3.4.3 Genetic diversity can arise as a result of mutation or during meiosis

Content:

Gene mutations involve a change in the base sequence of chromosomes.

They can arise spontaneously during DNA replication and include base deletion and base substitution.

Due to the degenerate nature of the genetic code, not all base substitutions cause a change in the sequence of encoded amino acids.

Mutagenic agents can increase the rate of gene mutation.

Mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis.

Meiosis produces daughter cells that are genetically different from each other.

The process of meiosis only in sufficient detail to show how:

two nuclear divisions result usually in the formation of four haploid daughter cells from a single diploid parent cell
genetically different daughter cells result from the independent segregation of homologous chromosomes
crossing over between homologous chromosomes results in further genetic variation among daughter cells.

Students should be able to:

complete diagrams showing the chromosome content of cells after the first and second meiotic division, when given the chromosome content of the parent cell
explain the different outcome of mitosis and meiosis
recognise where meiosis occurs when given information about an unfamiliar life cycle
explain how random fertilisation of haploid gametes further increases genetic variation within a species.

Mutations

Define ‘gene mutation’. (2)

Change in the base / nucleotide sequence of chromosomes / DNA

Resulting in the formation of new allele.

Name one mutagenic agent. (1)

high energy radiation / ionising particles

named particles / α, β, γ

colchicine

x rays / cosmic rays

uv light

carcinogen / named carcinogen

Explain how a gene mutation can have no effect on an individual. (2)

1.Because the genetic code is degenerate so amino acid sequence may not change.

Accept description of ‘degenerate’ i.e. some amino acids are coded by more than one triplet / codon.

OR Mutation is in an intron so amino acid sequence may not change.

2.Mutation changes the amino acid but has no effect on tertiary structure.

3.New allele is recessive so does not influence phenotype.

Explain how a gene mutation can have a positive effect on an individual. (2)

1.because a mutation can result in a change in polypeptide that positively changes the properties of the protein.

For ‘polypeptide’ accept ‘amino acid sequence’

2.May result in increased reproductive success or increased survival chances.

Not all mutations in the nucleotide sequence of a gene cause a change in the structure of a polypeptide.

Give two reasons why. (2)

Different DNA triplets can code for the same amino acid (the code is degenerate).

Reject: codons as question states within genes

The mutation may occur in a non-coding region (intron), so it does not affect the polypeptide.

Explain how a change in the DNA base sequence for a protein may result in a change in the structure of the protein. (3)

Change in amino acids primary structure

Change in hydrogen / ionic / disulfide bonds

Alters tertiary structure

Reject: ‘Different amino acid is formed’ – negates first marking point.

Neutral: Reference to active site.

Explain how a change in the DNA base sequence may result in a non-functional enzyme. (3)

Change in amino acids primary structure

Change in hydrogen / ionic / disulfide bonds in the tertiary structure / active site of enzyme

Substrate cannot bind / no enzyme-substrate complexes form

Reject: ‘Different amino acid is formed’ – negates first marking point.

Describe how a deletion mutation alters the structure of a gene. (1)

One or more bases are removed from the DNA sequence, causing a frameshift (base sequence change) from the point of mutation.

Errors in the precise location of splicing in the DNA molecule can lead to mutations.Explain why. (1)

Change in base sequence (of exons) occurs

OR Deletion/addition of bases occurs

OR Deletion of exons

The figure below shows part of a pre-mRNA molecule. Geneticists identified two mutations that can affect this pre-mRNA, as shown in the figure.

a. Mutation 1 leads to the production of a non-functional protein. Explain why. (3)

1. Mutation changes triplets / codons after that point / causes frame shift;

Accept changes splicing site

Ignore changes in sequence of nucleotides / bases

2. Changes amino acid sequence after this / codes for different amino acids after this

Accept changes primary structure

Reject changes amino acid formed / one amino acid changed

3. Affects hydrogen / ionic / sulfur bond not peptide bond

4. Changes tertiary structure of protein so non-functional

b. What effect might mutation 2 have on the protein produced? Explain your answer. (2)

1. Intron are non-coding DNA / only exons are coding

2. So not translated / no change in mRNA produced / no effect on protein / no effect on amino acid sequence

3. Prevents / changes splicing

4. So faulty mRNA formed

Accept exons not joined together / introns not removed

5. Get different amino acid sequence

Suggest how a mutation can lead to the production of a protein that has one amino acid missing. (2)

Loss of 3 bases / triplet / codon

Or triplet for last amino acid is changed to a stop codon

Or 3 bases / triplet forms an intron / non-coding DNA

‘Stop codon / code formed’ = 1 mark max unless related to the last amino acid

Loss of bases = 1 mark

During which part of the cell cycle are gene mutations most likely to occur? (1)

Suggest an explanation for your answer. (1)

Interphase / synthesis phase

because DNA replication occurs which is the longest stage

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