Lecture 10 - Mitosis and Meiosis

All living things pass on their genetic heritage by common processes
Mitosis and Meiosis: Lecture Outline 10:

1. The eukaryotic cell cycle.

Phases of cell cycle: Gap 1, DNA Synthesis, Gap 2 and Mitosis (G₁, S, G₂, and M)
    During G₁, cells grow with no DNA synthesis.
    During S, DNA is replicated.
    During G₂, cells continue to grow with no DNA synthesis.
    During M, cells divide.

2. Mitosis: Cell division

    Prophase: Chromosomes, duplicated during S, condense into sister chromatids at the centromere. Spindles of microtubules form outside the nucleus.
    Metaphase: Nuclear membrane vesiculates. The chromatids, attached to kinetochore microtubules from opposite poles, align between the poles.
    Anaphase: The chromatids are separated into the opposite poles by shortening of the kinetochore microtubules.
    Telophase: The separated sister chromatids are now chromosomes enclosed within the restored membrane.
    Cytokinesis: The parental cell divides into two identical daughter cells.

3. Meiosis: Reduction Division

Meiosis I:

Prophase: Chromosomes condense into sister chromatids as in mitosis.
Metaphase: Synapsis: homologous chromosomes align at metaphase plate.
                        Chiasmata: exchange between homologous chromatids.
Anaphase: Homologous chromosomes segregate into separate daughter cells.
Telophase: Nuclear membrane encloses nuclei containing only one of each of the homologous chromosomes (1N).
Cytokinesis: Formation of two 1 N cells with various combinations of maternal and paternal chromosomal homologues (for 23 pairs, 2²³ (over 8 million) possibilities!)
Meiosis II: A mitotic division of the 1N cells produced in Meiosis I.

4. Human Gametogenesis
A. Oogenesis

Arrested primary oocytes: Primordial germ cells migrate into embryonic gonads (oogonium). After limited multiplication, differentiate into primary oocytes blocked in prophase of meiosis I. Of 500,000 potential eggs, ca. 400 mature, one per month.

Maturation to secondary oocytes: Primary oocytes go through meiosis I and are blocked in metaphase of meiosis II as secondary oocytes in mature follicles.

Ovulation: Secondary oocytes released from ruptured, mature follicles.

Fertilization: The secondary oocyte is released from the metaphase arrest by fertilization.

B. Hormonal Control of the Ovarian Cycle

Hormones:

Hypothalamus: Gonadotropin Releasing Hormone (GnRH): Pituitary

Anterior Pituitary: Follicle Stimulating Hormone (FSH): Primary oocyte
                                  Luteinizing Hormone (LH):

Follicular: Estrogen: Uterus: Proliferative phase.
                        Secondary sex characteristics

Corpus luteum: Estrogen and Progesterone: Uterus: Secretory phase.

Feedback controls:

Estrogen:      Negative on Pituitary FSH secretion and Hypothalamus.
                        Positive on Pituitary LH secretion

Progesterone: Negative on Pituitary LH secretion and Hypothalamus

C. Spermatogenesis

Four haploid sperm are produced by meiosis in about 64 days.

D. Hormonal Control of Spermatogenesis

GnRH releases FSH, and LH.
FSH stimulates spermatogenesis in seminiferous tubules.
LH stimulates testicular release of Testosterone.
Testosterone: Secondary sex characteristics