Dalvoy Logo
Dalvoy
UPSC MainsBOTANY-PAPER-II201310 Marks
Q12.

Pachytene

How to Approach

The question asks for a detailed note on Pachytene, a crucial stage in meiosis. The answer should focus on defining Pachytene, detailing the events occurring during this stage (synaptonemal complex, crossing over, recombination), its significance in genetic diversity, and potential abnormalities. A structured approach covering the definition, events, significance, and related issues will be effective. Diagrams, though not directly requested, can be conceptually described to enhance understanding.

Model Answer

0 min read

Introduction

Meiosis is a specialized type of cell division that reduces the chromosome number by half, creating haploid gametes from diploid cells. This process is essential for sexual reproduction and genetic diversity. Pachytene is a critical substage within Prophase I of meiosis, representing a period of intense genetic exchange. It’s characterized by the close pairing of homologous chromosomes, allowing for the crucial process of crossing over, which ultimately contributes to the genetic variation observed in offspring. Understanding Pachytene is fundamental to comprehending the mechanisms driving evolution and inheritance.

Defining Pachytene

Pachytene (from Greek *pachys* meaning "thick" and *tene* meaning "stretched") is the fifth substage of Prophase I in meiosis. It follows Zygotene and precedes Diplotene. During Pachytene, homologous chromosomes are fully synapsed, forming a structure called the synaptonemal complex. This complex facilitates the precise alignment of genes along the chromosome length, enabling genetic recombination.

Events Occurring During Pachytene

1. Synaptonemal Complex (SC) Formation & Structure

The synaptonemal complex is a protein structure that forms between homologous chromosomes during Prophase I. It consists of five lateral elements and a central element. The lateral elements run along the chromosomes, while the central element holds the homologous chromosomes together. The SC provides a scaffold for crossing over and ensures accurate pairing.

2. Crossing Over and Recombination

Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes. This occurs at specific points called chiasmata (singular: chiasma), which are visible manifestations of crossing over. The process involves breaking and rejoining of DNA strands, leading to the recombination of alleles. Recombination is not simply an exchange of equal segments; it involves precise enzymatic processes to ensure accurate repair and prevent mutations.

  • Initiation: Double-strand breaks are introduced in the DNA of both chromosomes.
  • Strand Invasion: One strand from each broken DNA molecule invades the other chromosome.
  • Holliday Junction Formation: A cross-shaped structure called a Holliday junction is formed.
  • Resolution: The Holliday junction is resolved, resulting in the exchange of genetic material.

3. Chromosome Condensation

While synapsis is complete, chromosomes continue to condense, becoming shorter and thicker. However, they are still closely associated due to the synaptonemal complex.

4. Ribonucleoprotein Structures (RNP)

Pachytene chromosomes exhibit distinct RNP structures along their length, representing sites of crossing over. These structures are visible under electron microscopy and are indicative of active recombination events.

Significance of Pachytene

1. Genetic Diversity

The primary significance of Pachytene lies in its contribution to genetic diversity. Crossing over during this stage shuffles alleles between homologous chromosomes, creating new combinations of genes. This genetic variation is crucial for adaptation and evolution.

2. Proper Chromosome Segregation

The chiasmata formed during Pachytene play a vital role in ensuring proper chromosome segregation during meiosis I. They physically link homologous chromosomes, preventing their random separation and ensuring that each daughter cell receives a complete set of chromosomes.

3. Genome Stability

Recombination during Pachytene helps to repair damaged DNA and maintain genome stability. The process can correct errors that may have occurred during DNA replication.

Abnormalities in Pachytene

1. Asynapsis and Azoospermia

Failure of homologous chromosomes to synapse (asynapsis) can lead to infertility, particularly in males (azoospermia). This is because proper chromosome pairing is essential for spermatogenesis.

2. Premature Dissolution of SC

If the synaptonemal complex dissolves prematurely, it can result in chromosome missegregation and aneuploidy (an abnormal number of chromosomes).

3. Non-allelic Homologous Recombination (NAHR)

NAHR occurs when recombination happens between non-allelic sequences on homologous chromosomes. This can lead to deletions or duplications of genetic material, contributing to genetic disorders.

Stage Key Events Significance
Pachytene Synaptonemal complex fully formed, crossing over occurs, chiasmata visible Genetic diversity, proper chromosome segregation, genome stability

Conclusion

Pachytene is a pivotal stage in meiosis, orchestrating the crucial events of synapsis and crossing over that drive genetic recombination. This process is fundamental to generating genetic diversity, ensuring accurate chromosome segregation, and maintaining genome stability. Understanding the intricacies of Pachytene is essential for comprehending the mechanisms of inheritance and the evolutionary processes shaping life. Disruptions in this stage can lead to infertility and genetic disorders, highlighting its biological importance.

Answer Length

This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.

Evaluate your handwritten answer in under a minute

Free trial available • Trusted by 1,00,000+ aspirants

Additional Resources

Key Definitions

Synapsis
The pairing of homologous chromosomes during Prophase I of meiosis, forming a bivalent or tetrad.
Chiasmata
The X-shaped structures formed during Pachytene where crossing over has occurred, representing the physical connection between non-sister chromatids of homologous chromosomes.

Key Statistics

Approximately 1-3 crossover events occur per chromosome pair in humans.

Source: Based on knowledge cutoff 2023 - standard textbook information

Studies suggest that approximately 90% of human genome is repetitive, and recombination rates vary significantly across the genome, with higher rates in regions with less repetitive DNA.

Source: International Human Genome Sequencing Consortium (2003)

Examples

Cystic Fibrosis Linkage

Genetic mapping studies using recombination frequencies during Pachytene have helped identify the location of genes responsible for genetic disorders like Cystic Fibrosis.

Frequently Asked Questions

▶What happens if crossing over doesn't occur?

If crossing over doesn't occur, genetic diversity is significantly reduced. The resulting gametes will only contain the parental combinations of alleles, limiting the potential for adaptation and evolution. It can also lead to increased susceptibility to genetic diseases.

Topics Covered

BiologyGeneticsMeiosisRecombinationChromosomes
Back to all BOTANY-PAPER-II 2013 questions