Download Cell division Mitosis

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.