Meiosis+and+Gametogenesis

=**Objectives**=


 * 1. Recognize the differences between mitosis and meiosis.** (p. 31-32, 36)


 * Meiosis**||**Mitosis**||
 * In germ (sex cells)||In somatic (body) cells.||
 * Two nuclear and cellular divisions form 4 daughter cells||Single nuclear and cellular division, forms 2 daughter cells||
 * Haploid nucleus||Diploid nucleus||
 * Prolonged prophase||Normal prophase||
 * Homologous chromosomes pair up||No pairing of homologous chromosomes||
 * Crossing over, chiasmata formation allows genetic exchange||None||
 * Two nuclear divisions with one division of centromere||Centromere divides in each division||
 * No specific interphase before second meiotic division||Interphase always occurs in between mitosis||
 * No DNA synthesis between first and second meiotic division||DNA synthesis always precedes mitosis||


 * 2. Recognize different stages of prophase I of meiotic division.** (p. 33, 35)

Leptotene -- ling thin chromosomes, no pairing, no synaptonemal complex (thin & delicate) Zygotene -- Axial/lateral elements of synaptonemal complex form, homologous pairing is initiated, recombination initiated (conjugation) Pachytene -- Fully formed synaptonemal complex, homologous paring, chromosomes shorter (thick) Diplotene -- Synaptonemal complex disappers, homologs repel, and chiasmatia is visible (double) Diakinesis -- End of prophase, start of metaphase

Human female primary oocytes are arrested at diplotene stage of prophase I at birth.


 * 3. Recognize the concept of mitotic and meiotic division during spermatogenesis.** (p. 40)

Mitosis allows for the duplication of genetic material, producing daughter cells that both have all 46 chromosomes. Meiosis allows for the generation of cells with only 23 chromosomes, half the normal amount so that two gamates can combined and restore the 46 total chromosome number and allow for genetic variation.


 * 4. Recognze the male organ where meiosis occurs.** (p. 38-39)

Meiosis occurs in the semineferous tubules of the testis. Spermatogonia reside outside of the blood/testis barrier on the basal compartment of the semineferous tubule. Primary spermatocytes are produced mitotically from spermatogonia and enter through the tight junction of the blood/testies barrier which temporarily loosens up. Meiosis procedes with more mature sperm moving towards the lumen of the semineferous tubules.


 * 5. List types of nuclear division in spermatogonia, spermatocytes, and spermatids.** (p. 40)

Spermatogonia undergo mitotic division to produce primary spermatocytes. Primary spermatocytes undergo the first meiotic division to produce secondary spermatocytes. Secondary spermatocytes undergo the second meiotic division to produce spermatids. Spermatids do not undergo further divison and differentiate into mature spermatozoa.


 * 6. List the functions of Sertoli cells and Leydig cells in the process of meiosis.** (p. 41, 42)

__Sertoli Cells__ (1) Froms the blood/testis barrier -- tight junctions separating the seminiferous tubule from the intertubule space to protect spermatocytes. When a spermatogonia divides to produce a primary spermatocyte, the tight junctions loosen up to allow the primary spermatocyte to enter. (2) Provides the overall archetecture of the seminiferous epithelium (3) Secretes testicular fluid including androgen binding protein -- this protein helps concentrate androgens secreated by Leydig cells to spermatogonia to help maintain androgen effects. (4) Release of mature sperm into the seminiferous lumen. (5) Support germ cell development

__Leydig Cells__ (1) Located outside the seminiferous tubules inthe intertubular compartment (2) Produce testosterone in response to LH -- hormonal stimulation is critical for normal sperm production


 * 7. Describe two phases of female meiosis.** (p. 45)

__Embryonic Phase__
 * Extragonadal origin of primordial germ cell migration and incorporation in gonand
 * Beginning of oogenesis
 * Arrest at primary oocyte stage in diplotene of prophase I

__Postnatal Phase__
 * Primary oocytes at quiescence
 * Follicular development
 * Completion of first meiotic division, 23 chromosomes with 46 chromatids
 * Arrest at metaphase II
 * First polar body forms
 * Ovulation after puberty


 * 8. Recognize the female organ where meiosis occurs.** (p. 47)

Meiosis begins in the embryonic stage but is halted at diplotene phase of prophase I until puberty when oocytes in the ovary completes the first meiotic division and forms the first polar body before being arrested at metaphase II. Secondary Oocyte ovulates and does not continue meiosis unless it is fertilized by sperm midway along the fallopian tube. If the oocyte is fertilized by spermatozoa, it completes the second meiotic division and produces the second polar body.

Oocytes degenerate by a process called artesia; women produce only a finite amount of eggs, many of which will never ovulate, degenerating by artesia instead.


 * 9. List the types of nuclear division in oogonia, oocytes, and zygote.** (p. 50)

Oogonia undergo mitosis during the 8-30th week of intrauterine life. By the 3rd month of intrauterine life, oogonia being meosis I to produce the primary oocyte which is arrested in diplotene of prophase I until puberty. At puberty, LH stimulation activates the primary oocytes and allows them to complete the first meiotic division and form a secondary oocyte and the first polar body. The secondary oocyte is arrested at metaphase II unless it is fertilized by a spermatozoa. If fertilized, the second meiotic division completes, the second polar body is formed, and the egg becomes a zygote.


 * 10. Recognize the events during ovulation and fertilization.** (p. 50, 51)

When the secondary occyte becomes feritlizedby a spermatozoa, it completes the second meiotic division and forms the second polar body. As soon as the sperm attaches to the ovum surface, the cortical granules release their enzymes into the perivitelline space in the cortical granule reaction. This reaction causes the zona reaction -- the zona pellucida hardens to prevent penetration by other spermatozoa.

The window of fertilization is only about 24 hours. After this window, the egg becomes senescent and the cortical granules move centrally. This prevents an effective cortical granule reaction and allows multiple sperm to penetrate the zona pellucida. The condition of polyspermy is lethal.

4-6 hours after fertilization, two sets of haploid chromosomes become surrounded into two membranes and form pronuclei. Each pronucleus moves to a more central location and synthesize DNA. At 12-21 hours after fertilization, the pronuclei membranes break down and chromosomes condense. A metaphase spindle forms and the chromosomes line up on the metaphase plate. Anaphase and telophase occur and the cleavage furrow forms, making the two-cell embryo, each cell having a full set of 46 chromosomes.