target DNA as a template. - Cycle is repeated.
U3.5.3 DNA profiling involves comparison of DNA.
- DNA profiling is a technique by which individuals are identified on the basis of their respective DNA profiles
- Within the non-coding region of an individual's genome, there exists satellite DNA - long stretches of DNA made up of repeating elements called short tandem repeats (STRs)
- These repeating sequences can be excised to form fragments, by cutting with a variety of restriction endonucleases (which cut DNA at specific sites)
- As individuals all have a different number of repeats in a given sequence of satellite DNA, they will all generate unique fragment profiles
- These different profiles can be compared using gel electrophoresis
U3.5.4 Genetic modification is carried out by gene transfer between species.
A3.5.2 Gene transfer to bacteria using plasmids makes use of restriction endonucleases and DNA ligase.
The full name for restriction enzyme is restriction endonucleases. Their job is to cut a strand of DNA at a specific area of the code called a recognition site. Genetic engineers have a large number of restriction endonucleases for different recognition sites so that cuts can be made in specific places. The opposite of that is DNA ligase which puts together two strands of DNA.
After the DNA has been removed using this enzyme, sticky ends are formed, which are at the end of the fragment and are classified as 'sticky' because they want to become part of a DNA strand again.
The plasmid in this type of genetic engineering is called the vector.
NOTE: The same restriction endonuclease is used to cut the genes from both the human and bacteria so that they are of equal lengths.
U3.5.5 Clones are groups of genetically identical organisms, derived from a single original parent cell.
U3.5.6 Many plant species and some animal species have natural methods of cloning.
Clones are genetically identical individuals. Bacteria, plants, and some animals, can reproduce asexually to form clones that are genetically identical to their parent. Identical human twins are also clones. Any differences between them are due to environmental factors.
Asexual reproduction only requires one parent, unlike sexual reproduction, which needs two. Since there is only one parent, there is no fusion of gametes, and no mixing of genetic information. As a result, the offspring are genetically identical to the parent, and to each other. They are clones.
Plants:
Asexual reproduction in plants can take a number of forms. Many plants develop underground food-storage organs that later develop into the following year’s plants. Potato plants and daffodil plants do this.
Animals:
Asexual reproduction in animals is less common than sexual reproduction. It happens in sea anemones and starfish, for example.
Natural cloning:
Twins are genetically identical because they are formed after one fertilised egg cell splits into two cells. They are natural clones.
U3.5.7 Animals can be cloned at the embryo stage by breaking up the embryo into more than one group of cells.
U3.5.8 Methods have been developed for cloning adult animals using differentiated cells.
A3.5.1 Use of DNA profiling in paternity and forensic investigations.
A3.5.3 Assessment of the potential risks and benefits associated with genetic modification of crops.
Potential Benefits
Potential Harmful Effects
A3.5.4 Production of cloned embryos produced by somatic-cell nuclear transfer.
Somatic Cell Nuclear Transfer (SCNT) is a method of reproductive cloning using differentiated animal cells
Dolly the sheep is the most common example of somatic-cell nuclear transfer.
After the DNA has been removed using this enzyme, sticky ends are formed, which are at the end of the fragment and are classified as 'sticky' because they want to become part of a DNA strand again.
The plasmid in this type of genetic engineering is called the vector.
NOTE: The same restriction endonuclease is used to cut the genes from both the human and bacteria so that they are of equal lengths.
U3.5.5 Clones are groups of genetically identical organisms, derived from a single original parent cell.
U3.5.6 Many plant species and some animal species have natural methods of cloning.
Clones are genetically identical individuals. Bacteria, plants, and some animals, can reproduce asexually to form clones that are genetically identical to their parent. Identical human twins are also clones. Any differences between them are due to environmental factors.
Asexual reproduction only requires one parent, unlike sexual reproduction, which needs two. Since there is only one parent, there is no fusion of gametes, and no mixing of genetic information. As a result, the offspring are genetically identical to the parent, and to each other. They are clones.
Plants:
Asexual reproduction in plants can take a number of forms. Many plants develop underground food-storage organs that later develop into the following year’s plants. Potato plants and daffodil plants do this.
Animals:
Asexual reproduction in animals is less common than sexual reproduction. It happens in sea anemones and starfish, for example.
Natural cloning:
Twins are genetically identical because they are formed after one fertilised egg cell splits into two cells. They are natural clones.
U3.5.7 Animals can be cloned at the embryo stage by breaking up the embryo into more than one group of cells.
U3.5.8 Methods have been developed for cloning adult animals using differentiated cells.
A3.5.1 Use of DNA profiling in paternity and forensic investigations.
- A DNA sample is collected (blood, saliva, semen, etc.) and amplified using PCR
- Satellite DNA (non-coding) is cut with specific restriction enzymes to generate fragments
- Individuals will have unique fragment lengths due to the variable length of their short tandem repeats (STR)
- The fragments are separated with gel electrophoresis (smaller fragments move quicker through the gel)
- The DNA profile can then be analysed according to need
A3.5.3 Assessment of the potential risks and benefits associated with genetic modification of crops.
Potential Benefits
- Allows for the introduction of a characteristic that wasn't present within the gene pool (selective breeding could not have produced desired phenotype)
- Results in increased productivity of food production (requires less land for comparable yield)
- Less use of chemical pesticides, reducing the economic cost of farming
- Can now grow in regions that, previously, may not have been viable (reduces need for deforestation)
Potential Harmful Effects
- Could have currently unknown harmful effects (e.g. toxin may cause allergic reactions in a percentage of the population)
- Accidental release of transgenic organism into the environment may result in competition with native plant species
- Possibility of cross pollination (if gene crosses the species barrier and is introduced to weeds, may have a hard time controlling weed growth)
- Reduces genetic variation / biodiversity (corn borer may play a crucial role in local ecosystem)
A3.5.4 Production of cloned embryos produced by somatic-cell nuclear transfer.
Somatic Cell Nuclear Transfer (SCNT) is a method of reproductive cloning using differentiated animal cells
- A female animal (e.g. sheep) is treated with hormones (such as FSH) to stimulate the development of eggs
- The nucleus from an egg cell is removed (enucleated), thereby removing the genetic information from the cell
- The egg cell is fused with the nucleus from a somatic (body) cell of another sheep, making the egg cell diploid
- An electric shock is delivered to stimulate the egg to divide, and once this process has begun the egg is implanted into the uterus of a surrogate
- The developing embryo will have the same genetic material as the sheep that contributed the diploid nucleus, and thus be a clone.
Dolly the sheep is the most common example of somatic-cell nuclear transfer.
S3.5.1 Design of an experiment to assess one factor affecting the rooting of stem-cuttings.
Design should be done in class.
Some factors affecting the growth of stem cuttings are:
- Concentration of growth hormone auxin
- Temperature of environment
- Water / no water
- Soil quality
S3.5.2 Analysis of examples of DNA profiles.
Paternity Testing: Children inherit half of their alleles from each parent and thus should possess a combination of their parents alleles
Forensic Investigation: Suspect DNA should be a complete match with the sample taken from a crime scene if a conviction is to occur
S3.5.3 Analysis of data on risks to monarch butterflies of Bt crops.
Design should be done in class.
Some factors affecting the growth of stem cuttings are:
- Concentration of growth hormone auxin
- Temperature of environment
- Water / no water
- Soil quality
S3.5.2 Analysis of examples of DNA profiles.
Paternity Testing: Children inherit half of their alleles from each parent and thus should possess a combination of their parents alleles
Forensic Investigation: Suspect DNA should be a complete match with the sample taken from a crime scene if a conviction is to occur
S3.5.3 Analysis of data on risks to monarch butterflies of Bt crops.