Question

How do chromosome movements differ between meiosis I and meiosis II? Is spindle composition the basis for this distinction? Compare cohesin function during mitosis and meiosis.

Answer 1

Meiosis is reductional division which occur is gametes and chromosome number is halved so that fertilization will restore the diploid complement. It occurs in meiosis I and meosis II. The reduction of chromosomes is achieved by one round of DNA replication and two rounds of chromosome segregation.

Meiosis I Prophase I of meiosis I is divided into five sub-stages :

Leptotene : Each chromosome consist of two sister chromatids and are quite close in this stage.

Zygotene : Chromosomes ends separate out and pairing between homologous chromosomes starts forming synapsis. As a result, bivalent is formed.

Pachytene : At synaptonemal complexes, crossing over takes place that leads to recombination of genes.

Diplotene : Desynapsis begins, homologous chromosomes in bivalent pull away.

Diakinesis : stage of transition to metaphase I and termination of synapsis.

Metaphase I : homologous chromosomes move together along metaphase plate.

Anaphase I : homologous chromosomes are separated move to opposite poles, sister chromatids stay together.

Telophase I : each daughter cell has half the no of chromosomes but each chromosome consists of a pair of chromatid.

Meiosis II : time gap between Meiosis I and meiosis II is called interkinesis. Meiosis II is similar to mitosis. Meiosis II separates the sister chromatids producing two daughter cells each with a haploid chromosome number.

This below diagrams shows comparison of mechanisms of chromosme alignment at metaphase and separation at Anaphase in meiotic division I and meiotic division II.

Spindle are microtubules each has tubulin. Kinetochore microtubules pulls the two sister chromatids towards opposite poles. Different forces along with spindle fibres move chromosomes during metaphase too.

Cohesin is a protein complex made of four sub-units. At centromere, the sister chromatids are kept together by cohesins. Do read following paper for more detail.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4495907/