DNA REPLICATION:
DNA replicates prior to cell division, so that chromosomes can be copied to give the same genetic code to every new diploid cell that is made. DNA replication occurs in a series of enzyme controlled reactions in which DNA is copied to produce two identical double stranded DNA molecules. Replication occurs in a series of steps controlled by enzymes with energy supplied as ATP.

Steps:
1.An enzyme unwinds the DNA double helix by breaking the Hydrogen bonds between base pairs. Another enzyme stops the unwinding strand from becoming knotted.
2.Nucleotides from within the nucleus assemble opposite each strand according to the base pairing rules, using the energy from ATP to join together.
3. Due to the way the deoxyribose and phosphate molecules are joined to each other one strand is the leading strand in which replication is continuous by an enzyme working in one direction binding the nucleotides one by one in a 5'3' direction. Where as the other strand is the lagging strand in which replication happens in the opposite direction and is fragmented. An enzyme called polymerase creates the fragments of nucleotides (Okazaki fragments) and joins them in a 5'3' direction. The enzyme ligase then joins the Okazaki fragments on the lagging strand by re-establishing the sugar-phosphate bonds.
4.New nucleotides bond to the corresponding base and two identical DNA molecules are formed which wind into their double helix.
The leading and lagging strand exist because DNA is anti-parallel, and runs in opposite directions, with a 3' 5' arrangement.

Replication is semiconservative because each new molecule contains one new strand and one parent strand.

MITOSIS:
Cells divide by mitosis for growth, and to replace worn or injured cells. Mitosis produces two genetically identical daughter cells, so the same cell functions and processes occur.

Mitosis occurs most rapidly when new cells are forming during periods of growth, such as in an infant, embryo, or plants coming out from periods of dormancy (seed germination etc. ) In plants the tips of roots, stems and side branches are where mitosis is most rapid. A high rate of mitosis is also required in damaged tissue, for example human bone marrow, lymph nodes and dermis of the skin divide.

Some areas if the body is constantly producing new cells. Here the mitotic rate is greater. For example the dermis of human skin is constantly actively, forming cells to replace those lost from the epidermis, and bone marrow constantly produces red blood