Meiosis

Meiosis = The resulting daughter cells have half the original number of chromosomes. They are haploid and can be used for SEXUAL REPRODUCTION. 

In sexual reproduction, the offspring are genetically different from each other and from the parents. 

Gametes = Special sex cells. In many organisms, the gametes are HAPLOID and are produced by MEIOSIS. 

Gametes fuse together at fertilisation to produce a zygote. 

Zygote = Cell formed, during sexual reproduction, from the fusion of two gametes. 

When 2 gametes fuse to make 1 cell, the chromosomes are combined into 1 nucleus. Therefore the chromosome number in each gamete needs to be haploid. This ensures after fertilisation that the original chromosome number is restored.

Haploid = Eukaryotic cell or organism having only 1 set of chromosomes. 

PROPHASE 1:

• Chromatin condenses and supercoils. 
• Chromosomes shorten and thicken. 
• They can take up stains and be seen with a light microscope. 
• Chromosomes come together in their homologous pairs to form a bivalent. 
• Each member of the pair has the same genes at the same loci. 
• Each pair consists of 1 maternal and 1 paternal chromosome. 
• Non-sister chromatids wrap around each other and attach at points called chiasmata.
• They may swap sections of chromatids with one another in crossing over. 
• Nucleolus disappears and nuclear envelope disintegrates. 
• Spindle forms made of protein microtubules.

Bivalent = Pair of joined homologous chromosomes during prophase 1 and metaphase of meiosis 1.

Chiasmata = Points where non-sister chromatids within a bivalent join, where they cross over. 

Crossing over = Non-sister chromatids exchange alleles in prophase 1 of meiosis 1. 

METAPHASE 1:

• Bivalents line up across the equator of the spindle attached to spindle fibres at the centromeres. Chiasmata are still present. 
• Bivalents are randomly arranged with each member of a homologous pair facing opposite poles.
• This allows the chromosomes to independently segregate when they are pulled apart in anaphase 1. 

ANAPHASE 1:

• Homologous chromosomes in each bivalent are pulled apart by spindle fibres to opposite poles. 
• Centromeres do not divide.
• Chiasmata separate and lengths of chromatid that have been crossed over remain with the chromatid to which they have become newly attached. 

TELOPHASE 1:

• In most animal cells, 2 new nuclear envelopes form. 
• One forms around each set of chromosomes at each pole and the cell divides by cytokinesis.
• There is a brief interphase and the chromosomes uncoil.
• In most plant cells the cell goes straight from anaphase 1 into meiosis 2.

MEIOSIS 2:

PROPHASE 2:

• If a nuclear envelope has reformed, it breaks down again.
• The nucleolus disappears, chromosomes condense and spindles form. 

METAPHASE 2:

• Chromosomes arrange themselves on the equator of the spindle. They are attached to the spindle fibres at the centromeres.
• Chromatids of each chromosome are randomly assorted.

ANAPHASE 2: 

• Centromeres divide and chromatids are pulled apart to opposite poles by the spindle fibres.
• The chromatids randomly segregate.

TELOPHASE 2:

• Nuclear envelopes reform around the haploid daughter nuclei. 
• In animals, the 2 cells now divide to give 4 haploid cells.
• In plants, a tetrad of 4 haploid cells is formed.