meiosis

• meiosis

  = production of gametes, one from each parent, fuse to produce a fertilised egg.

  • nucleus divides twice to produce 4 daughter cells- the gametes. Each contain half the chromosomes number of the parent= haploid.
• homologous chromosomes

  -each nucleus contains matching sets of chromosomes= determined diploid.

  -each chromosome in a homologous pair has the same gene.
• crossing over

  -during prophase, homologous pairs of chromosomes form bivalents (pair closely)

  -chromatids twist around each other, point they join= chiasma

  -fragments of non-sister chromatids swap over, so genes are exchanged.
• meiosis1

  = 1st division= reduction division, when pairs of homologous chromosomes are separated into 2 cells.

  • each intermediate cell will only contain 1 full set of genes instead of 2. so cells are haploid.
• meiosis 2

  = 2nd division (similar to mitosis), the pairs of chromatids present in each daughter cell are separated, forming two more cells.

  • 4 haploid daughter cells produced.
• independent assortment 


  = maternal or paternal chromosomes can face either poles.
• prophase 1

  1 chromosomes condense, nuclear envelope disintergrates, the nucleolus disappears and spindle formation begins.

  2 homologous pairs form bivalents.

  3 chromosomes entangle during crossing over.
• metaphase 1

  1 homologous pairs line up along metaphase plate.

  • what chromosome is facing what pole is random. = independent assortment.
• anaphase 1


  1 homologous pairs are separated.

  2 sections of DNA broken off at chiasmata re-join to form recombiant chromatids.
• telophase 1


  1 chromosomes reach the poles, the nucleus reforms and the cell undergoes cytokinesis.
• prophase 2 


  -chromosomes condense again and the nuclear envelope breaks down.
• metaphase 2

  -individual chromosome line up along the metaphase plate.

  -again the chromosomes are independently assorted.
• anaphase 2


  -individual chromatids are pulled to opposite poles.
• telophase 2


  -the chromosomes reach the poles, the nucleus reforms and the cell undergoes cytokinesis

  -this produces 4 haploid daughter cells

  -they are therefore genetically different.