Genetic diversity can arise as a result of mutation or during
AQA
A-Level
A gene mutation is a change to the base sequence of the DNA in a chromosome, producing a new allele of that gene. Because the base sequence determines the sequence of codons on mRNA, a change can alter which amino acid is inserted at that point in the polypeptide. A different amino acid sequence changes the primary structure. Since the R groups of the amino acids determine where ionic bonds, hydrogen bonds and disulfide bridges form, the tertiary structure may also change. For an enzyme, this altered 3D shape means the active site may no longer be complementary to its substrate, preventing enzyme-substrate complexes from forming. Some mutations are silent: because the genetic code is degenerate, a changed triplet may still code for the same amino acid, so the primary structure is unchanged.
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Objectives
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Exam Tips
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Pitfalls
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Key Terms
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Mark Points
Subtopics in this area
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.
Genetic diversity can arise as a result of mutation or during Revision Guide
Learning Objectives
What you need to know and understand
- Define a gene mutation as a change in the base sequence of DNA within a chromosome.
- Explain how a change in the base sequence of a gene leads to a change in the tertiary structure of the protein it codes for.
- Explain why some changes to the base sequence of a gene have no effect on the polypeptide produced.
- Describe how base substitutions and base deletions change the base sequence of a gene.
- Explain why a base deletion usually has a greater effect on the polypeptide than a base substitution.
- Predict the amino acid sequence produced after a stated substitution or deletion, using a supplied codon table.
- Explain, using the degenerate nature of the code, why some base substitutions do not change the polypeptide produced.
- Use a codon table to decide whether a stated base substitution is silent.
- Distinguish the degenerate and universal properties of the genetic code and state which applies in a given context.
- Name two mutagenic agents and describe how each increases the rate of gene mutation.
- Explain why a mutagen increases the frequency of mutation but does not determine which mutation occurs.
- Explain why exposure to an antibiotic cannot create an allele for antibiotic resistance.
- Explain how non-disjunction in meiosis I and in meiosis II produce gametes with abnormal chromosome numbers.
- Complete a diagram showing the chromosome content of the gametes when non-disjunction occurs at a stated division.
- Explain why a zygote formed from a gamete carrying an extra chromosome has forty-seven chromosomes.
- Describe two events within meiosis that make the daughter cells genetically different from each other.
- Explain how crossing over produces new combinations of alleles on one chromosome.
- Calculate the number of different chromosome combinations possible in the gametes of an organism using $2^n$.
- Describe the outcome of each meiotic division in terms of chromosome number and chromatid content.
- Explain how independent segregation and crossing over together produce genetically different gametes.
- Explain why the chromosome number must be halved during gamete formation in a sexually reproducing species.
- Complete diagrams of the chromosome content of cells after the first and second meiotic divisions from a given parent cell.
- Explain the different outcomes of mitosis and meiosis in terms of cell number, chromosome number and genetic variation.
- Identify where meiosis occurs in an unfamiliar life cycle and justify the choice using chromosome number.
- Explain how random fertilisation of haploid gametes further increases genetic variation.
Marking Points
Key points examiners look for in your answers
- A gene mutation involves a change in the base sequence of the DNA or a change in a DNA triplet.
- This can result in a change in the sequence of amino acids, altering the primary structure of the polypeptide.
- A change in primary structure can lead to a consequent change in the tertiary structure of the protein due to different bond locations.
- The altered tertiary structure can affect protein function, such as an active site no longer being complementary to the substrate.
- Some mutations are silent because the genetic code is degenerate, so a changed triplet may still code for the same amino acid and the primary structure is unchanged.
- Mutations can arise spontaneously as errors during DNA replication.
- A base substitution involves one base being replaced by a different base, affecting one triplet only.
- A base deletion involves a base being removed so that all subsequent triplets are shifted, causing a frameshift.
- A deletion changes most amino acids after the point of mutation, so the resulting protein is often non-functional.
- A substitution may not change the amino acid sequence because the genetic code is degenerate.
- States that the code is degenerate because more than one triplet (codon) codes for the same amino acid.
- Explains that a substituted base may still produce a codon specifying the same amino acid, so the primary structure is unchanged.
- Concludes that if the amino acid sequence is unchanged, the tertiary structure and function of the protein, and hence the phenotype, are usually unaffected (a silent mutation).
- Uses a codon table to show that a stated substitution gives the same amino acid, e.g. identifying a third-base change that remains synonymous.
- Notes that a substitution at the third base of a triplet is the most likely to be silent, whereas first- or second-base changes are more likely to alter the amino acid.
- Recognises that not all substitutions are harmless, e.g. one creating a premature stop codon truncates the polypeptide.
- Distinguishes degenerate (more than one codon per amino acid) from universal (the same codons specify the same amino acids in almost all organisms).
- one mark for stating that mutagenic agents increase the rate or frequency of mutation rather than causing a particular mutation
- one mark for naming a mutagenic agent such as ultraviolet light, ionising radiation or a named chemical
- one mark for describing a mechanism, such as bases being chemically altered or an agent inserting between the bases
- one mark for linking an increased mutation rate in proto-oncogenes or tumour suppressor genes to the development of cancer
- one mark for stating that the mutations produced are still random with respect to their usefulness
- one mark for defining non-disjunction as the failure of homologous chromosomes, or of sister chromatids, to separate
- one mark for stating that both chromosomes or chromatids move to the same pole
- one mark for gametes being produced with one more, or one fewer, chromosome than normal
- one mark for the zygote having an abnormal chromosome number after fusion with a normal gamete
- one mark for correctly identifying how many of the four daughter cells are affected for the division named
- Homologous chromosomes pair up to form bivalents during prophase I.
- Crossing over occurs at chiasmata, exchanging equivalent lengths of chromatids between homologous chromosomes.
- Independent segregation of homologous chromosomes occurs randomly at the equator during metaphase I.
- These meiotic processes result in new combinations of alleles in the daughter cells.
- The formula $2^n$ can be used to calculate the number of possible chromosome combinations, where $n$ is the number of homologous pairs.
- one mark for two nuclear divisions producing four haploid daughter cells from one diploid parent cell
- one mark for homologous chromosomes separating in the first division and chromatids separating in the second
- one mark for independent segregation of homologous chromosomes producing different combinations of maternal and paternal chromosomes
- one mark for crossing over between homologous chromosomes producing new combinations of alleles
- one mark for stating that the daughter cells are genetically different from one another
- one mark for showing half the number of chromosomes, each as two chromatids, after the first meiotic division
- one mark for showing the correct number of single chromosomes in all four cells after the second division
- one mark for stating mitosis gives two genetically identical diploid cells and meiosis four genetically different haploid cells
- one mark for identifying meiosis in an unfamiliar life cycle at the point where the chromosome number is halved
- one mark for explaining that random fertilisation combines any two genetically different gametes, creating new allele combinations
Examiner Tips
Expert advice for maximising your marks
- 💡Logically sequence your answers: state the change in base sequence, then amino acid sequence, then tertiary structure, and finally the effect on function.
- 💡Say allele, not gene, whenever you mean one version of a gene.
- 💡When explaining why a mutation has no effect, attribute this to the degeneracy of the genetic code, not to the mutation itself.
- 💡If asked to compare the two types of mutation, contrast their scope: a substitution affects one triplet, whereas a deletion affects every triplet downstream.
- 💡Demonstrate a frameshift in application questions by rewriting the sequence in threes both before and after the deletion.
- 💡State the meaning of degenerate as 'more than one codon codes for the same amino acid'; equivalent wording is accepted, so focus on the meaning rather than memorising a phrase.
- 💡Follow the chain right to the end: same amino acid, same primary structure, same tertiary structure, no change in function.
- 💡If a codon table is supplied, use it to prove the specific substitution in the question is silent, rather than asserting it.
- 💡In any natural selection answer, put the mutation before the selection pressure in your sequence of events.
- 💡To explain increased cancer risk, route the answer through proto-oncogenes or tumour suppressor genes and uncontrolled cell division.
- 💡Use the words rate or frequency rather than number when describing the effect of a mutagen.
- 💡In diagram questions count chromosomes, each of which has two chromatids after replication, not chromatids.
- 💡Say both which division failed and which structures failed to separate; each can earn credit.
- 💡Where a question says the second division proceeds normally except in one cell, track each of the four cells separately.
- 💡When asked how meiosis creates genetic variation, focus strictly on independent segregation and crossing over; do not include random fertilisation unless the question asks about variation in the whole life cycle.
- 💡Always conclude descriptions of meiotic variation by explicitly stating that these processes produce 'new combinations of alleles'.
- 💡Keep chromosome and chromatid strictly separate in every sentence; most lost marks here are vocabulary, not understanding.
- 💡Name meiosis I as the reduction division explicitly.
- 💡Learn the sequence as two lines: I - pair up, cross over, separate homologues; II - separate chromatids.
- 💡Count before you draw: the chromosome number halves once only, at the end of meiosis I.
- 💡When explaining random fertilisation, state that 'any' male gamete can fuse with 'any' female gamete to produce new allele combinations.
Common Mistakes
Pitfalls to avoid in your exam answers
- Writing that 'different amino acids are formed'; the mutation changes the sequence in which existing amino acids are joined.
- Writing '3D structure' or 'shape' instead of the specific term 'tertiary structure' when describing protein folding.
- Saying the mutation makes a different gene rather than a different allele of the same gene.
- Saying a mutation is 'degenerate'; degeneracy is a property of the genetic code, so the correct phrasing is that a mutation is silent because the code is degenerate.
- Saying a substitution causes a frameshift, when only insertions and deletions shift the reading frame.
- Describing spontaneous mutation as being caused by the organism's need to adapt to an environment.
- Confusing a base deletion (a gene mutation) with a chromosome deletion or with non-disjunction.
- Defining degenerate as one codon coding for more than one amino acid; the correct meaning is the reverse: more than one codon codes for the same amino acid.
- Using degenerate where the required term is universal, which would be rejected; universal means the same codons specify the same amino acids in almost all organisms.
- Saying a silent substitution produces a slightly different protein; the protein is identical because the amino acid sequence is unchanged.
- Assuming every substitution is silent, so no substitution ever matters; a substitution creating a stop codon truncates the polypeptide, and first- or second-base changes often alter the amino acid.
- Forgetting to follow the chain to the end: same amino acid, same primary structure, same tertiary structure, no change in function.
- writing that exposure to a drug, antibiotic or pathogen causes the mutation that protects against it, which AQA explicitly rejects
- saying mutagens 'cause mutations' when the creditable point is that they increase the rate of mutation
- claiming mutagens affect only gametes, when mutations in body cells cause cancer
- confusing a mutagen with a carcinogen, or with an antigen
- assuming every mutation a mutagen produces is expressed, when many are silent or lie in non-coding DNA
- saying non-disjunction changes the base sequence, when it changes chromosome number
- confusing non-disjunction with crossing over or with independent segregation
- stating that all four gametes are abnormal when non-disjunction occurs in meiosis II
- counting chromatids instead of chromosomes when giving the chromosome content of a cell
- describing Down's syndrome as the result of a gene mutation
- Stating that crossing over occurs between sister chromatids. Correction: Specify that crossing over occurs between non-sister chromatids of homologous chromosomes.
- Including random fertilisation as a mechanism of meiosis. Correction: Remember that random fertilisation increases genetic variation in a population but occurs after meiosis, not during it.
- Confusing genes and alleles when describing crossing over. Correction: State that crossing over produces new combinations of alleles, not new genes.
- saying the chromosome number halves during meiosis II rather than meiosis I
- describing homologous chromosomes as identical, when they carry the same genes but may carry different alleles
- placing crossing over in meiosis II instead of prophase I
- writing that four daughter cells are produced by each division rather than four in total
- forgetting that each haploid cell after meiosis I still has two chromatids per chromosome
- drawing chromosomes after meiosis I as single structures instead of pairs of chromatids
- answering 'meiosis makes gametes' when asked for the difference in outcome between mitosis and meiosis, without stating haploid/diploid and genetic variation
- saying random fertilisation increases variation without stating that the fusing gametes are genetically different and create new allele combinations