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Case 1: Should we worry?,1. What is Gametogenesis?
Gametogenesis: is the process that converts primordial germ cells to mature
male(spermatozoa) and female gametes(oocytes). Gametogenesis consists of 4
phases:
1. The migration of germ cells (identical in males and females)
2. The increase of primordial germ cells via mitosis
3. The reduction of chromosomes number via meiosis
4. The maturation of the eggs and spermatozoa
1. Migration of germ cells:
Primordial germ cells (PGC) are the precursors of the gametes, they exit from
the yolk sac into the hindgut and migrate to the primordia of the gonads.
2. Increase of primordial germ cells via mitosis:
When PGC reach the gonads they start a phase of rapid mitotic
proliferation.
Oogonia go through a period of intense mitotic activity in the ovary from
the 2nd to the 5th month of pregnancy. (from few thousand to 7 million)
For spermatogonia mitosis starts early in the embryonic testes. Male
germ cells maintain the ability to divide throughout postnatal life.
Mitosis: is the process whereby one cell gives rise to 2 identical cells, each with a
complete complement of 46 chromosomes.
, Prophase :
- Chromosomes consist of 2 chromatids, the condensation of chromosomes
continues.
Metaphase:
- The chromosomes line up in the equatorial plane attached by microtubules.
Anaphase:
- The chromatids migrate to opposite poles of the mitotic spindle.
Telophase:
- Uncoiling of the chromosomes
- Nuclear envelope reformation
- Cytoplasm division
3. Reduction of chromosomes number:
The biological significance of meiosis is:
- the reduction in chromosomal number from diploid(2n) to haploid(n)
- independent reassortment of maternal and paternal chromosomes.
- the redistribution of maternal and paternal genetic information via crossing over
Meiosis: involves 2 meiotic cell divisions, it happens only in germ cells and gives rise
to haploid gametes.
, Meiosis I: called the reductional division
Prophase I: leads to the pairing of the homologous chromosomes and crossings-over which
take place at sites called hot spots. The location of hot spots is based on the configuration of
certain proteins that organize the chromosomes early in the meiosis.
Cohesin: a protein that helps to hold sister chromatids together during division.
Condensin: a protein involved in the compaction of the chromosomes.
Metaphase I: the chromosome pairs line up at the equatorial plate.
Anaphase I: each chromosome of the pair moves toward one pole of the spindle.
Telophase I: the 2 obtained daughter cells are genetically unequal and contain the haploid
(1n) number of chromosomes but still consisting of 2 chromatids(2c).
Meiosis II: called the equational division and is similar to a mitotic division except that for
the division the cells are haploid.
Metaphase II: the centromeres between sister chromatids divide.
Anaphase II: each chromatid migrates to a pole of the spindle.
Telophase II: each daughter cell is haploid (1n,1c)
Meiosis in females:
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