cell division

How does meiosis ensure the chromosome number is halved? (OCR-style 3 marks)

• Homologous chromosomes separate in anaphase I (reduction division).

• Sister chromatids separate in anaphase II.

• One of each homologous pair enters each gamete → haploid cells produced.

Define bivalent and explain its role.

• A bivalent is a pair of homologous chromosomes associated during prophase I.

• Allows crossing over and correct segregation of homologous chromosomes.

• Ensures genetic variation and proper distribution to daughter cells.

Explain why crossing over only occurs between non-sister chromatids.

• Non-sister chromatids carry different alleles because they come from different parents.

• Exchange produces new allele combinations.

• Sister chromatids are identical → no variation would be produced.

What is the role of chiasmata in meiosis?

• Hold homologous chromosomes together during prophase I and metaphase I.

• Prevent premature separation.

• Allow crossing over and formation of recombinant chromatids.

Difference between homologous chromosomes and sister chromatids? (2 marks)

• Homologous chromosomes: same genes, different alleles. (1)

• Sister chromatids: identical copies formed during DNA replication. (1)

Why must meiosis halve the chromosome number? (2 marks)

• Prevents doubling of chromosome number at fertilisation. (1)

• Ensures diploid number is restored in zygote. (1)

Why are meiotic daughter cells genetically non-identical? (2 marks)

• Crossing over generates recombinants. (1)

• Independent assortment creates random chromosome combinations. (1)