Question

Crossing over occurs between
A. Sister chromatids
B. Non-sister chromatids
C. Homologous chromosomes
D. Any two chromosomes

Answer 1

Hint: During pachytene stage of meiosis I, the chromatids of the synapsed chromosome exchange the genetic segments. It produces new gene combinations and one of the sources of genetic variations in sexually reproducing organisms.

Complete answer:
We will start the discussion with the introduction of homologous chromosomes and sister chromatids.
Homologous chromosomes are similar in morphology and genetic content. A somatic human cell has a total of 23 pairs of homologous chromosomes.
After DNA replication during the S phase, each chromosome has two chromatids called sister chromatids. They are held together at the centromere.

Let’s discuss the events of prophase I of the meiosis I.
Meiosis is the reduction division that produces four haploid daughter cells from a diploid parent cell. It includes two successive nuclear divisions called meiosis I and meiosis II.

Meiosis I includes prophase I, metaphase I, anaphase I, and telophase I. Pairing of homologous chromosomes during the zygotene stage of prophase I is mediated by the synaptonemal complex. The paired chromosomes are called bivalents or tetrads as each pair consists of four chromatids (two sister chromatids of each chromosome). Synapsis or pairing of homologous chromosomes facilitates the crossing over during pachytene of prophase I.

Crossing over is the process during which the non-sister chromatids of a homologous pair exchange the genetic segments between them. Here, non-sister chromatids represent the chromatids of two different chromosomes. However, crossing over can occur only between the non-sister chromatids of the paired homologous chromosomes which are brought closer by synapsis.

Hence, the correct answer is option (B).

Additional information:
Each chromosome is a thread-like structure and consists of a single chromatid and the primary constriction called the centromere. S phase of the cell cycle includes duplication of the DNA content of the cell. To accommodate the duplicated DNA content, each chromosome now has two sister chromatids. The duplicated chromosomes enter the meiosis I and become visible during the leptotene stage.

Note: Synapsis aligns the non-sister chromatids of two homologous chromosomes lengthwise and thereby, allows the exchange of genetic material between them. The site of the crossing over appears as chiasma during the diplotene stage of prophase I during which synaptonemal complex dissolves and the homologous chromosomes are joined only at chiasma.