B5.2 How does an organism produce new cells?
Mitosis and meiosis are two ways that cells reproduce
MITOSIS is the process by which a cell divides to produce two new cells with identical sets of chromosomes to the parent cell – the new cells will also have all the necessary organelles. The purpose of mitosis is to produce new cells for growth and repair and to replace old tissues.
Mitosis leads to the production of two new cells, which are identical to each other and to the parent cell. Mitosis can only take place when a cell is ready to divide this means that cells go through a cell cycle.
The cell cycle consists of a growth stage (G1) where the cell gets bigger and the number of organelles increase, then a synthesis stage (S) where the DNA is copied, followed by another very short growth stage (G2) immediately before mitosis (M)
Both new cells need to have a full complement of organelles and DNA to function properly. Therefore the number of organelles needs to increase and the DNA has to be copied.
The chromosomes are copied when the two strands of each DNA molecule separate and new strands form alongside them.
Meiosis only takes place in the testes and ovaries – it is a special type of division that produces gametes (sex cells i.e. eggs and sperm) for sexual reproduction.
Gametes contain half the number of chromosomes of the parent cell – this is important because it means that when the male and female gametes fuse, the number of chromosomes will increase back to the full number. The resulting zygote has a set of chromosomes from each parent.