Meiosis

is a type of cell division
produces gametes
occurs only in gamete mother cells
results in variation

Gamete mother cells and gametes

I) GAMETE MOTHER CELLS:

are diploid (2 sets of chromosomes)
undergo meiosis to produce gametes

II) GAMETES

are haploid (1 set of chromosomes)
fuse together during fertilisation to form a zygote

Location of gamete mother cells and gametes
Mammals

testes- sperm
ovaries- ova

Flowering plants

anther- pollen
ovary- ovules

Process of meiosis

consists of 2 consecutive divisions
reduces the chromosome number by half
produces 4 genetically different gametes

First meiotic division:

1. Chromosomes are long, thin and uncoiled.
Each chromosome replicates to form 2 identical chromatids.

2. Chromosomes coil becoming short and thick making them visible.
Homologous chromosomes pair up.
Crossing over occurs at points where the chromosomes touch- these points are called chiasmata.

3. Homologous pairs of chromosomes line up at the equator with their centromeres attached to spindle fibres.

Independent assortment occurs at these points.

4. Homologous pairs move to opposite poles.

Each chromosome is still made up of two chromatids joined at centromere.

5. Cytoplasm divides and nuclear membrane is formed.

Two haploid cells have been formed.

END OF FIRST MEIOTIC DIVISION

Second meiotic division:

1. The nuclear membranes disappear, chromosomes line up at equator with their centromeres attached to spindle fibres.

2. The centromeres split allowing the chromatids to be pulled to opposite ends of the poles.

3. Nuclear membranes reform, cytoplasm divides.

END OF SECOND MEIOTIC DIVISION

Result meiotic division:

4 haploid gametes have been formed (contain half the amount of genetic material in a normal cell)
Each gamete contains a different genetic combination

Variation

helps a species survive and adapt to a changing environment and so is important in long term evolutionary change
is determined by genes, carried on chromosomes
is increased by meiosis
is increased at fertilisation during sexual reproduction

Meiosis and Variation

Meiosis increases variation by allowing new combinations of existing genetic material to be formed in gametes. This happens in 2 ways:

A: Crossing Over

occurs during the first meiotic division
occurs between members of a homologous pair of chromosomes
exchanges parts of 1 chromatid with another
is random
results in chromosomes with new combinations of genes
results in variable gametes being formed at the end of meiosis

B: Independent Assortment

occurs during first meiotic division
is random
homologous chromosomes line up along the equator with the arrangement of any one pair unaffected by that of the others
the way the chromosomes line up along the equator will determine the genetic combinations present within a particular gamete

Fertilisation

occurs when a male haploid gamete fuses with a female haploid gamete
produces a diploid zygote which has a new combination of genes (one set from each parent)
is random
leads to a huge potential variability of offspring

Comparison between mitosis and meiosis

Feature, Mitosis, Meiosis

Place where it occurs: Body tissues, Sex organs

Pairing of homologous chromosomes: no, yes

Chiasmata formation: no, yes

Number of divisions of nucleus: 1, 2

Number of daughter cells: 2, 4

Ploidy of daughter cells: diploid, haploid

Effect on chromosome number: none, halved

Process providing variation: none, crossing over & independent assortment

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Notes on first and second meiotic divison

Meiosis