1. To learn the cell cycle as it relates to mitosis and meiosis.
2. To update information about gametogenesis.
3. To predict the usefulness of this information in genetics.
: The Cell Cycle
, or cell duplication, is only a short segment of the cell cycle. During the interphase or time between, duplications, the cell synthesizes materials, undergoes growth and carries on its catabolic functions. The entire cycle, in a mouse cell for instance, would take 18 to 24 hours. All but one hour, which corresponds to the actual mitotic cycle, would correspond to interphase. The DNA is duplicated before the onset of mitOsis so the daughter cell will have the same genetic information as the mother cell.
(figure available in print form)
Appearance of the chromatin material during mitosis
1. Interphase-material evenly dispersed.
2. Prophase—coiling to give thread like appearance. The thread is stranded, therefore the duplication of the DNA has already taken place (2 strands are called chromatids).
3. Metaphase—nuclear membrane breaks down, spindle of microtubules forms, chromatids are held together at centromeres.
4. Anaphase—centromere divides, homologues (chromatids) separate, centromere first, to opposite poles of spindle.
5. Telophase—new membrane forms around each set of daughter chromosomes.
Stages of meiosis-First division
is the division of gametes or germ cells. A second division takes place to reduce the number of chromosomes to the haploid number where only one of each kind of chromosome ends up in the “daughter” cells. The
I of meiosis is further broken down into leptotence, zygotene, pachytene, diplotene and diakinesis.
1. Leptotene—threadlike structure—2 of each (homologues)
2. Zygotene—homologues begin to pair (zipperlike, point for point.)
—complete pairing, thicker, tightly coiled, called bivalent.
—bivalent opens lengthwise, becomes four chromatids;
centromeres are not split
or points of contact exist along the chain, (possible sites of
—shortening at height, appears tightly coiled.
same as mitosis, 2 centromeres for each tetrad, independently arranged (not as pairs of homologues). Crossings over have taken place.
—undivided centromeres and sister chromatids move to opposite sides.
Meiosis II. second division
as in mitosis.
—centromere finally divides—and 4 cells have been produced each with one half the original number of chromosomes.
1. The sex cells in the human female are originated during fetal life. At the time of birth, the normal female has all the primary cocytes present. Of these only 3 to 4 hundred will be released throughout her life between menarche and menopause. They are at a resting stage in meiosis and must undergo the second meiotic division as long as 40 years after their first division.
2. Most of the cytoplasm in gametes is in the ovum about 20 or 50 times as much as in the sperm.
3. Male gametes or germ cells, (sperm) production, begins at puberty and is more subject to insult from external stimuli.
Relate the information above to the sheet illustrations of phases of meiosis in an organism from Levine,
, Holt, Rinehart and Winston, 1968, pages 36-37.