Cell Division

 

All cells are derived from preexisting cells. New cells are produced for growth and to replace damaged or old cells. DNA must be replicated before cell division. Duplicated chromosomes are called chromatids and are held together by centromere.

   

Cell cycle

 

        

    

            Interphase

Interphase is the period between cell division. In this phase cell prepares for division. So DNA replication, organelles replication, cell increases in size are occurred. In the end of the interphase, chromatin replicates and form chromatids.

Many damaged, dead and worn out cells can be replaced by growth and division of other similar cells. Most body cells have 46 chromosomes and divide by mitosis, result in two new daughter cells. The daughter cells are identical to one another and to the original parent cells.

Mitosis has 4 different stages

1.    Prophase

2.    Metaphase

3.    Anaphase

4.    Telophase

 

Prophase

In this phase, cell prepares for nuclear division. Chromatids become visible in the nucleus. Mitotic apparatus become appear, consists of 2 centrioles separated by the mitotic spindle. The centrioles migrate one to each end of the cell. Nuclear envelops disappear.

 

Metaphase

In this phase, cell prepares chromosomes for division. The chromatids align on the center of the spindle, attached by there centromeres.

 

Anaphase

In this phase, chromosomes are divided, centromeres separate. One of each pair of sister chromatid migrate to each pole of the spindle as the microtubules form the mitotic spindle contract. Spindle fibers pull chromosomes apart. Half of each chromosomes moves to each daughter cells.

 

Telophase

In this phase, cytoplasm divides. The mitotic apparatus disappears. The chromosomes uncoil and nuclear envelops reforms. Following telophase the cytoplasm and plasma membrane dividing in half forming two identical daughter cells.

The formation of gametes (ova and spermatozoa), takes place by meiosis and the four daughter cells are produced. They are genetically different from parent cells and each other. Meiosis involves two distinct cell division and it produces four daughter cells all different from parent cells and from each other. This is the basic of genetic diversity and the uniqueness of ach human individual.  

 

First meiotic division

This stage produces two genetically different daughter cells. DNA replication occurs beforehand. Each pair of chromosomes have 4 chromatids and they gather together into a tight bundle. Exchange of genes – crossing over, and result in the four chromatids acquiring different combination of genes. Each pair of chromosomes separates and one travel to each end of the cell, guided by a spindle and the cytoplasm divides, producing two genetically unique dipolar daughter cells.

 

Second meiotic division

The chromosomes separate and the two sister chromatids travel to opposite ends of the cell, which then divides. Each of the four haploid daughter cells now has only one chromosome form each original pair.