Topic 4: Variation: Meiosis

Meiosis

Type of cell division in which a parent 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. In human diploid cell, 23 pairs of homologous chromosomes – 46 replicated chromosomes in total- 92 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)

Interphase

• Chromosomes are present as homologous pairs (diploid– 2n), one from father and one from mother

• Before division the DNA has replicated – we know this because each chromosome is made up of two sister chromatids (4n)

• Identical sister chromatids are joined at the centromere

Prophase I

• One chromosome from mother and one from father (tetraploid-4n, as each chromosome made up of two sister chromatids)

• 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

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

• The pairs 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 pairs 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) - however 2n DNA content as each chromosome has two sister chromatids

First Cytokinesis

• divides cytoplasm

• Each new cell has half the number of chromosomes of original (i.e. 23 chromosomes) - each made up of two chromatids

Bivalent pairs

Homologous chromosomes

Chromatin

Unwound DNA

Prophase II

• The chromosomes condense and are now visible under a microscope

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

• The nucleus disappears and the nuclear envelope starts to breakdown

Metaphase II

• The chromosomes lineup at the equator of the cell (so in humans, 23 chromosomes lineup)

• Each chromosome attaches to the spindle by their centromere

Anaphase II

• The centromere divides and separate each pair of chromatids, and moved to opposite poles (usually indifferent plane to 1st division i.e. horizontal versus vertical)

• The centromere splits to allow each chromatid to move

• The spindle fibres contract and shorten to pull the chromatids to opposite poles of the cell

Telophase II

• chromatids reach the opposite poles of the cell where they uncoil to become long and thin again

• A nuclear envelope forms around each set of chromosomes to form two nuclei and the nucleolus starts to reform

• The cytoplasm divides (cytokinesis) and four cells are produced

Second Cytokinesis

• cytoplasm of both cells divides

• Forms four genetically different haploid cells with half the number of chromosomes than the original

• Each cell is genetically different and contains half the number of chromosomes as the original cell (haploid)

After meiosis

You now have thousands or even millions of genetically different gametes. Which gamete fertilises which gamete is usually a random process. This random fertilisation also increases the genetic diversity of organisms.

Mitosis differences

• growth and repair

• Asexual

• Diploid cells produced

• Two daughter cells

• One cell division

• Doesn’t create genetic variation

• Doesn’t separate homologous chromosomes

Meiosis differences

• production of gametes

• Sexual

• Haploid cells are produced

• Four daughter cells

• Two cell division

• Does create genetic variation

• Does separate homologous chromosomes - anaphase II

Only one interphase in meiosis, though it divides twice

i.e. DNA only replicated once