I. Meiosis

II. Recombination

III. Gametogenesis

Terms

Notes Page 39, Fig. 2.22, under mitosis, anaphase is missing a light blue chromosome. Page 40, definition of recombination: Recombination of genes occurs either by independent assortment of chromosomes if the genes are on different chromosomes or by crossing over if they are on the same chromosome.

I. Meiosis is the form of cell division that underlies sexual reproduction. It occurs only in germ cells.

A. In asexual reproduction, one mother cell (or organism) gives rise to daughter cells that are genetically identical to the mother cell or organism. Asexual reproduction is based on mitosis.

B. In sexual reproduction, two parental cells (or organisms) give rise to daughter cells that receive a portion (usually half) of their genes from each parent by means of meiosis. Sexual reproduction generates new combinations of genes and alleles. Asexual reproduction, such as cloning, reproduces pre-existing combinations.

C. Meiosis involves one cycle of DNA replication but two cell divisions: meiosis I and meiosis II. [Refer to Fig. 2.20.]

1. In meiosis I (MI), homologous chromosomes pair (synapse) in prophase I and undergo crossing over. The location of crossovers can be seen microscopically and are called chiasmata (s. chiasma).

2. Following metaphase I, homologues are distributed to daughter cells (disjunction) without division of the centromere. One of each homologous pair of chromosomes goes into each daughter cell.

3. The assortment of chromosomes into daughter cells is random, i.e. the original parental combinations are not maintained.

4. In meiosis II (MII), the centromeres divide at the end of metaphase II and, in anaphase II, each daughter cell receives one of each chromosome to form a complete haploid complement.

A. Genes that are on different chromosomes recombine by independent assortment of chromosomes at anaphase I.

B. Genes that are on the same chromosome recombine by crossing over that occurs between them during prophase I.

A. Gonial cells are the designation given to germ cells before they enter meiosis. They replicate mitotically; hence they are genetically identical to the original zygote. When they begin meiosis, they are called spermatocytes and oöcytes.
1. In mammals, female gonial cells (oögonia) expand in number during embryonic development. At approximately 3 months gestation, all the oögonia become oöcytes, indicating the beginning of the meiotic cycle.

2. In mammalian males, the gonial cells expand in number during the prepubertal period. Beginning at puberty, the spermatogonia behave as stem cells, i.e. at the end of each mitosis, one product becomes a spermatocyte and the other remains a spermatogonial cell, capable of additional mitoses.

3. Mature haploid germ cells, either male or female, are called gametes.

B. Oögenesis (mammals)
1. Meiosis begins simultaneously in all primary oöcytes in late embryonic/early fetal period, proceeding to late prophase I.

2. All oöcytes then go into a resting phase until an ovarian follicle develops further.

3. At that point, meiosis resumes. The mature ovum that is released is a secondary oöcyte.

4. In telophase I, one of the meiotic products becomes a polar body, with very little cytoplasm; the other product receives virtually all the cytoplasm.

5. If fertilization occurs, MII (meiosis II) is completed, with formation of a second polar body.

6. Only one of the four haploid products is functional.

7. In female embryos, there are several million ovarian follicles; at birth, only 2 million; only 400 mature during lifetime.

C. Spermatogenesis

1. Meiosis in males is a continual process from puberty until death.

2. When spermatogonia reproduce, one daughter cell remains a stem cell and the other becomes a spermatocyte, ready to undergo meiosis.

3. At the completion of meiosis, there are four haploid spermatids, which resemble other cells in having cytoplasm and nucleus.

4. By the process of spermiogenesis, the spermatids change their physical structure to become mature spermatozoa (sperm). This involves getting rid of cytoplasm and developing an acrosomal body and tail. The time required is about 50 days from primary spermatocyte to spermatozoon.

5. All four haploid products become functional sperm.

6. Ca. 200-500 million sperm/ejaculate several times per week; ca. 2-5 trillion per lifetime.

D. Fertilization, the joining of a sperm and an ovum to form a diploid zygote, will be covered in a later lecture.

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