MCAT Biology Review

Chapter 2: Reproduction

Conclusion

In this chapter, we explored one of the key tenets of the cell theory—how cells produce more copies of themselves. We first examined mitosis, which results in genetically identical diploid daughter cells. We then moved on to meiosis, which results in genetically nonidentical haploid daughter cells, or gametes. We then looked at the male and female reproductive systems, which form these gametes, each of which contains half of the normal complement of genetic information. Finally, we explored basic reproductive endocrinology and saw how testosterone and estrogen are key for development of the reproductive systems and the secondary sex characteristics that develop at puberty.

Formation of gametes is only half the story, of course. It serves us no good as a species to form sex cells if the cells cannot interact to form another human. Ultimately, gametes must accomplish their purpose: passing on the genes, the instructions for life, from one generation to another. Thus, we turn our attention in the next chapter to the next steps of fertilization, embryogenesis, and birth. The development of another human is a humbling and inspiring process. Indeed, it is through the union of one egg and one sperm that every human being on the planet today—and since the beginning of the human race—came to be in existence.

Concept Summary

The Cell Cycle and Mitosis

· Diploid (2n) cells have two copies of each chromosome; haploid (n) cells have one copy.

· The cell cycle contains five stages. The G₁, S, and G₂ stages are collectively called interphase, during which the DNA is uncoiled in the form of chromatin.

o In the G1 stage (presynthetic gap), cells create organelles for energy and protein production, while also increasing their size. The restriction point, during which the DNA is checked for quality, must be passed for the cell to move into the S stage.

o In the S stage (synthesis), DNA is replicated. The strands of DNA, called chromatids, are held together at the centromere.

o In the G2 stage (postsynthetic gap), there is further cell growth and replication of organelles in preparation for mitosis. Another quality checkpoint must be passed for the cell to enter into mitosis.

o In the M stage (mitosis), mitosis and cytokinesis occur.

o In the G0 stage, the cell performs its function without any preparation for division.

· p53 plays a role in the two major checkpoints of the cell cycle (G₁ to S, and G₂ to M).

· Cyclins and cyclin-dependent kinases (CDK) rise and fall during the cell cycle. Cyclins bind to CDKs, phosphorylating and activating transcription factors for the next stage of the cell cycle.

· Cancer occurs when cell cycle control becomes deranged, allowing damaged cells to undergo mitosis without regard to quality or quantity of the new cells produced. Cancerous cells may begin to produce factors that allow them to escape their site and invade or metastasize elsewhere.

· Mitosis produces two genetically identical diploid daughter cells from a single cell and occurs in somatic cells.

· Mitosis has four phases:

o In prophase, the chromosomes condense, nuclear membrane dissolves, nucleoli disappear, centrioles migrate to opposite sides of the cell, and the spindle apparatus begins to form. The kinetochore of each chromosome is contacted by a spindle fiber.

o In metaphase, chromosomes line up along the metaphase plate (equatorial plate).

o In anaphase, sister chromatids are separated and pulled to opposite poles.

o In telophase, the nuclear membrane reforms, spindle apparatus disappears, and cytosol and organelles are split between the two daughter cells through cytokinesis.

Meiosis

· Meiosis occurs in gametocytes (germ cells) and produces up to four nonidentical haploid sex cells (gametes).

· Meiosis has one round of replication and two rounds of division (the reductional and equational divisions).

· In meiosis I, homologous pairs of chromosomes (homologues) are separated from each other. Homologues are chromosomes that are given the same number, but are of opposite parental origin.

o In prophase I, the same events occur as in prophase of mitosis, except that homologues come together and intertwine in a process called synapsis. The four chromatids are referred to as a tetrad, and crossing over exchanges genetic material from one chromatid with material from a chromatid in the homologous chromosome.

o In metaphase I, homologous chromosomes line up on opposite sides of the metaphase plate.

o In anaphase I, homologous chromosomes are segregated to opposite poles of the cell. This accounts for Mendel’s first law (of segregation). The recombination of genes during crossing over also accounts for Mendel’s second law (of independent assortment).

o In telophase I, the chromosomes may or may not fully decondense, and the cell may enter interkinesis after cytokinesis.

· In meiosis II, sister chromatids are separated from each other in a process that is functionally identical to mitosis. Sister chromatids are copies of the same DNA held together at the centromere.

The Reproductive System

· Biological sex is determined by the 23rd pair of chromosomes in humans, with XX being female and XY being male.

o The X chromosome carries a sizeable amount of genetic information; mutations of X-linked genes can cause sex-linked disorders. Males are hemizygous with respect to the unpaired genes on the X chromosome, so they will express sex-linked disorders, even if they only have one recessive disease-carrying allele. Women with one copy of the affected allele are called carriers.

o The Y chromosome carries little genetic information, but contains the SRY (sex-determining region Y) gene, which causes the gonads to differentiate into testes.

· The male reproductive system contains both internal and external structures.

o Sperm develop in the seminiferous tubules in the testes. They are nourished by Sertoli cells.

o Interstitial cells (of Leydig) secrete testosterone and other male sex hormones (androgens).

o The testes are located in the scrotum, which hangs outside of the abdominal cavity and has a temperature 2° to 4°C lower than the body.

o Once formed, sperm gain motility in the epididymis and are stored there until ejaculation.

o During ejaculation, sperm travel through the vas deferens to the ejaculatory duct to the urethra and out through the penis.

o The seminal vesicles contribute fructose to nourish sperm and produce alkaline fluid.

o The prostate gland also produces alkaline fluid.

o The bulbourethral glands produce a clear viscous fluid that cleans out any remnants of urine and lubricates the urethra during sexual arousal.

o Semen is composed of sperm and seminal fluid from the glands above.

· In spermatogenesis, four haploid sperm are produced from a spermatogonium.

o After S stage, the germ cells are called primary spermatocytes.

o After meiosis I, the germ cells are called secondary spermatocytes.

o After meiosis II, the germ cells are called spermatids.

o After maturation, the germ cells are called spermatozoa.

· Sperm contain a head, midpiece, and flagellum.

o The head contains the genetic material and is covered with an acrosome—a modified Golgi apparatus that contains enzymes that help the sperm fuse to and penetrate the ovum.

o The midpiece generates ATP from fructose and contains many mitochondria.

o The flagellum promotes motility.

· The female reproductive system only contains internal structures.

o Ova (eggs) are produced in follicles in the ovaries.

o Once each month, an egg is ovulated into the peritoneal sac and is drawn into the fallopian tube or oviduct.

o The fallopian tubes are connected to the uterus, the lower end of which is called the cervix.

o The vaginal canal lies below the cervix and is the site where sperm are deposited during intercourse. Birth also occurs through the vaginal canal.

o The external female anatomy is known as the vulva.

· In oogenesis, one haploid ovum and a variable number of polar bodies are formed from an oogonium.

o At birth, all oogonia have already undergone replication and are considered primary oocytes. They are arrested in prophase I.

o The ovulated egg each month is a secondary oocyte, which is arrested in metaphase II.

o If the oocyte is fertilized, it will undergo meiosis II to become a true ovum.

o Cytokinesis is uneven in oogenesis. The cell receiving very little cytoplasm and organelles is called a polar body.

o Oocytes are surrounded by the zona pellucida, an acellular mixture of glycoproteins that protect the oocyte and contain the compounds necessary for sperm binding, and the corona radiata, which is a layer of cells that adhered to the oocyte during ovulation.

· Gonadotropin-releasing hormone (GnRH) from the hypothalamus causes the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH), the functions of which depend on the sex of the individual.

o In males, FSH stimulates the Sertoli cells and triggers spermatogenesis, while LH causes the interstitial cells to produce testosterone. Testosterone is responsible for the maintenance and development of the male reproductive system and male secondary sex characteristics (facial and axillary hair, deepening of the voice, and changes in growth patterns).

o In females, FSH stimulates development of the ovarian follicles, while LH causes ovulation. These hormones also stimulate production of estrogens and progesterone.

· The menstrual cycle is a periodic growth and shedding of the endometrial lining.

o In the follicular phase, GnRH secretion stimulates FSH and LH secretion, which promotes follicle development. Estrogen is released, stimulating vascularization and glandularization of the decidua.

o Ovulation is stimulated by a sudden surge in LH. This surge occurs because estrogen stops having negative feedback effects at a certain threshold and begins to have positive feedback effects.

o In the luteal phase, LH promotes the ruptured follicle to become the corpus luteum, which secretes progesterone that maintains the uterine lining. High estrogen and progesterone levels cause negative feedback on GnRH, LH, and FSH.

o Menstruation occurs if there is no fertilization. As the estrogen and progesterone levels drop, the endometrial lining is sloughed off, and the block on GnRH production is removed.

o If fertilization does occur, the blastula produces human chorionic gonadotropin (hCG) which, as an LH analogue, can maintain the corpus luteum. Near the end of the first trimester, hCG levels drop as the placenta takes over progesterone production.

· Menopause occurs when the ovaries stop producing estrogen and progesterone, usually between ages 45 and 55. Menstruation stops and FSH and LH levels rise. Physical and physiological changes accompanying menopause include flushing, hot flashes, bloating, headaches, and irritability.

Answers to Concept Checks

· 2.1

1. 

2. 

· 2.2

1. After meiosis I, there are two haploid daughter cells. After meiosis II, there are up to four haploid gametes.

2. Homologous chromosomes are related chromosomes of opposite parental origin (such as maternal chromosome 15 and paternal chromosome 15, or—in males—the X and Y chromosomes). Sister chromatids are identical copies of the same DNA that are held together at the centromere. After S phase, a cell contains 92 chromatids, 46 chromosomes, and 23 homologous pairs.

3. 

· 2.3

1. The interstitial cells of Leydig secrete testosterone and other male sex hormones (androgens). Sertoli cells nourish sperm during their development.

2. A primary oocyte is arrested in prophase I, while a secondary oocyte is arrested in metaphase II.

3. The acrosome contains enzymes that are capable of penetrating the corona radiata and zona pellucida of the ovum, permitting fertilization to occur. It is a modified Golgi apparatus.

4. 

Shared Concepts

· Behavioral Sciences Chapter 1

o Biology and Behavior

· Biochemistry Chapter 6

o DNA and Biotechnology

· Biology Chapter 1

o The Cell

· Biology Chapter 3

o Embryogenesis and Development

· Biology Chapter 5

o The Endocrine System

· Biology Chapter 12

o Genetics and Evolution