Questions & Answers

What develops between the homologous chromosomes during zygotene.
(a)Cohesion force
(b)Synaptic force
(c)Both A and B

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Hint: During Meiosis-I of cell division, homologous chromosomes pair and take part in a process known as crossing over. The process of homologous chromosomes pairing is known as synapsis and crossing over leads to genetic variations in the offsprings.

Complete answer:
Synaptic force is the force that develops between the homologous chromosomes during the zygotene stage. In this stage, the homologous chromosomes pair together with the help of specialized proteins known as synaptonemal proteins that help stabilize the chromosomes and promote crossing over. This process is known as synapsis which is followed by crossing over in the pachytene stage. This crossing over leads to new genetic combinations which alter the phenotype of the offsprings as well. Such variations, in the long run, lead to the evolution of organisms.

Additional Information: The prophase I of Meiosis I is further divided into 5 stages as there are a lot of events taking place in it.
Leptotene: It is the first stage of prophase I and the process of condensation of chromosomes begin in it. Homologous chromosomes start pairing with each other.
Zygotene: It is the second stage of prophase-I in which synapsis takes place with the formation of the synaptonemal complex. The paired chromosomes are known as bivalents or tetrads.
Pachytene: It is the third stage in which recombination of the genetic material takes place due to the crossing over of non-sister chromatids of homologous chromosomes.
Diplotene: It is the fourth stage in which crossing over gets fully completed. But there are still visible junctions between the chromosomes where crossing over took place known as chiasmata.
DIakinesis: It is the fifth stage in which the full chromosome condensation occurs and the chromosomes move towards the equator for the next stage of meiosis-I known as metaphase-I.
So, the correct option is ‘Synaptic force’.

Note: -Meiosis is known as reductional division as chromosomes number becomes half after it occurs. It takes place when there is a gamete formation.
-The term zygotene means paired threads because in this stage the pair homologous chromosomes look like paired threads.
-The zygotene stage is also known as the bouquet stage because all the telomeres of chromosomes cluster at one end of the nucleus.