Meiosis is a special kind of cell division that produces haploid sex cells
(gametes) after two nuclear divisions.
The diploid chromosome number is restored in fertilization when male and
female cells unite.
Chromosome numbers:
- A haploid (N) set of chromosomes has one chromosome of each type.
- Two haploid sets make a diploid (2N) number.
- In diploid cells, all chromosomes exist in homologous pairs.
- Familiar animals and plants all have diploid cells, except for gametes.
Meiosis is a special type of cell division in which diploid cells divide twice
to produce haploid sex cells (gametes). Stages of meiosis include:
- Prophase I:
- Chromosomes first become visible to light microscopy (leptotene)
- Homologous chromosomes pair up (zygotene or synapsis)
- Chromosomes thicken (pachytene); crossing-over may occur
- Individual chromatids become visible to light microscopy (diplotene),
with chromosomes arranged in tetrads (each with 2 homologous chromosomes
or 4 chromatids)
- Chromosomes of each homologous pair begin to separate a bit (diakinesis),
with an X-shaped chiasma marking the location of each crossover.
- Prometaphase: spindle forms and enlarges; centrioles move to opposite
poles; nuclear envelope dissolves
- Metaphase I: paired chromosomes line up on metaphase plate
- Anaphase I: homologous chromosomes separate and move toward poles
- Telophase I: homologous chromosomes reach opposite poles but remain
condensed (short and thick)
- Interkinesis (may not occur): cytoplasm divides in half
- Prophase II: new spindles form, perpendicular to that of division I
- Metaphase II: chromosomes line up on new metaphase plate
- Anaphase II: centromeres divide; sister chromatids separate as in mitosis,
forming separate chromosomes
- Telophase II: new chromosomes reach opposite poles and begin to lengthen
and thin out; nuclear envelope reappears
- Cytokinesis: cytoplasm divides, resulting in 4 haploid cells
Spermatogenesis (meiosis in males): cytoplasmic divisions are equal,
resulting in 4 equal cells.
- Primary spermatocytes (diploid) undergo first division of meiosis,
producing haploid secondary spermatocytes.
- Secondary spermatocytes then undergo second division of meiosis;
4 haploid spermatids result.
- Each spermatid undergoes maturation (growth of tail, loss of most
cytoplasm) to become a mature sperm cell (spermatozoon).
Oogenesis (meiosis in females): cytoplasmic divisions are very unequal,
resulting in one ovum only and several smaller cells (polar bodies).
- In the first division, the primary oocyte (2N=diploid) divides into
a secondary oocyte (N=haploid) and a first polar body (N)
- The secondary oocyte then divides into a second polar body (N)
and a mature ovum (egg cell) (N).
Fertilization: A small haploid sperm penetrates the membrane surrounding
the much larger egg. Sperm tail remains outside; only the sperm head (with nucleus)
enters.
- Primary (fast) block: A wave of negative electric polarity
spreads across surface of egg from point of sperm entry, reducing (but not
eliminating) probability of a second sperm entering (polyspermy).
- Secondary (slow) block: Vesicles pop open, separating membrane
from egg cytoplasm, reducing probability of polyspermy to zero.
- Haploid sperm pronucleus then approaches haploid egg pronucleus;
they fuse to form diploid nucleus of fertilized egg (zygote), which
immediately begins mitosis (first cleavage).
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