Topic 4: Variation: Meiosis

Meiosis

Type of cell division in which apparent cell divides to form haploid cells, each genetically distinct from one another. Meiosis involves a reduction division in which the chromosome number is halved to form haploid cells these haploid cells form the gametes (egg and sperm cells) in animals and plant cells.

Before meiosis starts, DNA is replicated during interphase so that each chromosome contains two chromatids

Cell then undergoes two divisions (meiosis I and meiosis II)

Meiosis I

Homologous chromosomes are separated (prophase I, metaphase I, anaphase I, and telophase I)

Meiosis II

Chromatids are separated (prophase II, metaphase II, anaphase II, telophase II)

Prophase I

• Before division the DNA has replicated – we know this because each chromosome is made up of two sister chromatids

• Identical sister chromatids are joined at the centromere

• Chromosomes are present as homologous pairs (diploid– 2n)

• One chromosome from mother and one from father

• Chromosomes condense and homologous chromosomes pair up

• Centrioles migrate to opposite poles of the cell where each centriole starts forming spindle fibre

• The nucleus disappears and the nuclear envelope starts to breakdown, leaving the chromosomes free in the cytoplasm

Metaphase I

• premium’s lineup along the equator of the cell in their homologous pairs (so in humans, 23 pairs lines up)

• The pears are close together, causing chromatids on chromosomal pairs to wrap around each other due to tension

• Causing crossing over at the chiasmata (recombination)

• Causing genetic variation (due to this chromosome not having existed before)

• Independent assortment when they lineup

• Each chromosome attaches to the spindle by their centromere

Anaphase I

• Homologous chromosome pears are separated and pulled to opposite poles of the cell (chromatids stay joined together)

• Independent segregation, as pulled apart by spindle fibres

• Reduction division as one pair of sister chromatids at each pole

Telophase I

• The chromosomes reached the opposite poles of the cell where they uncoil

• The nuclear envelope forms around each set of chromosomes and the nucleolus starts to form

• Cytoplasm divides to form two cells (cytokinesis)

• Each new cell now only has one of each homologous pair of chromosomes as is therefore haploid (n)

Bivalent pairs

Homologous chromosomes