Meiosis

Meisosis

Fertilization, halves number of chromosomes (2N—> 1N)

Pro: Diversity, greater likelihood of survival. Stability for population
Con: 50% of DNA is passed on, requires energy to find a mate

Present in all eukaryotes, but pathway differs in eukaryotic kingdoms

Diplontic

Animals

Products of meiosis are the gametes used for fertilization to create a zygote that divides into a multicellular organism (using mitosis)

Haplontic

Fungi

After fertilization, the zygote will divide by meiosis into four daughter cells (spores). Spores don’t fertilize, they divide by mitosis into a multicellular haploid organism that then produces gametes. The haploid cell produces two daughter cells that are haploid, creating two daughter gametic cells

Haplodiplontic

Plants

Zygote divides into a multicellular plant via mitosis. Within the plant, some cells will go through meiosis to create spores that divide into a multicellular plant via mitosis. The plant (haploid) will then form gametes via mitosis.

Animal meiosis ploidy

Start with one diploid cell with one nucleus (double copied chromosomes, two copies of each homologous pair).

End with 4 haploid cells, each with one copy of one of the homologues.

Prophase I

DNA condenses from chromatin into chromosomes. Homologous pairs align, 4 chromatids become a tetrad and crossing over occurs (equal genetic exchange between non-sister chromatids). 
Nuclear envelope breaks down into fragments and spindle fibers move towards opposite poles, microtubules begin building up, growing the spindle fibers. 

Metaphase I

Pairs of homologues are pushed until they line up at the equator. Each homologous pair gets ONE spindle fiber (1 fiber per two chromosomes)

Anaphase I

Homologous pairs break and are pulled into opposite poles, daughter cells are still diploid

Telophase I

Chromosomes unpack into chromatin (in females), and generally stay as chromosomes in males. Nuclear envelope will reform in chromatin is present

Prophase II

No S phase occurs, chromatin goes back into chromosomes, nuclear envelope breaks down. Centrosomes replicate

Metaphase II

Chromosomes line up in the metaplate, two spindle fibers attach to each pair (one per sister chromatid, two per chromosome)

Anaphase II

Sister chromatids separate, haploid cells temporarily becomes diploid

Telophase II

Nuclear membrane reforms, cytokinesis occurs shortly after, producing 4 haploid daughter cells

After meiosis

Further cell maturation occurs:

• Spores: increase cell wall for protection

• Eggs: All cytoplasm ends up in 1 egg, with 3 extra useless polar bodies that will be discarded during the menstrual cycle

• Sperm: decrease in cytoplasm and organelles (except for mitochondria) and grow a flagellum