Meiosis divides a single cell into four daughter cells with half the number of chromosomes. In animals, it occurs in germ cells to form gametes (sperms and eggs), while in plants, it occurs in spore mother cells to produce spores.
Meiosis is a series of two divisions, Meiosis I and Meiosis II which result in the formation of four haploid cells.
First meiotic division is the reduction division during which the chromosomes number is reduced to half.
Meiosis Prophase I:
It is the longest phase with following steps:
Leptotene: Chromatin network breaks into thin beaded threads called leptotene.
Zygotene: Homologous chromosomes pair up by synapsis to form bivalent.
Pachytene: Synapsis is completed, chromosomes separate, crossing over occurs, and bivalent tetrad is formed.
Diplotene: Homologous chromosomes exchange genetic material at chiasmata.
Diakinesis: Nucleoli and nuclear membrane disappear, spindle is completed, and chiasmata move towards the ends of chromosomes.
Meiosis Metaphase I:
The bivalent chromosomes align at the equatorial plane and attach to spindle fibers.
Meiosis Anaphase I: Homologous chromosomes separate and move to opposite poles by the contraction of spindle fibers.
Meiosis Telophase I: Nuclear membrane forms around chromosomes at each pole, chromosomes uncoil, and two daughter nuclei form.
Cytokinesis It may or may not occur.
If Telophase I is present, there's a brief Interphase before meiosis II. It's like the Interphase in between mitotic divisions, but without DNA replication since each chromosome already has two chromatids.
Meiosis II divides each haploid cell from meiosis I into two daughter cells. Meiosis II has four stages:
Prophase II: Spindle fibers form and the nuclear membrane/nucleolus disappear.
Metaphase II: Chromosomes attach to half spindle fibers and align at the equatorial plane.
Anaphase II: Spindle fibers pull the chromatids of chromosomes apart to position a set of chromosomes at each pole.
Telophase II: Spindle fibers disappear and chromosomes uncoil. Nuclear envelopes form around each set of chromosomes.
Cytokinesis: Cytokinesis then occurs, resulting in four haploid cells.
Without meiosis, chromosomes would double, causing abnormal growth, changes in species characteristics, and possibly death.
Constant number of chromosomes: Meiosis keeps chromosome number consistent between generations by halving the number of chromosomes from diploid to haploid in gametes. Fertilization restores the diploid number of chromosomes.
Responsible for genetic variation among species: Meiosis enables gene exchange between homologous chromosomes through crossing over, resulting in genetic variations that are the basis of evolution.
Generally, during meiosis, homologous chromosome pairs separate into two gametes. However, non-disjunction can occur when a pair of homologous chromosomes fails to separate during meiosis I, resulting in gametes with abnormal chromosome numbers. If these abnormal gametes combine with normal gametes, the resulting zygote will also have an abnormal chromosome number.
