 Today, we are going to talk about cell division in germline cells, also called meiosis. As we know that all of the cells need to divide for growth, for regeneration and for reproduction. There are two types of cell divisions, mitosis in which the cell, one cell also called a mother cell divides into two daughter cells and these two daughter cells have all the properties just same as that of the mother cell. The other type and this type of division mitosis occur in the somatic cells, all cells of the body other than the germline cells the gametes. But in the germline cells the gametes for the production of gametes, the other type of cell division occur which is called meiosis, meiosis so meiosis is a cell division in which occur in the germline cells to produce the gametes for a sexual reproduction, meiosis is also called reduction division because in this type of division the chromosome number of the mother cell when it divides into cells becomes one half, exactly one half. Meiosis occur in the germline cells to make gametes, sperms and the eggs. Meiosis is a reduction division in which the chromosome number become one half and one cell divides into four cells ultimately and the end of meiosis unlike mitosis the cell one cell divides into four cells and all of these four cells have a half chromosome number in comparison of the mother cell. For example human beings have 46 chromosomes in total, these 46 chromosomes are present in 23 pairs. In each pair the two chromosomes which are present are just alike in their generalized appearance though there are certain differences in the set of genes because one part of gene on one of the member of the pair may be a dominant one and the gene present on the other one may be the recessive one or both are dominant or vice versa. These pairs are called homologous pairs, homologous because they look alike in their general appearance and shape particularly when the cell is dividing because we already know that the chromosomes at interface are very thin and they are not visible. These chromosomes are visible only when the cell is dividing maybe during mitosis or maybe during meiosis. So when there is a division phase the chromosomes appear in the pairs, we call these pairs the homologous pairs and we call these individual chromosomes that is two in a pair homologous. We call this chromosome number the total chromosome number for example like 46 total chromosomes in human beings 2n. When the cell divide by meiosis into four daughter cells this 2n number of chromosomes becomes one half that is now each daughter cell have exactly half number of chromosomes as the mother cells had. We call this number n and the parent cells the mother cell which have that 2n number we call that cell a diploid cell. Diploid means that it have two sets of chromosomes and the daughter cells the sperm or eggs they are called haploid. They have a n that is a half set of chromosomes. So we call them haploid. The mother cell is called diploid the daughter cells are called haploid that which means that they have a exact half number of chromosomes sometimes we call this ploidy animal cells and many of the plant cells they are diploid their germ cells are diploid and when they divide for a sexual reproduction into sperms and eggs they become half in their number divided by meiosis and we call them haploid. But in some plant cells have even more sets of chromosomes for example if they have 18 chromosomes these are divided into three sets we can call them triploid that is triploid three sets of chromosomes there are different types of plants which even have even more sets of chromosomes now we call them polyploids and we call this phenomena polyploidy. Now to understand this phenomena just look at a diagram you can see a set of chromosomes these two are actually the homologous chromosomes when they are not duplicated or replicated during the S phase as you can see after replication or duplication the contents of both of these chromosomes will be duplicated and now each one chromosome consists of two sister chromatids and the other part the other homologue the other chromosome which is a part of the pair of this homologous chromosome consist of now two other sister chromatids this is the reason that we call them diploid half double number of chromosomes. Now then what happen that when the gametes are formed the sperms and the egg they have to join together each from one parent and when these cells join together which is called fertilization in this after this process the sperm have a half number of chromosome N number and the egg have a half number of chromosome N number when they combine the normal two N number of chromosomes or we can say that diploidy is retained in the zygote formed and the next individual the embryo which is formed by fertilization again have the same number of chromosomes two N as its parents did have but this process of myosas actually offers a great number of variations because half set of chromosomes are coming from one parent and the other half is coming from the other parent the result is a lot many a lot many changes or a lot many different sets of genes which may result in different characteristics now look at another diagram this diagram shows the set of chromosomes in the human beings if you look at the diagrams it shows the chromosomes in pairs you can see in the pair one there are two chromosomes just like that in the pair two there are two other chromosomes which have different forms but in the pair one both chromosomes they look like each other just like that in the pair two they also look like each other just like that the chromosomes 3 4 5 6 and so on up to 22 they are alike just the one pair as you can see the last pair in the right corner down you can see that there is an X and there is an Y this only set the XY pair is different in human beings in the males and females males have set of XY so in this pair the chromosomes are not homologous they are not alike Y is different from X but in females this is again a homologous pair which makes X X so in females all the pairs are homologous but in males one pair is not homologous now we talk about the meiosis meiosis occur in the germline cells in the ovary of the female or in the testis of the male there are the germline cells divide into four daughter cells each as we know now haploid in number