Question

At what stage of the meiotic cell cycle and during what chromosomal configuration does crossing over occur?

A) during synapsis in prometaphase
B) at the four-strand stage of meiosis, after synapsis of homologous chromosomes, and before the end of
prophase I
C) in S phase of meiosis
D) during synapsis at the four-strand stage of meiosis
E) during bivalent formation, after synapsis of homologous chromosomes, and before the end of prophase I

Answer 1

Crossing over occurs during prophase I of meiosis, particularly during synapsis when homologous chromosomes pair tightly and form a synaptonemal complex, enabling the exchange of genetic material.

Step-by-step explanation:

Crossing over occurs during prophase I of meiosis, more specifically during the sub-stage known as synapsis. This is the stage when homologous chromosomes pair up very tightly and form a structure known as a synaptonemal complex. The close pairing is called synapsis, which allows for the alignment of genes on the chromatids of homologous chromosomes and supports the exchange of chromosomal segments between non-sister homologous chromatids, a process called crossing over. After crossing over, the homologous chromosomes remain connected at points called chiasmata, and because four sister chromatids are visible, they form a structure known as a tetrad.

So, crossing over occurs: B) at the four-strand stage of meiosis, after synapsis of homologous chromosomes, and before the end of prophase I.