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What happens in the chiasma and why is it important?

What happens in the chiasma and why is it important?

At the chiasmata, homologous chromosomes exchange genes, allowing genetic information from both the paternal and maternal chromatids to be exchanged, and a recombination of paternal and maternal genes can be passed down to the progeny. This process is important in diploid organisms to ensure variation in the progeny.

What is chiasma and what is its role in crossing over?

The chiasma is a structure that forms between a pair of homologous chromosomes by crossover recombination and physically links the homologous chromosomes during meiosis.

What event causes chiasmata formation?

Completion of reciprocal recombination/crossing-over between parental half chromosomes (chromatids) together with chromatid cohesion, leads to the formation of chiasmata, i.e. physical connections that hold parental homologs (bivalents) together.

How is chiasmata formed?

In many species, chiasmata (the physical attachments between homologous chromosomes) are formed after the recognition of homologous chromosomes (pairing), the close association of paired chromosomes by the synaptonemal complex (SC), and the reciprocal exchange of sequences by the homologous recombination (HR) process.

What happens during Pachytene?

During the pachytene phase, the chromosomes become shorter and thicker and split into two chromatids joined by the centromere. Pachytene is a lengthy phase, lasting about 12 days in the rat; during this time there is a marked increase in cellular and nuclear volume.

What holds Chiasmata together?

Chiasmata are specialized chromatin structures that link homologous chromosomes together until anaphase I (Figs. 45.1 and 45.10). This DNA complex is held in place on the chromosome by cohesion of the distal sister chromatid arms between the chiasma and the telomeres.

Why is crossing over important?

Crossing over is essential for the normal segregation of chromosomes during meiosis. Crossing over also accounts for genetic variation, because due to the swapping of genetic material during crossing over, the chromatids held together by the centromere are no longer identical.

What happens during pachytene?

How many chiasmata are formed?

Humans have 39 such arms on the 23 pairs of homologous chromosomes, if one excludes the five acrocentric short arms, which do not normally undergo crossovers. Remarkably, there is typically only one chiasma produced for most arms; human males typically have 46 to 53 chiasmata (Fig. 45.11).

What stage does chiasmata appear?

The chiasmata become visible during the diplotene stage of prophase I of meiosis, but the actual “crossing-overs” of genetic material are thought to occur during the previous pachytene stage.

What does chiasmata hold together?

At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole. The homologous chromosomes are still held together at chiasmata.

What is the importance of pachytene stage?

The long pachytene stage begins with the completion of synapsis and is associated with further thickening and shortening of chromosomes. During this stage, exchanges of chromosome material between maternal and paternal homologous chromosomes occur by crossing over.