A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: Meiosis and Genetic Variation — mark scheme explained
The short answer
Meiosis is a specialized form of cell division that occurs in the germ-line (reproductive) cells found in the gonads of organisms to produce gametes. It gives rise to gametes; it is not a division of the gametes themselves.
The question
A cell with a chromosome content of 2n = 8 undergoes meiosis. Draw and label the chromosome content of the cells after the first and second meiotic divisions. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the initial chromosome content.
- S2
Initial chromosome content: 2n = 8 (4 pairs of homologous chromosomes), with each chromosome having been replicated to consist of 2 sister chromatids before division.
- S3
Step 2: After meiosis I (the reduction division), homologous chromosomes separate.
- S4
Chromosome content after meiosis I: n = 4 (two cells, each with 4 chromosomes, but each chromosome still consists of 2 sister chromatids, as homologues have separated but chromatids have not). This halves the chromosome number from diploid to haploid.
- S5
Step 3: After meiosis II, sister chromatids separate.
- S6
Chromosome content after meiosis II: n = 4 (four cells, each with 4 chromosomes that now consist of single chromatids, as sister chromatids have separated).
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the initial chromosome content.
- S2
Initial chromosome content: 2n = 8 (4 pairs of homologous chromosomes), with each chromosome having been replicated to consist of 2 sister chromatids before division.
- S3
Step 2: After meiosis I (the reduction division), homologous chromosomes separate.
- S4
Chromosome content after meiosis I: n = 4 (two cells, each with 4 chromosomes, but each chromosome still consists of 2 sister chromatids, as homologues have separated but chromatids have not). This halves the chromosome number from diploid to haploid.
- S5
Step 3: After meiosis II, sister chromatids separate.
- S6
Chromosome content after meiosis II: n = 4 (four cells, each with 4 chromosomes that now consist of single chromatids, as sister chromatids have separated).
Final answer: After meiosis I: 2 cells, each n = 4, with each chromosome composed of 2 sister chromatids (meiosis I is the reduction division in which homologues separate). After meiosis II: 4 cells, each n = 4, with chromosomes now consisting of single chromatids (sister chromatids have separated). The diagram answer must show chromatid number, not just chromosome number.
Common mistakes
- Confusing meiosis with mitosis in terms of chromosome content and number of daughter cells. — Always remember that meiosis results in four haploid daughter cells, while mitosis produces two diploid daughter cells.
- Failing to understand the stages of meiosis and their significance. — Practice drawing and labeling the stages of meiosis, focusing on key events like crossing over and independent assortment.
- Misunderstanding the concept of genetic variation from meiosis. — Review the processes of independent assortment and crossing over, and practice explaining their roles in generating genetic variation.
- Confusing base substitution with base deletion mutations. — Clearly define base substitution (replacement of one nucleotide) and base deletion (removal of one or more nucleotides), and practice identifying their potential impacts on protein function.
- Failing to recognize the importance of random fertilization in genetic variation. — Practice explaining how random fertilization contributes to genetic diversity by creating unique combinations of alleles in each zygote.
- Not understanding the degenerate nature of the genetic code and its implications for mutations. — Review the concept of codon degeneracy and practice explaining how it can buffer against certain types of mutations.
Where the marks go
- Full worked solution (all marking points)4 marks