In my channel Ashalatha Kothapalli @biologyintelugu Iam adding a new video on Meiosis in Telugu. In this video I am explaining how the wonderful cell division maintains the chromosome number from generation to generation. If mitosis takes place while forming gametes, produce offspring with double the chromosome number. So to maintain chromosome number from generations to generations some special cell division must take place before formation of gametes with half of the chromosomes. In zygote chromosomes will be the same in number when such gametes unite. That special division is nothing but Meiosis.
In any cell division, the first nucleus division takes place, called Karyokinesis. In Meiosis karyokinesis takes place twice, Meiosis 1 and Meiosis 2. Meiosis 1 is with Four stages, Prophase 1,Metaphase 1,Anaphase 1,and Telophase 1. Meiosis 2 is with 4 stages Prophase 2,Metaphase 2, Anaphase 2 and Telophase 2.
In Prophase 1 there are 5 substages, Leptotene, zygotene,Pachytene, Diplotene, Diakinesis. During interphase in gametocytes in S stage replication of DNA takes place and chromosomes are divided into Chromatids but are connected with the centromere. The chromatids are called sister chromatids. During Leptotene stage chromosome number is clear and telomeres of those chromosomes are attached to the nuclear membrane.
In Zygotene stage pairing of chromosomes which have the same genes. Such chromosomes are called homologous chromosomes. These chromosomes are chromatids connected by centromeres. They are called bivalent chromosomes. They are with four chromatids so another name for them are tetrads. These bivalent chromosomes come together by the formation of the synaptonemal complex. This is called synapse.
In pachytene stage the synaptonemal complex loosens overlapping of non-sister chromatids. This is called crossover. At this time breaking and repairing of DNA takes place. Due to exchange of genes between non sister chromatids chromosomes differ from parent chromosomes. Variation forms may play a key role in evolution.
In the diplotene stage the synaptonemal complex still loosens and chromosomes condense more and we can see Chiasmata between Bivalent chromosomes. We can see many chiasmata between chromosomes shows the level of recombination of genes. At this stage the disappearance of the nuclear membrane and nucleolus starts.
In the final stage of prophase 1, diakinesis chiasma moves towards the end of the chromosomes. Nuclear membrane and Nucleolus disappears and formation of centrioles with spindle fibers begins.
In Metaphase 1 all the bivalent chromosomes reach to the median plate and spindle fibers attached to the kinetochore of the chromosomes.
In Anaphase 1 by the condensation of spindle fibers chromosomes reach to the opposite poles. We can see half the number of chromosomes at the pole.
During the telophase formation of nucleus and Nucleolus takes place. Later on Cytokinesis takes place and we can see two haploid daughter cells. In most of the cases, the cell takes some gap to start cytokinesis is Known as interkinesis.
Immediately Meiosis 2 takes place and it resembles the mitosis stages. Prophase 2 with clear chromosomes with chromatids. In Metaphase all the chromosomes form in a line at the median plate. In Anaphase chromatids move to the opposite poles due to pull of spindle fibers. In the final stage telophase 2 we can see four daughter nuclei with haploid number of Chromosomes. Now the Cytokinesis takes place and forms four haploid cells. In males all four cells form into male gametes but in females only one develop into female gamete and other three as polar cells.
Last clip I explained the difference between the metaphase of Mitosis and meiosis 1. How the arrangements of chromosomes at median plates in these two cases are the main causes of forming daughter cells with the same chromosome number or haploid chromosomes. Finally explained the differences between mitosis and meiosis and asked to note down as an evaluation of the topic.