Cell Division: An Essential Life Process :
- Due to this property only -
- A new organism is formed from existing one,
- A multicellular organism grows up
- Emaciated body can be restored.
There are two types of cell division as :
- mitosis
- meiosis.
- Mitosis occurs in somatic cells and stem cells of the body.
- Meiosis occurs in germ cells.
- Before any type of cell division, the cell doubles up its chromosome number present in its nucleus.
- if chromosome number is 2n, it is doubled up to 4n.
Mitosis :
- Somatic cells and stem cells divide by mitosis.
- Mitosis is completed through two main steps -
- Karyokinesis (nuclear division)
- Cytokinesis (cytoplasmic division).
- Completed through four steps.
A. Prophase :
- Condensation of basically thin thread-like chromosomes starts.
- Due to this, they become short and thick
- They start to appear along with their pairs of sister chromatids.
- Centrioles duplicate.
- Each centriole moves to opposite poles of the cells.
- Nuclear membrane and nucleolus start to disappear.
B. Metaphase :
- Nuclear membrane completely disappears.
- Chromosomes complete their condensation.
- They become clearly visible along with their sister chromatids.
- All chromosomes are arranged parallel to equatorial plane (central plane) of the cell.
- Special type of flexible protein fibers (spindle fibers) are formed between centromere of each chromosome and both centrioles.
C. Anaphase :
- Centromeres split.
- Sister chromatids of each chromosome separate.
- They are pulled apart in opposite directions with the help of spindle fibers.
- Separated sister chromatids are called as daughter chromosomes.
- Each set of chromosomes reach at two opposite poles of the cell.
D. Telophase :
- The chromosomes which have reached at opposite poles of the cell now start to decondense.
- They again become thread-like thin and invisible.
- Nuclear membrane is formed around each set of chromosomes reached at poles.
- Thus, two daughter nuclei are formed in a cell.
- Nucleolus also appears in each daughter nucleus.
- Spindle fibers completely disappear.
In this way, karyokinesis completes and cytokinesis begins.
cytokinesis :
- The cytoplasm divides by cytokinesis.
- Two new cells are formed which are called as daughter cells.
- In this process, a notch is formed at the equatorial plane of the cell which deepens gradually and thereby two new cells are formed.
- However, in case of plant cells, instead of the notch, a cell plate is formed exactly along midline of the cell.
- Thus cytokinesis is completed.
Meiosis :
- Meiosis is completed through two stages.
- Those two stages are
- Meiosis-I
- Meiosis- II.
- Recombination / crossing over occur between homologous chromosomes
- Thereafter those homologous chromosomes (Not sister chromatids) are divided into two groups.
- Thus two haploid cells are formed.
Meiosis-II :
- Just like mitosis.
- In this stage, the two haploid daughter cells formed in meiosis-I undergo division by separation of recombined sister chromatids.
- Four haploid daughter cells are formed.
- Process of gamete production and spore formation occurs by meiosis.
- In this type of cell division, four haploid (n) daughter cells are formed from one diploid (2n) cell.
- During this cell division, crossing over occurs between the homologous chromosomes.
- Thereby genetic recombination occurs.
- Due to this, all the four daughter cells are genetically different from parent cell and from each other too.