Meiosis - the production of haploid cells with unpaired chromosomes - word means "to diminish".

Key points of Meiosis

• The process results in 4 daughter cells
• Daughter cells are haploid
• Daughter cells have unique combinations of chromosomes
• Daughter cells do not have homologous pairs
• Meiosis creates gametes (sperm and eggs)
• Meiosis ensures variability in offspring

Meiosis occurs in two stages - Meiosis I and Meiosis II

The stages are similar to mitosis.

Prophase I - homologous pairs join together - Crossing over can occur Pairs are called a TETRAD Metaphase I - homologous pairs line up along the equator Spindle Completely formed, fibers arrange tetrads Anaphase I - homologous pairs separate   Telophase & Cytokinesis - two daughter cells are formed, these daughter cells are haploid and do not contain homologous pairs   Prophase II - cell prepares for second division   Metaphase II - chromosomes line up along the equator   Anaphase II - chromatids separate   Telophase II & Cytokinesis - two daughter cells are formed from each parent cell - for a total of FOUR haploid cells   View the following animations: Meiosis Animation I Meiosis Animation II Meiosis Animation III Meiosis Animation IV

Independent Assortment and Crossing-Over

Crossing-Over occurs during prophase I - two homologous chromosomes switch peices of each other. This adds to the variability of the gametes formed.

Independent Assortment

Depending on how the chromosomes arrange themselved during metaphase I, the cells formed can have a variety of different chromosome combinations.

Life Cycles

Haploid Life Cycle (protists, algae, fungi)

 

Diploid Life Cycle (animals)

Alternation of Generations (plants)

the organism can exist in both haploid and diploid states; the zygote grows into a diploid sporophyte in which some cells undergo meiosis to produce spores; the spores germinate and grow into a haploid gametophyte that produces male and/or female gametes; the gametes fuse to produce a new zygote

In Ulva lactuca (Sea lettuce) the haploid and diploid generations are well developed and look identical until they produce gametes or spores, respectively: