Meiosis - the production of haploid cells with unpaired chromosomes - word means "to diminish".
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Key points of Meiosis
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Meiosis occurs in two stages - Meiosis I and Meiosis II
The stages are similar to mitosis.
**NOTE: PLANT CELLS DO NOT HAVE CENTRIOLES
View the following animations:
Meiosis
Animation I
Meiosis Animation
II
Meiosis
Animation III
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:
Meiosis is not flawless
It is estimated that from 10-20% of all human fertilized eggs contain chromosome abnormalities, and these are the most common cause of pregnancy failure (35% of the cases).
These chromosome abnormalities- arise from errors in meiosis, usally meiosis I;
- occur more often (90%) during egg formation than during sperm formation;
- become more frequent as a woman ages.
- Aneuploidy;
the gain or loss of whole chromosomes is the most common chromosome
abnormality. It is caused by nondisjunction, the failure of chromosomes
to correctly separate:
- homologues during meiosis I or
- sister chromatids during meiosis II
- Zygotes missing one chromosome ("monsomy") cannot develop to birth (except for females with a single X chromosome).
- Three of the same chromosome ("trisomy") is also lethal except for chromosomes 13, 18, and 21 (trisomy 21 is the cause of Down syndrome).
- Three or more X chromosomes are viable because all but one of them are inactivated.