17 reproduction 118 - Biology 118 17. REPRODUCTION AND...

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Biology 118 17. REPRODUCTION AND DEVELOPMENT Why reproduce? Individuals are generally finite, so new individuals must be made from from existing ones. SEXUAL REPRODUCTION : Fusion of two haploid sex cells, gametes. Sperm is small and motile, the ovum (egg) is large and sessile. The resulting zygote represents a combination of genes from both gametes. GAMETOGENESIS : the production of haploid gametes, in the female and the male. In the human female : oogenesis Ovaries – each about an inch long, bumpy. Bumps are follicles; each consists of a developing egg cell (primary oocyte) surrounded by follicle cells. Every 28 days or so, ovulation occurs – when an egg cell (secondary oocyte) is released from its follicle. After ovulation, the leftover follicle cells make up the corpus luteum. Secretes estrogen and progesterone (helps maintain uterine lining during pregnancy). If secondary oocyte goes unfertilized, corpus luteum degenerates, making way for the next follicle to mature and release a new oocyte. Oogenesis the ovaries contain about 2 million follicles at birth. Oogenesis starts even prior to birth. At birth, each follicle contains a diploid, dormant primary oocyte. It starts meiosis I, but it gets arrested before it can finish. At puberty, FSH stimulates one follicle to develop – follicle enlarges, primary oocyte grows. As LH levels rise, the oocyte divides. (i.e., the LH releases the primary oocyte from its arrested state, and so it completes meiosis I). One daugher cell gets most of the cytoplasm – this is the secondary oocyte. The other daughter cell is the first polar body (polar body formation allows for the generation of haploid cells with the maximum amount of cytoplasm). This secondary oocyte will begin meiosis II, but will become arrested before it can finish. During ovulation (stimulated by LH), the secondary oocyte is released into the oviduct. If fertilized, this oocyte undergoes meiosis II – giving rise to the ovum and the second polar body (haploid). The zygote forms when the haploid nucleii of the sperm and ovum unite. A follicle that expels its secondary oocyte becomes a corpus luteum – this will degenerate if the ovum is not fertilized. Uterus is the womb – this is where the embryo implants and develops into a fetus. The uteral lining, the endometrium, is densely supplied with blood vessels and contains uterine glands. Endocrine control of the ovary and uterus Menstrual bleeding occurs for 3-5 days; discharge includes blood and endometrium (the ‘functional zone’). At the same time, a primary oocyte starts to develop back in the ovary.
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17 reproduction 118 - Biology 118 17. REPRODUCTION AND...

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