Cell Division;Biology Chapter 5

Necessary-helps to heal wound
                 -hair and nail growth

Cell division can be divided into stages:
1.Nuclear division
2.Cytoplasmic division(cytokinesis)

Mitosis
2n=diploid

• 2 daughters cell which are genetically identical to parent cells are formed
• Occurs only in somatic cells
• Occurs actively in the meristems of plants
• Helps in growth and ellongation of plants

Meiosis

n=haploid

• 4 daughter cells which are genetically not identical to parent cells are formed
• Occurs only in reproductive cells helps to produce gametes

Significance of Mitosis

• Chromosomal number in a species must be maintained

• Somatic cells contain 2 sets of chromosomes(human:2n=46)

• Each pair of chromosomes is known as a pair of homologous chromosomes and it is genetically identical
• Gene-an unit of inheritance(passed to the offspring from the parent cell)

• Mitosis doubles the number of chromosomes without changing the genetic content

Cell Cycle

• Can be divided into two major phases:

1) Interphase

2)  Mitotic cell division or the all phase(consists of mitosis and cytokinesis)

Interphase

• Stage for the cells to grow larger and prepare for cell division

• During the period,the nucleus is big and well defined

• A pair of centrosomes is also formed in the cytoplasm of animal cells

• Each centrosome consists of a pair of centrioles

• It is divided into three shorter stages or sub phases

1. G1 (Growth phases 1)

2. S (DNA synthesis)

3. G2 (Growth phases 2)

G1 (Growth phases 1)

• A newly formed cell enters the G1 portion of interphase and begins to acquire and synthesis materials needed for the next cycle of cell division

• Proteins and new cytoplasmic organelles such an mitochondria and chloroplasts are synthesized during this stages

• The metabolic rate of the cell is high

• The chromosomes are not condensed and appear as thread like structures called chromatin

S (DNA synthesis)

• Synthesis of DNA occurs during the S phase

• The DNA in the nucleus undergoes replication

• Each duplicated chromosomes now consists of two identical sister chromatids which contain identical copies of the chromosomes DNA molecule

G2 (Growth phases 2)

• The cell continues to grow,accumulaters energy and carries out final preparation for cell division

G1-Mitochondrion,Chloroplast

S-Synthesis DNA -Carried out replication

G2 - Store E - Energy ATP