Question 1

At which stage of meiosis does crossing over occur?

Accepted Answer

Crossing over occurs during pachytene, the third sub-stage of prophase I. It is mediated by the enzyme recombinase and involves the exchange of segments between non-sister chromatids of homologous chromosomes. The resulting X-shaped structures (chiasmata) become visible later, at diplotene.

Question 2

Does crossing over happen between sister chromatids?

Accepted Answer

No. Crossing over happens between non-sister chromatids of homologous (one maternal, one paternal) chromosomes. Sister chromatids are genetically identical copies, so an exchange between them would produce no new gene combinations and serves no purpose in generating variation.

Question 3

What is a synaptonemal complex?

Accepted Answer

The synaptonemal complex is a proteinaceous structure that forms during zygotene between paired homologous chromosomes, holding them tightly together in synapsis. The paired structure is called a bivalent or tetrad. The complex dissolves at diplotene, after which chiasmata become visible.

Question 4

What separates in anaphase I versus anaphase II?

Accepted Answer

Anaphase I separates homologous chromosomes — whole chromosomes move to opposite poles while sister chromatids remain joined at their centromeres (reductional division). Anaphase II separates sister chromatids, similar to mitosis but in haploid cells (equational division).

Question 5

How does meiosis create genetic variation?

Accepted Answer

Through two mechanisms: crossing over in pachytene, which recombines segments between maternal and paternal chromatids to produce recombinant chromatids, and independent assortment at metaphase I, where each homologous pair orients randomly, producing 2ⁿ possible gamete combinations (2²³ ≈ 8.4 million in humans). Even without crossing over, independent assortment alone still generates 2ⁿ distinct gamete types — what crossing over adds is recombinant chromatids, combinations no parent carried.

Crossing Over Lab — Prophase I Se Four Gametes Tak, 3D Mein

Practice

Learned it? Now prove it under the clock.