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