AQA A-level Biology: Mutation and Meiosis (7402) – Revision Notes

These are condensed revision notes for AQA A-level Biology (7402) section 3.4.3, Genetic diversity can arise as a result of mutation or during meiosis, from the AQA AS and A-level Biology specification for first teaching 2015. For full explanations read the study guide, then test yourself with the practice questions. See also the AQA A-level Biology hub, the printable checklist and the free diagnostics.

Key definitions

Term	Meaning
Gene mutation	A change in the base sequence of DNA
Base substitution	One base replaced by another; the length of the gene is unchanged
Base deletion	One base lost; the reading frame shifts
Degenerate code	Several amino acids can be specified by two or more different triplets
Mutagenic agent	Anything that increases the rate of gene mutation
Non-disjunction	Failure of chromosomes (or chromatids) to separate in meiosis
Homologous chromosomes	A pair, one from each parent, with the same genes at the same loci
Haploid (n) / diploid (2n)	One set of chromosomes / two sets
Chiasma	A point where non-sister chromatids have crossed over
Gamete	A haploid sex cell that fuses at fertilisation

Gene mutations at a glance

• Arise spontaneously during DNA replication.
• Mutagenic agents (for example ionising radiation, ultraviolet light, some chemicals) raise the rate; they do not choose the gene.
• Two types in the specification: substitution and deletion.

	Substitution	Deletion
Bases after the change	Unaffected	All shifted by one place
Triplets affected	One	Every triplet from the deletion onward
Likely effect on polypeptide	None, one amino acid changed, or a premature stop	Many amino acids changed; often a very different polypeptide
Why it might have no effect	Degenerate code	Rarely; only if near the end of the coding sequence

Method: predict the effect of a mutation

1. Write the mRNA as triplets.
2. Translate it using the data you are given (codons are not recalled from memory).
3. Apply the change to the DNA or mRNA and rewrite the triplets.
4. Translate again and compare.
5. State the effect: same, one amino acid, or frameshift from that point.
6. Link to the protein: a different primary structure can change the tertiary structure and so the function.

Small worked reminder

Given AUG AAA CAC UAA with AAA = Lys, AAG = Lys, CAC = His, UAA = stop. A change of AAA to AAG leaves Met-Lys-His, the same, because the code is degenerate. Deleting the first A of AAA gives AUG AAC ACU AA, which changes every codon after AUG.

Quick check on gene numbers

A gene of 1200 bases with a supplied rate of 2 × 10⁻⁸ per base per replication has 2.4 × 10⁻⁵ expected mutations per replication. Over 5 × 10⁵ replications the expected number is 12.

Non-disjunction

• Arises spontaneously in meiosis.
• Gives gametes with n + 1 or n − 1 chromosomes.
• Fusion with a normal gamete (n) gives 2n + 1 or 2n − 1.

Failure at	Gametes produced
Meiosis I (homologous pair stays together)	Two n + 1 and two n − 1
Meiosis II (sister chromatids stay together)	One n + 1, one n − 1, two normal

Human example: 23 chromosomes in a normal gamete, 24 with an extra chromosome 21, giving 47 in the zygote.

Meiosis in outline

1. Interphase: DNA replicated, so each chromosome has two chromatids.
2. Meiosis I: homologous chromosomes pair, crossing over occurs, pairs line up at the equator and homologous chromosomes are pulled apart. Two haploid cells.
3. Meiosis II: chromatids are pulled apart. Four haploid cells, which are genetically different from one another.

Where the variation comes from

Source	When	What it does
Independent segregation	Meiosis I	Maternal and paternal chromosomes of each pair go to either pole at random, giving 2ⁿ combinations
Crossing over	Early in meiosis I	Non-sister chromatids of homologous chromosomes swap DNA, giving new allele combinations on a chromatid
Random fertilisation	Fertilisation	Which male gamete fuses with which female gamete is left to chance

Completing chromosome diagrams

Parent cell 2n = 6 (three pairs), after replication:

Cell	Number of chromosomes	Chromatids in each chromosome	One of each pair?
After meiosis I	3	2	Yes
After meiosis II	3	1	Yes

Rules: halving occurs at meiosis I. Each cell after meiosis I has one member of every pair, never both. Meiosis II separates chromatids and does not change the number.

DNA amount per cell (arbitrary units): 2 before replication, 4 after replication, 2 after meiosis I, 1 after meiosis II.

Mitosis versus meiosis

	Mitosis	Meiosis
Divisions	1	2
Cells	2	4 (usually)
Chromosome number	Same as the parent	Halved
Genetic result	Identical	Different
Pairing and crossing over	No	Yes
Use	Growth, repair, asexual reproduction	Gamete or spore formation

Life cycles

• 2n to n means meiosis. n to 2n means fertilisation. No change in number means mitosis.
• Animal: meiosis makes gametes.
• Moss or fern: diploid plant makes spores by meiosis; haploid plant makes gametes by mitosis.
• Some algae and fungi: only the zygote is diploid, and meiosis follows at once.

Method: write the chromosome number next to every cell in the cycle and mark the points where it changes.

The 2ⁿ calculations

n is the number of homologous pairs, not the diploid number.

Quantity (no crossing over)	Formula	2n = 20 (n = 10)
Gamete types from one individual	2ⁿ	2¹⁰ = 1024
Zygote types from two such parents	2ⁿ × 2ⁿ = 2²ⁿ	1024 × 1024 = 1 048 576

Reverse: if a species can make 4096 types of gamete, 2ⁿ = 4096, so n = 12 and 2n = 24.

Human: n = 23, 2²³ = 8 388 608 gamete types; 2⁴⁶ = 70 368 744 177 664 zygote types.

Must-know distinctions

• Gene mutation versus non-disjunction. A gene mutation changes the base sequence within a gene. Non-disjunction changes the number of whole chromosomes.
• Independent segregation versus crossing over. Segregation shuffles whole chromosomes between cells. Crossing over shuffles alleles between chromatids of one homologous pair.
• Homologous chromosomes versus sister chromatids. Homologous chromosomes are a pair from two parents and separate in meiosis I. Sister chromatids are identical copies joined at the centromere and separate in meiosis II (and in mitosis).
• Silent substitution versus no mutation. The DNA has changed, but the polypeptide has not. Describe both facts when you answer.
• Mitosis versus meiosis in a life cycle. Mitosis can make haploid cells (for example plant gametes) as well as diploid ones, so look at the chromosome number rather than the cell type.

Worked reminders

Counting chromosomes after non-disjunction. A species has 2n = 8, so a normal gamete has 4. A gamete with an extra chromosome has 5 and one that lacks a chromosome has 3. Fusing with a normal gamete gives 9 or 7 in the zygote.

Percentage increase from mutagen data. Plates without the chemical: 9, 11, 13 and 7 colonies, so the mean is 40 / 4 = 10. Plates with it have a mean of 30. The increase is (30 − 10) / 10 × 100 = 200%. Always divide by the control value.

Explanation frame for variation. Name the process, say when it happens, say what moves, and state the result. For example: in meiosis I, homologous pairs line up at the equator in a random orientation, so each gamete gets a different mixture of maternal and paternal chromosomes.

Quick self-test

1. Name the two types of gene mutation in the specification.
2. Why can a base substitution leave a polypeptide unchanged?
3. Why is a base deletion usually more damaging than a substitution?
4. How does a mutagenic agent affect mutation?
5. At which division does the chromosome number halve?
6. Between which chromatids does crossing over occur?
7. A cell has 2n = 12. How many chromosomes are in each cell after meiosis II?
8. Calculate the number of gamete types for a species with 2n = 20, ignoring crossing over.
9. Where does meiosis occur in a moss life cycle?
10. If non-disjunction occurs in meiosis II, how many of the four gametes are normal?
11. A mutagen test gave mean colony counts of 8 without and 32 with the chemical. Calculate the percentage increase.

Answers

1. Substitution and deletion.
2. The code is degenerate, so the changed triplet can still specify the original amino acid.
3. It shifts the reading frame, so every triplet after the deletion changes.
4. It increases the rate of mutation.
5. Meiosis I.
6. Non-sister chromatids of a homologous pair.
7. 6 chromosomes, each with one chromatid.
8. 2¹⁰ = 1024.
9. In the diploid plant, in the spore-producing structure, when spores are made.
10. Two of the four.
11. (32 − 8) / 8 × 100 = 300%.

Where marks are usually lost

• Writing “the DNA changes” without saying which bases and what effect on amino acids.
• Not mentioning the degenerate code when explaining a silent substitution.
• Saying a deletion changes “one amino acid”.
• Putting the halving at meiosis II.
• Writing “chromosomes cross over” without non-sister chromatids and homologous pairs.
• Using the diploid number as n in 2ⁿ.
• Calling every gamete a product of meiosis in plants.
• Giving a percentage increase using the new value as the baseline.

Official syllabus

AQA AS and A-level Biology specification (7401 and 7402), for first teaching 2015, section 3.4.3 (version 1.5), published by AQA.