Problem 90 - Objective Biology for NEET

Question

Repulsion of homologous chromosomes takes place in (a) Diplotene (b) Zygotene (c) Diakinesis (d) Leptotene

Step-by-Step Solution

Answer

None of the options given - Diplotene, Zygotene, Diakinesis, Leptotene - are correct. The separation or repulsion of homologous chromosomes occurs during Anaphase I of meiosis.

Step 1: Understanding the Terminologies

It is essential to understand the terminology used. 'Homologous chromosomes' are a pair of chromosomes, one from each parent, that carry the same genes. Repulsion of homologous chromosomes refers to the process in which these chromosomes separate from each other.

Step 2: Identifying the Stage of Repulsion

Now, let's look at each option: \n\n(a) Diplotene: During the diplotene stage, the homologous chromosomes further condense and the chiasmata (locations where genetic information is exchanged between chromosomes) move towards the ends of the chromosomes. Homologous chromosomes remain connected at the chiasmata and do not repel.\n\n(b) Zygotene: The zygotene stage is characterized by the pairing or synapsis of homologous chromosomes, not the repulsion.\n\n(c) Diakinesis: Diakinesis is the final stage of Prophase I, homologous chromosomes move apart from each other, but do not yet completely separate or repel each other.\n\n(d) Leptotene: The leptotene stage is characterized by the condensation of chromosomes, not the repulsion. \n\nTherefore, none of the choices indicate the stage where homologous chromosomes repel each other, this actually happens at the start of anaphase I.

Step 3: Conclusion

Based on the descriptions given of each stage, it's clear that the repulsion of homologous chromosomes doesn't occur in any of the stages listed in the options. This separation of homologous chromosomes takes place during Anaphase I, which is not mentioned in the options.

Key Concepts

Diplotene

The diplotene stage is a crucial part of meiotic prophase I. It is recognized for significant chromosomal events. During this stage, homologous chromosomes have already undergone genetic recombination.
The chromosomes begin to move apart, but remain connected at points called chiasmata. These chiasmata are sites where genetic information has been exchanged.

• Homologous chromosomes begin to repel each other.
• Chiasmata become visible.
• Chromosomes separate except at the chiasmata.

This gradual process allows the stages following diplotene to eventually complete chromosomal separation. Though chromosomes start to separate, they are not completely repelled just yet.

Prophase I

Prophase I is a complex stage in meiosis where several crucial events occur to set the stage for successful genetic segregation. It is further divided into sub-stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.
It is during these steps that homologous chromosomes pair and exchange genetic material. Let's explore some key points:

• Chromosomes condense and become visible.
• Synapsis occurs, forming pairs of homologous chromosomes.
• Genetic recombination happens, increasing genetic diversity.

Prophase I ensures each gamete receives a unique set of genetic instructions, which is vital for evolution and diversity.

Anaphase I

Anaphase I is the meiotic stage where homologous chromosomes are pulled apart, which is sometimes referred to as their 'repulsion'. This is a key step in genetic reduction, as it ensures that each daughter cell receives only one chromosome from each pair of homologs.
Here's what happens during Anaphase I:

• Spindle fibers attach to chromosomes and pull them toward opposite poles.
• Chromosomes remain as sister chromatids, not yet separated into individual arms.
• The reduction division—halving the number of chromosomes—occurs.

This separation is essential for creating haploid cells necessary for sexual reproduction. Proper anaphase movement ensures genetic integrity in the offspring.

Genetic Information Exchange

Genetic information exchange is a critical process in meiosis that increases genetic diversity. It primarily occurs during the pachytene stage of Prophase I, through a process known as crossing over.
This exchange involves:

• The breaking and rejoining of DNA between homologous chromosomes.
• Chiasmata formation, which are the visible sites of exchange.
• Creating new chromosome combinations, contributing to genetic variation.

This variation is crucial for adaptation and evolution, ensuring that species can survive changing environments.

Chromosome Condensation

Chromosome condensation is the process by which chromosomes become tightly packed and visible under a microscope. It begins in the leptotene stage of Prophase I.
Key characteristics of condensation include:

• Chromosomes shortening and thickening.
• Improvement in visibility and distinction of individual chromosomes.
• Preparation for the subsequent steps in meiosis.

Condensation helps facilitate the precise alignment and separation of homologous chromosomes, ensuring accurate genetic division and successful meiosis.