Question

describe the stages of meiosis, good explanation that is easy to understand please

Answer 1

Meiosis can be defined as the reductional division occurring only in diploid cells for the formation of haploid cells in which the number of chromosomes and the nuclear DNA content are reduced to half and there is recombination of hereditary material. It involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication.

Meiosis I: It is the reductional or heterotypic division which starts after the interphase of the cell cycle where DNA duplication occurs at S phase. It results in the reduction of chromosome number to half. It consists of four stages-Prophase-I, Metaphase-I, Anaphase-I and Telophase-I.

(a) Prophase-I: It is very complex and of very long duration. It is divided in the following five substages as follows: (i) Leptotene- The duplicated centrioles start moving apart and aster formation occurs. The chromatin fibres undergo progressive condensation, coiling, shortening and thickening to form thin condensed filamentous chromosomes. Here the chromosomes appear to form loops with ends attached to nuclear membrane. This specific arrangements is often called the bouquet stage.

(ii) Zygotene- The homologous chromosomes come to lie in pairs and this pairing of homologous chromosomes is called synapsis.and these paired chromosomes are called bivalents. This is acompained by the formation of synaptonemal complex.

(iii) Pachytene- The bivalent chromosomes become condensed, thickened and shortened and each chromosome of a bivalent consists of two chromatids and are called sister chromatids. This stage is characterized by the crossing over between non-sister chromatids of the homologous chromosome. Crossing over is the exchange of genetic material between two homologous chromosome.

(iv) Diplotene- Longest phase of prophase-I. There is dissolution of synaptonemal complex, so the recombined homologous chromosomes will start to separate, i.e desynapsis. But they remain attached to one or more points where crossing over has occured. This points of attachment are called chiasmata.At this stage the chromosomes decondense and are engazed in rapid synthesis of RNA. Terminalization of chiasmata starts.

(v) Diakinesis- This stage is marked by complete terminalization of chiasmata. The nucleoli and the nuclear membrane disintegrate and disappear. The spindle fibres extend from one pole of paired centrioles to the other pole of paired centrioles. Astral rays and asters are fully developed.

(b) Metaphase-I: The bivalents arrange themselves on the equator of the bipolar spindle. Since, there are two centromeres in each bivalents, the centromeres of all bivalents produce a double equatorial or metaphasic plate. The microtubule from the opposite poles of the spindle attach to the pair of homologous chromosomes. (c) Anaphase-I: One chromosome of each homologous pair moves to the opposite poles with recombined characters of both paternal and maternal chromosomes. The movement of chromosomes occurs along the path of their tractile fibres. There is no division of centromere. At the end of anaphase-I half of the chromosome reach one pole and other half reach on to the other pole. There occurs true reduction in the number of chromosomes at this stage.

(d) Telophase-I: The haploid number of chromosomes which has reached at each pole, undergo elongation, uncoiling and decondensation and changes into chromatin network. The nuclear envelope develops from the elements of ER and new nucleolus is formed. The astral rays and spindle fibres disintegrate and disappear. The two daughter nuclei, each containing haploid chromosome number are formed. The end of Telophase-I marks the end of karyokinesis-I also.

Karyokinesis-I may be followed by cytokinesis-I or may undergo direct Meiosis-II.

Meiosis-II: It is equational or homotypic division and is necessary beacuse each daughter cell formed from meiosis-I contains chromosomes each having two chromatids, each chromatid with 2C amount of DNA. This reaction in DNA from 2C to 1C DNA occurs only in meiosis-II. It resembles mitotic division and comprise of Prophase-II, Metaphase-II, Anaphase-II and Telophase-II.