Meiosis

1) The chromatin condenses and undergoes supercoiling so that chromosomes shorten and thicken. They can take up stains and can be seen under a microscope.

2) The chromosomes come together in their homolgous pairs to form a bivalent. Each member of the pair has the same genes at the same loci. Each pair consists of one maternal and one paternal chromosome. 

3) The non-sister chomatids wrap around each other and attach at points called chiasmata.

4) They may swap sections of chromatids with one another in a process called crossing over.

5) The nucleolus disappears and the nuclear envelope disintegrates.

6) A spindle forms. It is made of protein microtubules.

7) Prophase 1 may last for days, months or even years, depending on the species and type of gamete being formed.

1) Bivalents line up across the equator of the spindle, attached to spindle fibres at the centromeres. The chiasmata are still present.

2) The bivalents are arranged randomly (random assortment) with each member of a homologous pair facing opposite poles.

3) This allows the chromosomes to independently segregate when they are pulled apart in anaphase 1.

1) The homologous chromosomes in each bivalent are pulled by the spindle fibres to opposite poles.

2) The centromeres do not divide.

3) The chiasmata separate and lengths of chromatid that have been crossed over remain with the chromatid to which they have become newly attached.

1) In most animal cells two new nuclear envelopes form- one around each set of chromosomes at each pole- and the cell divides by cytokinesis. There is a brief interphase and the chromosomes uncoil.

2) In most plant cells the cell goes straight from anaphase 1 into meiosis 2.

1) If a nuclear envelope has reformed, it breaks down again.

2) The nucleolus disappears, chromosomes condense and spindles form.

1) The chromosomes arrange themselves on the equator of the spindle, turned at a 90 degree angle compared to metaphase 1.

2) They are attached to the spindle fibres at the centromeres.

3) The chromatids of each chromosome are randomly assorted. 

1) The centromeres divide and the chromatids are pulled to opposite poles by the spindle fibres. 

2) The chromatids randomly segregate. 

1) Nuclear envelopes reform around the haploid daughter nuclei.

2) In animals, the two cells now divide to give four haploid cells.

3) In plants, a tetrad of four haploid cells is formed.