- - AGRICULTURAL CORE CURRICULUM - - (CLF200) Core area: ANIMAL SCIENCE (CLF250) Unit title: GENETICS AND BREEDING ____________________________________________________________________________ (CLF251) Topic: FERTILIZATION time taught in years 3 hours 1/2 ____________________________________________________________________________ Topic objectives: Upon completion of this lesson the student will be able to: Learning outcome # (D-1) - Describe the process of fertilization in mammals. (D-2) - Discuss the transmission of genes through meiotic division to form sperm and ova. (D-4) - Define the terms phenotype, genotype, haploid, diploid, homozygous, heterozygous, segregation, and homologous and use them correctly in discussing gametogenesis and the genetic makeup of an animal. (B-2) - Review the steps of meiosis and mitosis. Special Materials and Equipment: Prepared slides and pictures of mammalian sperm and ovum; live sperm for inspection under the microscope; microscopes; and Supplemental Handout #1. References: See unit directory Evaluation: Quiz by instructor TOPIC PRESENTATION: FERTILIZATION (See also CLF222, "Conception/Gestation" and CLF243, "Reproductive Systems.") I. Gametogenesis: Formation of sperm and ova. (Gamete = the male or or female reproductive cell . . . sperm and egg; genesis = the beginning.) A. Review meiosis and mitosis. B. SPERMATOGENESIS 1. The process by which male sex cells are produced. a. This process occurs continuously once the animal reaches puberty. b. Once a cell begins spermatogenesis it takes approximately 60 days to turn into a mature sperm cell ready to fertilize an egg. 2. Sperm are formed within the seminiferous tubules from cells called SPERMATOGONIA. a. The process is complex and involves cell division and differentiation. b. The final cell has half the number of chromosomes of a normal "somatic" or body cell, making it a sex or "germ" cell. 3. Steps in spermatogenesis: ____________________________________________________ ACTIVITY: 1. Go over Supplemental Handout #1. "Diagram of Spermatogenesis" as you discuss the following steps. ____________________________________________________ Step 1: Mitotic division of spermatogonia. Two mitotic divisions to yield four cells called SPERMATOCYTES. Step 2: Spermatocytes go through meiosis I. (The number of chromosomes is now halved.) Step 3: Spermatocytes go through meiosis II to become SPERMATIDS. Step 4: There is a metamorphosis (change) of the spermatids as they become SPERMATOZOA (singular - spermatozoon). Step 5: The spermatozoas mature as they pass through the epididymis. ___________________________________________________ ACTIVITY: 1. Set up a microscope and have a slide of live sperm available for viewing. Have them diagram a sperm cell as seen through the microscope. 2. Provide the students with microscopic photos of sperm cells of other types of mammals to emphasize variations and similarities between species. ____________________________________________________ C. OOGENESIS 1. This is the process by which the female egg is produced. 2. In the female, all eggs are present in the ovaries by the time the animal is born. a. The egg at this point is called an OOCYTE and it is in a resting stage after completing Meiosis I. 3. Once a female reaches puberty, a hormone causes an oocyte to begin meiosis II. The oocyte is ovulated while in the metaphase of meiosis II. a. A new oocyte begins meiosis II each time the animal goes through a heat cycle. 4. The sperm fertilizing the oocyte causes the completion of meiosis II. _____________________________________________________ ACTIVITY: 1. Have students view slides of an egg. 2. Have photos of an ovary to show the developing egg. _____________________________________________________ II. FERTILIZATION A. Fertilization is the fusion of male and female gametes to form a a single cell, the ZYGOTE. B. Female gamete ("egg"): The ovum - 1. The ova is in metaphase of meiosis II at the time of ovulation. 2. The ova is surrounded by the ZONA PELLUCIDA which is a protective covering around the ova. 3. The ova is usually fertile for about 25 hours. 4. The ova travels to the upper third of the oviduct where it is fertilized, if sperm are present. C. Male gamete: The sperm - 1. Millions of sperm are found in each ejaculate, but only a few thousand will reach the site of fertilization. 2. Only one sperm can penetrate and fertilize the ova. 3. Sperm can reach the site of fertilization as quickly as within 15 minutes of ejaculation. 4. In some species, spermatozoa undergo changes in the female tract before fertilization. a. It is called "sperm capacitation", this activates the sperm so it can fertilize the egg. This occurs in rabbits, mice, and sheep. 5. The fertile life of a sperm is 2-3 days. D. The Fertilization Process 1. Entry of sperm into ovum. a. Sperm must penetrate the ZONA PELLUCIDA of the ovum. This causes the completion of meiosis II in the ovum. b. The sperm head attaches to the membrane that surrounds the ova and the genetic material from the sperm is taken into the the cell (which already is holding the female's contribution of genetic material). 2. Pronuclei formation. a. Male and female "pronuclei" are seen as separate entities before uniting to form a single nucleus. (Again we have a cell with pairs of chromosomes ---> "2n".) b. Once the pronuclei have united, the process of fertilization is complete. The zygote nows begins to undergo mitotic divisions to develop into the embryo. E. The outcome of fertilization, based on chromosome count. male gamete (1n) -------------------> + = (2n) ------> (2n) zygote adult female gamete (1n) -----------------> III.Segregation A. Segregation is the separation, during gametogenesis of the pairs of of chromosomes. If there are different "instructions" on each of the two homologous (similar in structure, controlling the same traits) one of set of instructions goes to one gamete and the other set carried on the other chromosome goes to the other gamete. 1. The diagram below shows briefly how the chromosome pairs are "segregated into specific sex or germ cells. 2. The segregation or separation begins during the reduction division shown below, and is completed when each germ cell has been produced and is in a haploid (1n) state. IV. A diagramatic summary of the entire process: testicle cell ovary cell _______ _______ / \ / \ | \ \ | | \ \ | | / / | <------ Chromosome pairs ------> | / / | | \ \ | (2n) | \ \ | | / / | | / / | \_______/ \_______/ | preparatory steps for | | reduction division | | | \|/ \|/ ___________________ ___________________ / \ / \ | \ \ | chromosomes begin | \ \ | | / <--- ---> / | to separate | / <--- ---> / | | \ \ | (segregation begins) | \ \ | | / / | | / / | \___________________/ \___________________/ | | | | --|-------------| --------- cell division -------|-------------|---------- | | | | \|/ \|/ \|/ \|/ _____ _____ _____ _____ / \ / \ The gametes are now / \ / \ | \ | | \ | formed (1n) and | \ | | \ | | / | | / | segregation is complete | / | | / | | \ | | \ | | \ | | \ | | / | | / | | / | | / | \_____/ \_____/ \_____/ \_____/ \ / \ / \ / ------------\--------------- conception -------------/------------------- \ / \ / \ / _______ / \ | \ \ | | / / | | \ \ | | / / | \_______/ ZYGOTE Supplemental Handout #1: EXAMPLE OF SPERMATOGENESIS Diagram of spermatogenesis in the ram and bull (differs in other species). (This is diagramed at the cell level, the chromosome number 1n/2n is noted.) ============================================================================ O Step 1: one SPERMATOGONIS (2n - full chromosome count - THERE ARE TWO OF EACH - paired) -------------------------------------------------------------------------- | | Step 2: goes through mitosis (still | | pairs of each chromosome - 2n) | | to form spermatocytes O O | | | | | | | | <---------- SPERMATOCYTES | | | | O O 0 O -------------------------------------------------------------------------- | | | Step 3: the SPERMATOCYTES goes O | | - through "meiosis I" (1n - now | | the pairs are divided in half SPERMATIDS ------> | | and segregation has occurred) O O | | | | SPERMATIDS ------> | | | | - goes through "meiosis II" | | | | (still 1n) O O O O -------------------------------------------------------------------------- Step 4: go through METAMORPHOSIS -------------------------------------------------------------------------- Step 5: go through MATURATION to become --- --O --O --O --0 SPERM (1n or diploid state) -------------------------------------------------------------------------- 9/26/88 gfv