1. Biology 2B
Cell division

2. Cell division by mitosis
Needed 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

3. Cell cycle

4. Stages of mitosis

5. Interphase Phase of growth and normal metabolism
Nucleus and nucleolus visible
DNA replication occurs just before cell division occurs

6. 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)

7. Metaphase
Chromosomes line up on spindle at the equator of the cell

8. Anaphase Chromatids pull apart at the centromere
One chromatid from each chromosome moves to each end of the cell

9. 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

10. Interphase Phase of growth and normal metabolism
Nucleus and nucleolus visible

11. Gametogenesis – cell division by meiosis
Sexual 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.

12. Events of meiosis

13. Meiosis 1 – the first division
prophase I
the chromosomes become visible - each chromosome contains two chromatids
homologous chromosomes move together & a spindle forms
metaphase I
homologous pairs of chromosomes line up on the equator, attached by their centromeres
anaphase I
one of each pair of homologous chromosomes is pulled to either end of the cell
telophase I
the chromosomes condense
cytoplasm is split of and cell divides into two daughter cells, each with 23 chromosomes
each chromosome consists of two chromatids

14. Meiosis 2 – the second division
prophase II
the chromosomes become visible - each chromosome contains two chromatids
a new spindle forms, at right angles to the old one
metaphase II
chromosomes line up on the equator, attached by their centromeres
anaphase II
one of each chromatid in each chromosome is pulled to either end of the cell
telophase II
the chromosomes condense
cytoplasm 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

15. Comparing mitosis and 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
Number of chromosomes in daughter cells
Same as parent (diploid)
Half that of parent (haploid)
Amount of variation in daughter cells
None
Lots

16. Comparing mitosis and meiosis 2
Advantages for reproduction
Simple
Rapid division
Allows variation
Disadvantages for reproduction
No variation
More complex
Slower reproduction