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Lecture 14

GENERAL BIOLOGY II Lecture 14 & 15 Notes

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Lecture 14: Reproduction I I. Asexual and Sexual Reproduction A. Sexual Reproduction 1. The creation of an offspring a. Fusion of a male gamete (sperm) and female gamete (egg) to form a zygote i. Female gamete is large and non-motile cell ii. Male gamete is small and motile cell b. Zygote can give rise to gametes by meiosis B. Asexual Reproduction 1. Generation of new individuals without the fusion of egg and sperm a. Reproduction relies entirely on mitotic cell division (in most asexual animals) i. Invertebrate reproduce asexually by fission, separation or a parent into two or more individuals of about the same size (exactly similar to the parent) 2. Mechanisms ofAsexual Reproduction a. Budding i. New individuals arise from outgrowths of existing ones b. Fragmentation i. Breaking of the body into pieces some or all of which develop into adults ii. Must be accompanied by regeneration: regrowth of lost body parts c. Parthenogenesis i. Ahaploid or diploid egg cell develops into an embryo without fertilization (1) If haploid, the offspring develop into adults that produce eggs or sperm without meiosis C. Sexual Reproduction:An Evolutionary Enigma 1. Twofold cost of sexual reproduction a. Sexual females have half as many daughters as asexual females b. Despite this, almost all eukaryotic species reproduce sexually 2. Advantage a. Sexual reproduction results in genetic recombination i. Increase variation in offspring (increases reproductive success of parents in changing environments) ii. Increase in rate of adaptation iii. Shuffling of genes and the elimination of harmful genes from a population D. Variation in Patterns of Sexual Reproduction 1. Finding a partner, for many animals, can be difficult 2. Hermaphroditism a. Each individual has male and female reproductive systems b. Two hermaphrodites can mate and some can self-fertilize 3. Individuals of some species undergo sex reversals a. Male to female reversal vs. female to male reversal II. Fertilization Depends on Binding Mechanisms A. Mechanisms of fertilization 1. Union of egg and sperm play an important part in sexual reproduction B. Internal Fertilization 1. Sperm deposited in/near female reproductive tract (fertilization occurs within the tract) 2. External fertilization: eggs shed by the female are fertilized by sperm in the external environment III. Internal Fertilization: Human Male ReproductiveAnatomy A. Organs 1. External Organs a. Scrotum i. Contains the testes, epididymis and vas deferens b. Penis 2. Internal Organs a. Gonads (testes); produce sperm and hormones b. Accessory gland (seminal vesicles, prostate and bulbourethral glands) i. Essential for sperm movement B. Testes (male gonads) 1. Seminiferous tubes a. Highly coiled tubes surrounded by connective tissues 2. Leydig cells a. produce hormones i. Testosterone and other androgens b. Scattered between the tubules 3. Production of sperm cannot occur at the body temperatures of most mammals a. Testes are held outside the abdominal cavity in the scrotum i. Temperature is lower than in the abdominal cavity (~35⁰C) ii. Scrotum maintains testis temperature 2 C less than body temperature C. Ducts- carry the sperm and glandular secretions 1. From the seminiferous tubules of a testis, sperm pass into the coiled tubules of the epididymis a. Takes 3 weeks for the sperm to pass the duct b. During the passage i. Sperm complete maturation and become motile ii. They acquire the ability to fertilize an egg only upon expose to the chemical environment of the female reproductive system 2. Pathway of sperm during ejaculation are: a. propelled from the epididymis through the muscular vas deferens i. vas deferens joins a duct from the seminal vesicle, forming a short ejaculatory duct ii. Via accessory glands (1) Seminal vesicles, prostate and bulbourethral glands b. Ejaculatory duct open into the urethra c. Urethra runs through the penis and opens to the outside at the tip of the penis D. Accessory Glands 1. As sperm move toward ejaculation, semen is produced 2. Semen composed of sperm plus secretions from three sets of accessory glands (which is ejaculated) a. Two seminal vesicles contribute about 60% of the total volume i. Contains mucus, fructose (provides most of the sperm’s energy), coagulation enzyme, ascorbic acid and prostaglandins (oviduct motility and gamete transport in female) (promotes survival/success of sperm) ii. b. Prostate gland i. Secretes its products directly into the urethra through several small ducts ii. Contains anticoagulants and citrate (sperm nutrient iii. Is the source of most common medical problem in men over age of 40 c. Bulbourethral gland i. Secretes a clear mucus before ejaculation (1) Neutralizes acidic urine remaining in the urethra ii. Also carries some sperm released from ejaculation E. Penis 1. Human penis is composed of three cylinders of spongy erectile tissues 2. Arousal a. Erectile tissue fill with blood form the arteries, causing an erection i. Head of the penis has a thinner skin covering than the shaft and is more sensitive to stimulation ii. Nitric Oxide: erection pathway that causes vasodilation in smooth muscles of penile blood vessels—and blood flow iii. Viagra prolongs the Nitric Oxide pathway; inhibitor of PDE5 which breaks down the cGMP F. Pathway of Sperm in Males 1. Testicles 2. Epidymis 3. Vas deferens 4. Seminal Vesicles 5. Prostate Gland 6. Cowper’s Gland 7. Urethra G. Gametogenesis 1. Production of gametes a. Sperm i. Small and motile and must pass from male to female (1) The acrosome in the sperm head contains enzymes that help sperm penetrate the egg b. Eggs i. Large; oogonia develop into primary oocytes (1) Decline over time and degenerate to only about 50 prior to menopause ii. Mature in synchrony with the tissues that will support the embryo H. Spermatogenesis 1. Development of sperm is continuous and prolific (millions per day: each takes ~7weeks to develop) 2. Cell division and maturation occur throughout the seminiferous tubules coiled in the testes 3. Process of Spermatogenesis a. Primordial germ cells of the embryonic testes divide and differentiate into stem cells b. Stem cells divide mitotically to form spermatogonia c. Spermatogonia generates spermatocytes (primary spermatocyte) by mitosis d. Each spermatocyte gives rise to four spermatocytes (early spermatid) by meiotic (meiosis I) cell division of the secondary spermatocyte produced through meiosis II i. Reduces the chromosome number from diploid to haploid e. Spermatids undergo extensive changes in cell shape and organization i. Steroli cells provide nutrients f. As spermatogenesis proceeds, cells move steadily inward as they pass through the spermatocyte stage i. In the last step, mature sperm are releases into the lumen (fluid-filled cavity) of the tubule, where they become motile 4. Steroli cells a. Somatic cells of the testis b. Essential for testis formation and spermatogenesis i. Facilitate progression of germ cells to spermatozoa via direct contact ii. Control the environment milieu within the seminiferous tubules 5. Differs from oogenesis in 3 ways a. All four products of meiosis develop into sperm while one of the four becomes and egg b. Spermatogenesis occurs throughout adolescence and adulthood c. Sperm are produced continuously whereas there are prolonged interruptions during oogenesis IV. Internal Fertilization: Human Female ReproductiveAnatomy A. Internal Organs 1. Pair of gonads a. Ovaries and a system of ducts and chambers 2. Oviducts and Uterus a. Carry gametes and house the embryo and fetus B. Ovaries- lies in the abdominal cavity 1. Outer layer of each ovary is packed with follicles a. Each consisting of an oocyte i. Partially developed egg (primary oocyte which is arrested in prophase 1), surrounded by a group of support cells ii. Surrounding cells nourish and protect the oocyte during much of the formation and development of an egg (1) Granulosa cells-producing estrogen and progesterone b. Only about 500 follicles fully mature between puberty and menopause 2. During Menstrual Cycle a. One follicle matures and expels its egg (Ovulation) i. Prior to Ovulation (1) Cells of the follicle produce the primary female sex hormone (estradiol) ii. After Ovulation (1) The residual follicular tissue grows within the ovary, forming a mass called the corpus luteum (yellow body) (2) Corpus luteum secretes additional estradiol and progesterone (to help maintain the uterine lining during pregnancy) iii. If egg is not fertilized, the corpus luteum degenerates and a new follicle mature during the next cycle C. Oviducts and Uterus 1. Oviduct (fallopian tube) extends from the uterus toward each ovary a. At ovulation i. Egg is released into the abdominal cavity near the funnel-like opening of the oviduct ii. Egg cell travels from ovary to uterus via oviduct b.
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