Question

Chromosome syndesis or bivalent formation occurs in
A. Leptotene
B. Zygotene
C. Pachytene
D. Diplotene

Answer 1

Hint: Synapsis also called syndesis is the pairing of two homologous chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation and possible chromosomal crossover between them. Syndesis takes place during prophase I of meiosis. When homologous chromosomes synapse, their ends are first attached to the nuclear envelope.

Complete answer:
Meiosis is a specialized type of cell division that reduces the chromosome number by the half which further results in the production of haploid daughter cells.
Meiosis involves two cycles of nuclear division and cell division that is:
- Meiosis 1
- Meiosis 2

Meiosis 1 is further divided into four stages:
- Prophase 1
- Metaphase 1
- Anaphase 1
- Telophase 1

Prophase-1 is first meiotic division and it is further divided into five subtypes:
> Leptotene: It is also known as thread stage because chromosomes become gradually visible under light microscope. In this stage, chromosomes arrange themselves like a bouquet which gets attached to the nuclear membrane.

> Zygotene: It is the second stage after leptone, here, paternal and maternal chromosomes come to lie together thus, pairing or synapses occur, bivalent formation and synaptonemal complex occurs in this stage but this stage is short lived so, whole of process moves towards the third stage.

> Pachytene: In this stage, Bivalent chromosomes are now clearly visible as tetrads. This stage is characterized by the appearance of recombination of the nodule. The crossing over takes place by an enzyme named recombinase.

> Diplotene: In this, Dissolution of synaptonemal complex and chiasma is clearly visible.

> Diakinesis: In this stage, terminalisation of plasmodesmata occurs which means separation of homologous chromosomes at the site of chiasma by slipping off.

So, according to the question chromosome syndesis or bivalent formation occurs pachytene phase of prophase 1.
Hence, the correct answer is option (C).

Note: Bivalent or tetrads is a complex formed by a pair of homologous chromosomes. Synaptonemal complex is that stage which indicates that chromosome synapsis is accompanied by the formation of complex structure. Both these complexes are formed in the zygotene phase but they are clearly visible in the pachytene phase.