Question

1) When/why do cells divide using meiosis?

2) Describe the events in the meiotic cell cycle including any stages, phases and/or subphases.

Answer 1

Ans. 1

Germ cells devide using meiosis at the time of gametogenesis and give rise to four haploid cells from one germ cells. Here meiosis occur to generate haploid cells because haploid gametes undergo fusion while fertilisation to form diploid cells again. Hence to maintain ploidy in offspring meiosis generate haploid gametes.

Ans. 2

Meiosis is the fashion of divison of cell where one cycle of replication is followed by two cycles of divison. It is devided into two stages Meiosis I and Meiosis II.

Meiosis I is also called reductional devison because in Anaphase I the genetic material is halved. It consist of four substages -

Prophase I

Prophase I is further devided into five stages-

Leptotene - in this chromosome start condensing and ends of chromosomes allign with there homolog. And this stage is called bocquet stage.

Zygotene - in this stage the chromosome are paured with there homologous chromosomes from end to end to form bivalents.

Pachytene -reciprocal Exchange of genetic material occur between the non sister chromatids (crossing over)

Diplotene - chiasma appear and chromosome remain attached at chiasma.

Diakinesis - here chromosomes condense and four separate chromatids start appearing. Deattach from nuclear envelope.

Metaphase I - chromosomes are arranged at equitorial plate with the help of spindle fibers.

Anaphase I - homologous chromosomes start moving towards opposite poles.

Telophase I - Nuclear envelope form to form nucleus of draughter cells.

Then cytokinesis occur and formed cells undergo second round of divison.

Prophase II - chromatin condense and nuclear envelope is disappeared.

Metaphase II- chromosome arranged at metaphase plate.

Anaphase II- sister chromatids move apart toward poles.

Telophase II - nuclear envelope is formed again.

Cytokinesis occur to form new daughter cell haploid in nature.