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Cell division Mitosis • Body cells divide to make new cells. • Mitosis - cell division which takes place in normal cells produces genetically identical daughter cells. • In asexual reproduction cells of offspring produced by mitosis – No variety – identical genetic info. Normal body cell – four chromosomes in two pairs As cell division starts a copy of each chromosome is made Cell divides into two to form two daughter cells. Each daughter cell has a nucleus containing four chromosomes identical to the ones in org. Parent cell. Differentiation • In early development of animal and plant embryos cells are unspecialised. – Each one of them can become any type of cell which is needed. • These are known as stem cells. • In many animals cells become specialised early in life. – They have differentiated. • Differentiated cells – some of their genes have been switched on and others switched off. – After differentiation each cell can only produce identical copies of itself. Differentiation • Plant cells can differentiate all through their life. • Cloning can easily produce huge numbers of plant clones from a tiny piece of leaf tissue. • In right conditions plant cell will become unspecialised and undergo mitosis many times. • Animal clones cannot be made easily because animal cells differentiate permanently early in embryo development and can not change back. Cell division in sexual reproduction • Mitosis takes place all the time in tissues all over the body. • Meiosis – cell division in sex cells (gametes). – Results in cells with half the original number of chromosomes. Cell in reproductive organ looks like normal cell before it starts to divide to form gametes As in normal cell division first step is that chromosomes are copied. Cell divides in two and these cells immediately divide again. This gives four sex cells each with a single set of chromosomes – two instead of four Why is meiosis important? • Normal body cells have 46 chromosomes (23 pairs): 23 from mum, 23 from dad. • If two normal body cells joined in sexual reproduction new cell would have 92 chromosomes. • As a result of meiosis sex cells only contain one set of chromosomes. – So when gametes join at fertilisation new cell formed contains 46 chromosomes. Why is meiosis important? • Each gamete produced is slightly different – Combination of chromosomes different. • During process of meiosis there will be some exchange of genes between the chromosomes. – No two sperm or eggs are the same. – Introduces variety into offspring.