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Biology 2B
Cell division
Cell division by mitosisNeeded for:• reproduction in unicellular organisms• growth in multicellular organisms• replacement of damaged or dying cells• repair of damaged organs or tissues
Involves:• replication of DNA• controlled division of the nuclear material so new each cell has
the same DNA• division of cytoplasm and organelles so each new cell has similar
contents
Cell cycle
Stages of mitosis
Interphase
• Phase of growth and normal metabolism
• Nucleus and nucleolus visible
• DNA replication occurs just before cell division occurs
Prophase
• First stage of division• Nucleus and nucleolus
disappear• Spindle starts to form• DNA condenses and
becomes visible as chromosomes
• Centrioles move to opposite poles (animal cells)
Metaphase
• Chromosomes line up on spindle at the equator of the cell
Anaphase
• Chromatids pull apart at the centromere
• One chromatid from each chromosome moves to each end of the cell
Telophase
• Chromosomes uncondense and a new nucleus forms around them
• The cytoplasm splits (cytokinesis) to form two new cells identical to the original except smaller
Interphase
• Phase of growth and normal metabolism• Nucleus and nucleolus visible
Gametogenesis – cell division by meiosisSexual reproduction involve the formation of gametes, which
are then fertilised to produce a new individual.Gametes are made by a process called gametogenesis.
Gametogenesis involves a special type of division called meiosis. In meiosis, the number of chromosomes is halved, so that when
gametes unite in fertilisation, they have the correct number of chromosomes.
Gametes are called haploid cell as they have only half the number of chromosomes as normal (diploid) cells.
Female gametes are made in the ovaries and are called ova.Male gametes are made in the testes and are called sperm.
In most species gametes then undergo a period of maturation before they are capable of fertilisation.
Events of meiosis
Meiosis 1 – the first divisionprophase Ithe chromosomes become visible - each
chromosome contains two chromatidshomologous chromosomes move together
& a spindle formsmetaphase Ihomologous pairs of chromosomes line up
on the equator, attached by their centromeres
anaphase Ione of each pair of homologous
chromosomes is pulled to either end of the cell
telophase Ithe chromosomes condensecytoplasm is split of and cell divides into two
daughter cells, each with 23 chromosomes
each chromosome consists of two chromatids
Meiosis 2 – the second divisionprophase IIthe chromosomes become visible - each
chromosome contains two chromatidsa new spindle forms, at right angles to the
old onemetaphase IIchromosomes line up on the equator,
attached by their centromeresanaphase IIone of each chromatid in each chromosome
is pulled to either end of the celltelophase IIthe chromosomes condensecytoplasm is split of and each daughter cell
divides into two (ie total of 4 daughter cells), each with 23 chromosomes - each chromosome consists of one chromatid
Gametes
Comparing mitosis and meiosisMitosis Meiosis
Where it occurs Body cells Gonads (reproductive organs)
Why it occurs Cell repair, growth, asexual division
Sexual reproduction
Number of cells produced 2 4
Number of divisions 1 2
Number of chromosomes in daughter cells
Same as parent (diploid)
Half that of parent (haploid)
Amount of variation in daughter cells
None Lots
Comparing mitosis and meiosis 2
Mitosis Meiosis
Advantages for reproduction
SimpleRapid division
Allows variation
Disadvantages for reproduction
No variation More complexSlower reproduction