Questions: Meiosis

Question 1

What is the key mechanistic difference between meiosis I and meiosis II?

• A. Meiosis I replicates DNA; meiosis II separates chromosomes
• B. Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids
• C. Meiosis I produces haploid cells; meiosis II produces diploid cells
• D. Meiosis I occurs in somatic cells; meiosis II occurs in germ cells

Answer: (B)

Meiosis I is the reductional division: it separates the two members of each homologous pair, cutting the chromosome number in half (diploid → haploid). Meiosis II is the equational division: it separates sister chromatids within each already-haploid cell, analogous to mitosis. DNA replication occurs before meiosis I (in S phase), not during. Both divisions occur in germ cells.

Question 2

Crossing over during meiosis occurs in prophase II, after the chromosome number has already been reduced.

• T. True
• F. False

Answer: False

Crossing over (recombination) occurs during prophase I, when homologous chromosomes are paired in a structure called the synaptonemal complex. This is the only time homologs are close enough to exchange segments at chiasmata. By prophase II, the cell is already haploid — homologs have separated — so there are no homolog pairs present for crossing over to occur between.

Question 3

Meiosis generates genetic diversity through two distinct mechanisms. Name both mechanisms and explain how each contributes to variation in gametes.

Model answer: Independent assortment: during meiosis I, each pair of homologous chromosomes aligns randomly at the metaphase plate, so the maternal or paternal version of each chromosome is independently inherited. Crossing over: during prophase I, non-sister chromatids of homologs exchange segments, creating recombinant chromosomes with new combinations of alleles that didn't exist in either parent.

Independent assortment alone can produce 2^23 (over 8 million) chromosome combinations in human gametes. Crossing over generates additional variation by shuffling alleles along individual chromosomes, so that even two gametes with the same chromosome set can carry different allele combinations. Together, these mechanisms ensure that virtually every gamete is genetically unique.