Question

Crossing over occurs during

• A. Metaphase I
• B. Anaphase II
• C. Prophase I
• D. Prophase II

Answer 1

The correct option is C

Prophase I

The correct option is C:

Explanation of the correct option

1. Crossing over is the exchange of the chromatic segments between the two chromosomes of a homologous pair.
2. During Prophase I homologous chromosomes come closer together and form bivalent.
3. This pairing is known as synapsis.
4. Synapsis is followed by crossing over.

Explanation of the incorrect options

Option A:

During Metaphase I the bivalents are arranged in the equatorial plane.

Option B:

During Anaphase I the homologous pair of chromosomes separate and move towards the opposite poles.

Option D:

1. During prophase II nuclear membrane and nucleolus disappear.
2. Condensation of chromatin into chromosomes takes place.
3. Appearance of the spindle fibers occurs.

Final answer:

Crossing over occurs during Prophase I.