Meiosis II: Introduction

o The living world has seen many numbers of generations and successions of organism evolved adaptively to the pertaining environmental changes; sustaining the diverse lifeforms on The Earth.

o Sustenance of a species and its lineage is the cumulative adaptations of the ancestors inherited over a million years to reach the present morpho – functional form of beingness.

o The inherited changes from parent to offspring, its proliferation supported by external growth factors and the environment; developed the organism to become fit and ensure viability for further generations to sustain.

o The fundamental cellular process supporting the growth and development is the inheritance of expressive genes from the parents to offspring mediated by the process of Meiosis.

o Cell division that leads to the contribution of one half of the chromosomes from each of the sexually reproducing parent to offspring is meiosis.

o Sexual reproduction is possible among the same species with same number of chromosomes and are equally contributed to the offspring so that half of maternal and paternal genes are contributed to the progeny.

o Meiosis is specialized because of the variability it provides to the offspring by the recombination of the homologous genetic material and the production of reduced chromosome where a diploid cell gives haploid daughter; on fusion of the male gamete (Sperm in animals and pollen in plants) and female gametes Eggs (plants and animals) produces diploid zygote proceeds with mitosis to develop into new organism.

Mechanism of Meiosis II

o Significance of meiosis is that it takes place twice to segregate the homologous pair and give 4 haploid cells in continuous division without any gap between the two division.

o At the end of Meiosis I, the single gamete on induction becomes divided into 2 diploid cells with varied genetic expression due to the recombination enter the meiosis II.

Meiosis Diagram in Sperm

o Meiosis II is a replica of Mitosis with a difference in chromosomal composition of the somatic cells and the meiotic cell.

o The Meiosis I and Meiosis II has no gap phase between them to duplicate the 46 chromosomes present in the cell at the end of the Telophase I.

o Other components are similar to that of mitosis; wherein prophase II has the same rule where the chromatin condenses faster to become a chromosome.

o Metaphase II includes the appearance of the spindle fibers from the organizing center attaches to the sister kinetochores of the same chromosomes; a difference from the METAPHASE I where one of the sister chromatids of the homologous are attached.

Meiosis Diagram in Oocytes

o The chromosomes are attached to opposite end fibers of the other sister kinetochores and aligns in the center of the cell forming equatorial plate.

o This phase has a regulator where all the chromosomes must be attached at the sister kinetochores of the opposite poles to initiate Anaphase where the real separation of haploid chromosomes takes place.

o At Anaphase II the sister chromosomes held by cohesins are removed by the cascade activity of separase; chromosomes when bound induces APC/C complex which eliminates the repressor of separase and separase on removal of repressor cleaves cohesins along the length of the sister chromatids including centromere.

o Cohesin degradation releases the tension maintained when held at the equator which pulls the chromatids to opposite ends separating the haploids to the opposite ends.

o Reaching poles, the last Meiosis II stage starts by forming a deep furrow between cells and formation of cell organelles and nuclear membrane and separates.

o Post Meiotic II involves maturation of the germ cells and re – organization of the structural and functional capability of the cell.

o In female, the cell matures to eggs and in male the cells undergo structural modification to attain a sperm with head, neck and tail and the transformation varies for different groups of eukaryotes.