• Meiosis is a form of?nuclear division?that results in the?production of?haploid?cells?from diploid cells
• It produces?gametes?in plants and animals that are used in?sexual reproduction
• It has many similarities to mitosis however it has?two divisions: meiosis I and meiosis II
• Within each division there are the following stages: prophase, metaphase, anaphase and telophase

Prophase I

• DNA condenses?and becomes visible as chromosomes
• DNA replication has already occurred so each chromosome consists of?two sister?chromatids?joined together by a centromere
• The chromosomes are arranged side by side in?homologous pairs
  • A pair of homologous chromosomes is called a bivalent

   

• As the homologous chromosomes are very close together the?crossing over?of non-sister chromatids may occur. The point at which the crossing over occurs is called the?chiasma?(chiasmata; plural)
• In this stage centrioles migrate to opposite poles and the?spindle is formed
• The?nuclear envelope breaks down?and the nucleolus disintegrates

Metaphase I

• The?bivalents line up along the equator of the spindle, with the spindle fibres attached to the centromeres

Anaphase I

• The homologous pairs of chromosomes are separated as?microtubules pull whole chromosomes to opposite ends?of the spindle
• The centromeres do not divide

Telophase I

• The chromosomes arrive at opposite poles
• Spindle fibres start to break down
• Nuclear envelopes form around the two groups of chromosomes?and nucleoli reform
• Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I

Cytokinesis

• This is when the?division of the cytoplasm?occurs
• Cell organelles also get distributed between the two developing cells
• In animal cells: the?cell surface membrane?pinches inwards creating a?cleavage furrow?in the middle of the cell which contracts, dividing the cytoplasm in half
• In plant cells, vesicles from the Golgi apparatus gather along the equator of the spindle (the cell plate). The?vesicles?merge with each other to form the?new cell surface membrane?and also secrete a layer of calcium pectate which becomes the?middle lamella.?Layers of cellulose are laid upon the middle lamella to form the primary and secondary walls of the cell
• The end product of cytokinesis in meiosis I:?two haploid cells
  • These cells are haploid as they contain half the number of?centromeres

   

Second division of Meiosis : Meiosis II

• There is no interphase between meiosis I and meiosis II so the DNA is not replicated
• The second division of meiosis is almost identical to the stages of mitosis
• Prophase II
  • The?nuclear envelope breaks down?and?chromosomes condense
  • A?spindle forms at a right angle to the old one

   

• Metaphase II
  • Chromosomes?line up in a?single file along the equator?of the spindle

   

• Anaphase II
  • Centromeres divide and individual?chromatids are pulled to opposite poles
  • This creates?four groups of chromosomes?that have half the number of chromosomes compared to the original parent cell

   

• Telophase II
  • Nuclear membranes form?around each group of chromosomes

   

• Cytokinesis
  • Cytoplasm divides?as new cell surface membranes are formed?creating four haploid cells
    • The cells still contain the same number of centromeres as they did at the start of meiosis I but they now only have half the number of chromosomes (previously chromatids)

     

   

The different stages of Meiosis I in an animal cell

Prophase II, Metaphase II and Anaphase II in Meiosis II of an animal cell

Telophase II and cytokinesis in Meiosis II of an animal cell