Question

At which stage of Meiosis-Ⅰ does crossing over occur?
A. Leptotene
B. Zygotene
C. Pachytene
D. Diplotene

Answer 1

Hint: In the process of meiosis, a single cell divides two times and produces four cells. These cells will have half the original amount of genetic information. MeiosisⅠis further divided into Interphase, Prophase-Ⅰ, Metaphase-Ⅰ, Anaphase-Ⅰ, Telophase-Ⅰ and cytokinesis. The Prophase-Ⅰhas 5 distinct sub-stages.

Complete answer:
> Option A: Lepoptene- It is the first stage of prophase-Ⅰ. At this stage, the chromosomes are not very distinct but they entangle and form mass of threads. Condensation of chromosomes begin to start here and they attach to nuclear membrane.

> Option B: Zygotene- It is the second stage soon after Lepoptene. Here, chromatin fibers start condensing. The homologous chromosomes will begin synapses and will be joined by a synaptonemal complex.

> Option C: Pachytene- Synaptonemal complex will be fully formed at the Pachytene stage. The chromosomes will thicken as well as shorten in this stage. It is here where the crossing over occurs between non-sister chromatids. Crossing over takes place by breakage and reunion of chromatid segments.

> Option D: Diplotene- There is disappearance of the synaptonemal complex. It should be noted here that the chromatids become separated but chiasmata does not.

Hence, the correct answer is option (C).

Note: The last stage of Prophase-Ⅰis the Diakinesis. Here, the full condensation of chromosomes happens. A disintegration of the nuclear membrane is observed. The process of meiosis is very essential to make sure that all organisms that are the product of sexual reproduction contain the correct number of chromosomes. Genetic variation that is seen amongst organisms is also due to meiosis.