Chiasma

Chiasma

A chiasma (plural: chiasmata) is a point at which paired chromosomes remain in contact during the first metaphase of meiosis, and at which crossing over and exchange of genetic material occur between the strands. This process is crucial for genetic diversity.

Structure and Formation

Chiasmata are formed during the prophase I stage of meiosis, specifically during the pachytene substage. During this time, homologous chromosomes pair up in a process known as synapsis, facilitated by the synaptonemal complex.

Synapsis

Synapsis is the pairing of two homologous chromosomes that occurs during meiosis. It allows for the exchange of genetic material between the chromosomes, a process known as genetic recombination.

Crossing Over

Crossing over is the process where homologous chromosomes exchange segments of genetic material. This occurs at the chiasmata and results in the recombination of genetic material, which increases genetic variation in the offspring.

Function

The primary function of chiasmata is to ensure the proper segregation of homologous chromosomes during meiosis. By holding the homologous chromosomes together, chiasmata help align them on the metaphase plate, ensuring that each daughter cell receives one chromosome from each homologous pair.

Importance in Genetic Diversity

Chiasmata and the associated process of crossing over are essential for genetic diversity. By exchanging genetic material between homologous chromosomes, new combinations of alleles are created, which can be passed on to offspring. This genetic variation is a key factor in evolution and adaptation.

Historical Context

The concept of chiasmata was first described by Franz Schrader and Barbara McClintock in the early 20th century. Their work laid the foundation for our understanding of genetic recombination and chromosome behavior during meiosis.

Related Concepts

• Synaptonemal complex
• Genetic recombination
• Meiosis
• Homologous chromosomes

References

• Bruce,

 Molecular Biology of the Cell, 
  
 Garland Science, 
 2014, 
  
  
 ISBN 978-0815344322,

• McClintock, Barbara,

 The Behavior in Successive Nuclear Divisions of a Chromosome Broken at Meiosis, 
 Proceedings of the National Academy of Sciences, 
 1931,
 Vol. 17(Issue: 8),
 pp. 485–491,
 DOI: 10.1073/pnas.17.8.485,

External Links

• Meiosis and Genetic Recombination