AS-LEVEL AQA BIOLOGY NOTES
TOPIC 3: genetic diversity can arise as a result of mutation or during meiosis
Gene mutations are natural occurrences that come about during DNA replication. There are 3 categories of gene mutations:
Substitution = a nucleotide base is replaced with another
Substitution = a nucleotide base is replaced with another
Insertion = an extra nucleotide base is inserted into the sequence causing ‘frameshift’ where all the subsequent bases are shifted down 1 place relative to the twin DNA strand
Deletion = the absence of a nucleotide, causing ‘frameshift’ where all the subsequent bases are shifted back 1 place relative to the twin DNA strand
The effects of these mutations can be..
Point mutation
Meiosis
Interphase I to telophase I and cytokinesis I are very similar as in mitosis. The main difference is that during prophase I, recombination occurs. This is what creates genetic diversity and involves chromatids swapping genes between themselves. Recombination, crossing over, and the segregations of homologous chromosomes ensure that the four daughter cells are not genetically identical.
The first division creates two daughter cells; the second division leads to the production of 4 daughter cells.
Prophase II
- Neutral
- May occur in phenotypically insignificant strand of DNA
- May not result in change of polypeptide primary sequence (because DNA is degenerate)
- May result in a change of polypeptide primary sequence that does not affect secondary/tertiary/quaternary structure of protein therefore protein function unaffected
- May results in a change of polypeptide secondary/tertiary/quaternary structure but where active site of protein remains the same therefore function still unaffected
- May occur in phenotypically insignificant strand of DNA
- Harmful
- May result in change in final protein shape where protein and active site is deformed and therefore cannot fulfil function
- May result in change in final protein shape where protein and active site is deformed and therefore cannot fulfil function
- Beneficial
- May result in change in final protein shape where the protein performs its function better than it would have without the mutation
- This is the basis of natural selection and evolution
- The individual is better suited to survival and will pass on the mutation to its offspring
- E.g. eye colour
- Blue eyes was a mutation that occurred about 7000 years ago
- In sunny areas, this would be harmful as the retina is more exposed
- However in cloudy regions this was beneficial as it enabled people to see better
- So the mutation was carried down generations and became widespread
- Blue eyes was a mutation that occurred about 7000 years ago
- May result in change in final protein shape where the protein performs its function better than it would have without the mutation
Point mutation
- Mutations can affect 1 nucleotide base, or more than one adjacent bases
- A point mutation is where only one base is affected
- There are 3 types: silent, nonsense, missense
- Silent mutation
- No change in amino acid sequence of polypeptide
- No change in amino acid sequence of polypeptide
- Missense mutation
- The mutation changes the code for 1 amino acid
- 1 amino acid in the sequence is changes
- The mutation changes the code for 1 amino acid
- Nonsense mutation
- The mutation changes the code turning the triplet into a stop codon
- Instructs the end of polypeptide synthesis
- The polypeptide is shorter than it would normally be
- The mutation changes the code turning the triplet into a stop codon
Meiosis
Interphase I to telophase I and cytokinesis I are very similar as in mitosis. The main difference is that during prophase I, recombination occurs. This is what creates genetic diversity and involves chromatids swapping genes between themselves. Recombination, crossing over, and the segregations of homologous chromosomes ensure that the four daughter cells are not genetically identical.
The first division creates two daughter cells; the second division leads to the production of 4 daughter cells.
Prophase II
- Chromosomes pair up
- Pair of sister chromatids can be seen using a light microscope
- Centrioles divides in 2, forming 2 daughter centrioles
- Nuclear membrane breaks down
- Replicated chromosomes line up down middle of cell
- Centrioles move to opposite poles of the cell, forming spindle
- Chromosomes move to central region of spindle
- Each becomes to spindle thread by its centromere
- Sister chromosomes pulled apart towards opposite ends of the cell
- Sister chromatids separated as centromere splits
- Each sister chromosomes effectively become individual chromosomes
- Each one genetically unique
- Spindle fibers shorten
- This pulls sister chromatids further and further away from each other
- New nuclear envelope forms around each chromatid
- Spindle breaks down and disappears
- Chromatids uncoil and can no longer be seen under a light microscope
- Cytoplasm and surface membrane divide, creating four independent haploid daughter cells.
Mitosis
Random fertilisation
- 2 daughter cells are genetically identical to parent cell
- Doesn’t contribute to genetic diversity
- Mutations sometimes occur
Random fertilisation
- Random fertilisation of gametes increases genetic diversity
- Gametes all have varied genetic content
- Combining gametes increases diversity of gene distribution