Meiosis is a greek word which means reduction. Obviously the next question arise is, reduction of what? A short answer to that question is, reduction of chromosomes. In the human body each and every cell that is not a gamete contains 46 chromosomes in the nucleus. These chromosomes occur in pairs. We have 23 pairs of chromosomes. Among them, 22 pairs are called autosomes and one pair of sex chromosome. Each pair contains identical copies of chromosomes except the sex chromosomes. They are different.

Meiosis produces 4 daughter cells. Each of them has 23 chromosomes. A cell that has 23 chromosomes is called a haploid cell. Before a cell goes into mitosis or meiosis cell divisions they go through a phase which is called the interphase. What happens in the interphase is the duplication of the DNA. Which results in the formation of two identical chromatids of a single chromosome. They're called sister chromatids. They remain joined together at the centromere. This way a cell nucleus gets 92 chromatids. It is safe to assume these chromatids as new chromosomes. Because once they get separated they become independent chromosomes. During this interphase stage we actually get 92 chromosomes.

Lets talk about the meiosis now. Meiosis cell division has two parts. Meiosis 1 and meiosis 2. A parent cell gets divided into two daughter cells in meiosis 1. each of these daughter cells get 46 chromatids. Later in meiosis 2 these 46 chromatids further gets seperated into two daughter cells each one containing only 23 chromatids. Meiosis one has 4 phases of its own. Prophase 1, metaphase 1, anaphase 1 and telophase 1. And meiosis 2 has 4 phases as well. prophase 2, metaphase 2, anaphase 2 and telophase 2. however meiosis one prophase one has five chronological events. They happen one after another. they are Leptotene, Zygotene, pachytene, diplotene and diakinesis. In the leptotene stage nothing really happens except the chromosomes become condensed and visible. In the Zygotene stage chromosomes start to pair with their homologues. this process is called synapses and a tetrad is formed. tetra is a greek prefix which means four. Tetrad is formed when four chromatids come close. Pairing is achieved by a protein structure which is called synaptonemal complex. As soon as the pairing is complete zygotene ends.

Pachytene is Probably the most fascinating stage of them all. Because in this stage crossing over happens. as the name suggests genes cross one chromosome to another at this stage. And as a result we get non identical chromatids. But remember this, crossing over only happens to the autosomes not in the sex chromosomes. So what we get after the crossing over is 88 non identical chromatids. If you are finding it difficult to understand then you can pause the video any time and try to appreciate the numbers before you proceed any further. Now we have reached to the meiosis one prophase one diplotene stage. in this stage synaptonemal complex disappears and the chromatids condense further. This stage can be lenghty in different species. Nuclear membrane starts to disappear in diakinesis stage and spindle apparatus appears. but the chromosomes still remain attached at the site of chiasma. With this, meiosis one prophase one stage ends. All the chromosomes with their chiasma align on the equitorial plane and the spindle fibres start to pick up chromosomal centromeres. After the alignment is complete metaphase one ends. Spindle fibers start to retract towards oppostie poles of the cell and they carry the Chromatids with them. When 46 chromatids reach towards the opposite poles the stage completes and nuclear membrane starts to form again. Nuclear membrane formation is now complete and the cell cytoplasm gets divided into two daughter cells. Each of these two daughter cells has 46 chromatids where 44 are autosomes and 1 pair of sex chromatids. as soon as the division completes meiosis one ends. Meiosis two is exactly similar to mitosis but there is a very important difference. In the mitosis a parent cell starts with 92 chromatids and meiosis 2 a parent cell starts with only 46 chromatids. Meiosis 2 starts with Prophase two and ends with telophase two. At the end 4 haploid daughter cells are created.

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