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Structure of cell cycle
- Cell cycle is composed of four phases- Interphase (G1, G2), S phase (synthetic phase), cell division (mitosis/meiosis).
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Factors controlling cell division
Factors controlling cell division are as follows:
1. Cell size
2. Karyoplasmic ratio
3. Mitogens
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Mitosis
- Mitosis is an equational division, in which one parent cell divides into two identical daughter cells.
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Prophase 1
In Prophase 1 of meiosis, Chromosomes become shorter and thicker, clearly visible in a microscope. The nucleus disappears during this phase. It has five phases-
1. Leptotene – The chromosomes condense and are visible under an electron microscope.
2. Zygotene – It is the sub-stage, where synapsis between homologous chromosomes begins.
3. Pachytene – Crossing over starts within the bivalents. The synaptonemal complex is complete, allowing chiasma to form.
4. Diplotene – The synaptonemal complex degrades and homologous chromosomes separate from one another a little.
5. Diakinesis – Chromosomes condense further during this stage. The four parts of the tetrads are actually visible.
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Meiosis 2
Phases of Meiosis II
The second meiotic division is very much similar to mitosis. It has the following steps:
1. Prophase II
The events of prophase II are similar to mitotic prophase.
Nucleolus and nuclear membrane disappear.
Spindle fibres are formed at each pole.
2. Metaphase II
Chromosomes move to the centre of the equatorial plane.
They get attached to spindle fibres centromere.
3. Anaphase II
The sister chromatids separate from one another and are pulled to opposite poles of the spindle due to the contraction of the spindle fibres.
4. Telophase II
The chromosomes begin to uncoil and become thin.
They reorganize into the nucleus with the reappearance of the nucleolus and nuclear membrane in each pole.
Cytokinesis follows and four haploid daughter cells are formed and thus the meiotic division is completed.
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Significance of mitosis
1. It is responsible for the growth and development of multicellular organisms from a single-celled zygote.
2. It repairs the damaged and wounded tissues.
3. It helps in maintaining the same chromosome number in daughter cells.
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Significance of meiosis
1. It maintains the same chromosome number in the sexually reproducing organisms. From a diploid cell, haploid gametes are produced which in turn fuse to form a diploid cell.
2. It restricts the multiplication of chromosome numbers and maintains the stability of the species.
3. Maternal and paternal genes get exchanged during crossing over. It results in variations among the offspring.
4. All the four chromatids of a homologous pair of chromosomes segregate and go over separately to four different daughter cells. This leads to variation in the daughter cells genetically.
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Cytokinesis
Cytokinesis is the physical process of cell division, which divides the cytoplasm of a parental cell into two daughter cells.
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