#107 Meiosis (English)

Meiosis is a type of reduction division of the nucleus to form haploid gametes. In other words, it is the process by which sex cells (gametes) are made in the reproductive organs.

·         Meiosis is preceded by interphase, a stage in which cells in general, spend most of their life in, which includes the replication of DNA (S phase) to create chromosomes with genetically identical sister chromatids.

·         The process of meiosis involves two divisions, both of which follow the same basic stages as mitosis (Prophase, Metaphase, Anaphase and Telophase)

1.      Prophase I

-        DNA (chromatins) supercoils and chromosomes condense.

-        Nuclear membrane dissolves and the centrioles migrate to the poles of the cell.

-        The homologous* chromosome associate with each other to for bivalents.

Homologous: chromosomes that share the same structural features and the same genes at specific loci positions.

In this case, crossing-over between non-sister chromatids took place resulting in the recombination of alleles, further leading to genetic variation. 

2.      Metaphase I

-        Random orientation takes place in this phase where each bivalent independently aligns resulting in the daughter nuclei receiving a different mix of chromosomes.

-        The bivalents line up at the equator.

3.      Anaphase I

-        Homologous pairs are separated and pulled to opposing poles.

-        Spindle microtubules (fibre) contract causing a reduction division where the bivalents have now been split in half. Each of these now single stranded chromosomes move to opposite poles.

4.      Telophase I

-        The nuclei, which are now Haploid (N), each contain one pair of sister chromatids for each of the species’ chromosomes.

-        The cytoplasm begins to divide by a process called cytokinesis.

-        Chromosomes decondense into chromatins.

5.      Prophase II

-        Nuclear membranes dissolves.

-        Sister chromatids (chromosomes) condense via supercoiling with the help of specialised proteins called histones.

-        Here, no crossing-over occurs.

-        Centrioles move to opposite poles.  

6.      Metaphase II

-        Pairs of sister chromatids align at the equator.

-        Spindle fibres form and attach at the centromeres.

7.      Anaphase II

-        The sister chromatids are now separated. The centromeres are split and the chromosomes are pulled to opposing poles through contractions of the spindle fibres.

8.      Telophase II

-        Nuclear membrane reforms.

-        Chromosomes decondense back into chromatins.

-        Cytokinesis begins, dividing the cells.

-        As a result, meiosis produced four haploid gamete cells.

-        Furthermore, fertilization of these haploid gametes will produce a diploid zygote.

 

SAG