Question

Chromosome syndesis or bivalent formation occurs in

• A. Leptotene
• B. Zygotene
• C. Pachytene
• D. Diplotene

Answer 1

The correct option is C Pachytene

Prophase I of meiosis I is divided into a series of substages which are named according to the appearance of chromosomes. The five stages are: leptotene, zygotene, pachytene, diplotene and diakinesis. The formation of a bivalent occurs during the pachytene stage. During this stage, chromosomal condensation makes it possible for the chromosomes to be viewed under the microscope. The homologous chromosomes, also known as bivalent chromosomes or bivalents, pair up side-by-side along the length of the chromosomes.This often results in chromosomal crossover which is nothing but the exchange of genetic material (occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome) between homologous chromosomes. It occurs during a process called “synapsis”. This physical strand exchange and the cohesion between the sister chromatids along each chromosome make sure that a strong pairing of the homologs occurs in diplotene phase. The structure hence formed and which is visible by microscopy, is called a bivalent. The correct answer is option C.