5.1 Cell Division

5. CELL DIVISION

The necessity for cell division in living organism  For growth  For replacing dead and worn out cells  For reproduction in unicellular organisms  To produce gametes cells

Stages in cell division  Two stages: 1. Nuclear division - two types: a) Mitosis b) Meiosis 2. Cytoplasmic division (Cytokinesis)

5.1 Mitosis

The meaning of mitosis  the process of nuclear division which form two genetically identical daughter cells

The Significance of Mitosis Essential in replacing dead cells allow damaged cell to be repaired, replace or regenerated increase the number of cells to allow growth in unicellular organism as the basis of asexual reproduction (binary fission). E.g. Amoeba sp.

Chromosomes and chromosomal number  Chromosomes - long, fine, threadlike structures found in the nucleus  Builds from molecules called deoxyribonucleic acid (DNA)  Every species has its own number of chromosomes per cell to store their heredity information  Humans possess 46 chromosomes

 2 types of chromosomes: 2. Autosomes – controls all somatic traits (blood group, body weight & skin colour) 3. Sex chromosomes – determine gender/sex  Chromosomes in nucleus (somatic cells) exist in pairs (diploid (2n))  Gamete cell contain only half the number of chromosomes (haploid (n))

 Somatic cells contain: 44 autosomes + 2 sex chromosomes = 46 chromosomes  Gamete cells contain: 22 autosomes + 1 sex chromosome = 23 chromosomes  2 types of sex chromosomes: XY – chromosomes in males XX – chromosomes in females

Chromosomes In Human

Male

Female

• Two chromosomes in each pair have the same structural feature (homologous chromosomes) – one chromosomes is of paternal origin and the other is maternal origin

The necessity for the production of new cells identical to parent cells  In chromosomes the DNA contains genes which encode information for the synthesis of protein  Each gene is a unit of inheritance which contain the genetic information and must be passed on to the new cells when the nucleus divide  Mitosis doubles the number of cells without changing the genetic content of the cell

Test Yourself 1. What are chromosomes? Answer: - Long, fine, thread-like structures found in the nucleus.

2. How many types of chromosomes and what are they? Answer: - 2 types = autosomes and sex chromosomes

3. How many chromosomes in human? Answer: - 46 = 23 pairs