Sexually Reproducing organisms begin their lives from a single cell called zygote. Our growth takes place by cell division. Cell is divided for various purposes at the initial stages of life almost all the cells divide and enlarge but later on the cell division is restricted for special purposes. For example : Reproductive Cell, Old and Worn out cells etc.
Every cell goes division through cell cycle. This cycle has long resting phase called as “Interphase” followed by a short division phase called “M Phase.”
Interphase has three phases
It is also called as pre DNA (Deoxyribonucleic Acid) synthesis phases. In this cell grows in size, does the synthesizing of RNA (Ribonucleic Acid) and proteins. There is no color change in its DNA content.
It is also called as DNA (Deoxyribonucleic Acid) synthesis phase. During this phase new DNA is synthesized as par as the new DNA template. Histone proteins are also synthesized. Thus each chromosome contains of two chromatids jointed at the centromere.
It is also called as post DNA (Deoxyribonucleic Acid) synthesis phase. Synthesis of RNA (Ribonucleic Acid) and proteins continues and after this new cells are ready for M phase.
Cell Division occurs in three ways :
- Discovered by Remak in 1841 in the RBCs (Red Blood Cells) of a chick.
- In this cell division formation of spindles and appearance of chromosomes take place and nuclear envelope remains intact.
- Nuclear division is followed by division of cytoplasm
- Amitosis is also known as direct division
- Amitosis is very rare, the macro nucleus of certain ciliates such as Paramecium is divided by Amitosis.
- Mitosis was described by a German Biologist, Edward Strasburger in the plant cell in 1875 and by another German Biologist, W. Flemming in the animal cell in 1882.
- It takes place in the somatic cells in animals. Hence is also known as somatic division.
- It occurs in gonads for the multiplication of undifferentiated germ cell.
- In plants it takes place in meristematic tissue.
- Mitosis is division of a parent cell into two identical daughter cell, each with the nucleus and same amount of DNA, and same amount of chromosomes. Hence is also known as equational division.
- There are two kinds of mitosis Karyokinesis which do the division of nucleus and cytokineses which do the division of cytoplasm. Karyokinesis has four phases that are Prophase, Metaphase, Anaphase and Telophase.
- The term meiosis was introduced by Fermer and Moore in 1905.
- Meiosis or reduction division is so named as it results in the formation of four daughter cells, each with the number of chromosomes as compared to the mother cell.
- All sexually reproducing organisms undergoes Meiosis
- Meiosis occurs in two stages that are Meiosis 1 and Meiosis 2.
- Reduction of Chromosomes number occur in Meiosis 1.
- In Meiosis 2 chromosome number of the cell remains the same as Meiosis 1.