Mitosis

Each body cell keeps the same number of chromosomes as the original cell. In humans, each body cell has 23 pairs of chromosomes or 46 chromosomes in total. At the end of mitosis, each daughter cell also has 46 chromosomes identical to the originals. Other organisms have different numbers of chromosomes e.g. most species of cat have 38 chromosomes and horses have 64.

Mitosis is an important process for all living things, not only does it allow the number of cells in an individual to increase and damaged cells to be replaced (growth and repair), it enables certain species to regenerate (regrow) body parts e.g. starfish can replace lost arms and some lizards can regrow their tails. It also ensures that all cells in an organism carry the same genetic code and allows plants and some single celled creatures to reproduce asexually.

Mitosis occurs in several phases with grand names such as prophase and telophase; for the exams you don't need to know these. But you do need to know the essentials of the steps themselves i.e. the chromosomes are copied, lined up at the center of the cell, become separated by the spindle fibers and a finally new cell wall grows, separating the two daughter cells.

Some exams will test if you know how the chromosomes copy themselves. This involves the DNA. A chromosome is made up from two strands of DNA in a double spiral (often referred to as a double helix) locked together by a specific sequence of bases. During mitosis, the double DNA spiral unzips down the center and the cell makes a new strand of DNA alongside the original which re-forms the original chromosome, making an exact copy. If you are a little uncertain about mitosis and how the chromosomes copy themselves, go back to basics and refresh your memory of the structure of chromosomes, DNA and genes.