Lecture Outline - Chapter 18

18.1 Chromosome Inheritance (p. 380, Fig. 18.1)
	1.	When a cell prepares for division, the normally diffuse chromatin condenses into short chromosomes.
		a.	Individual chromosomes can be recognized by their size, location of the centromere, and staining bands. A karyotype shows an individual's chromosomes.
		b.	Humans have twenty-two pairs of autosomes and one pair of sex chromosomes. Females are XX, and males are XY.
	2.	Down Syndrome and Cri du Chat (p. 381)
		a.	Down Syndrome (p. 381, Fig. 18.2)
			i.	Mental retardation and a number of physical traits, such as short stature, eyelid fold, and stubby fingers, are associated with Down syndrome.
			ii.	This condition results from trisomy 21, or three copies of chromosome 21 as a result of nondisjunction.
			iii.	The genes causing Down syndrome are located on the bottom third of chromosome 21. In particular, the Gart gene has been identified with mental retardation. It leads to a high level of purines, which may contribute to mental impairment.
		b.	Cri du Chat Syndrome (p. 382, Fig. 18.3)
			The infant with this condition has a moon face, small head, and malformed larynx that produces the sound of a cat. Severe mental retardation is evident. The cause of this condition is a missing portion of one chromosome 5.
	3.	Sex Chromosome Inheritance (p. 382)
		a.	The sex chromosomes are X and Y. Women (XX) produce eggs with an X. Men (XY) produce sperm that bear either an X or a Y; the male determines the gender of the offspring.
		b.	Fragile X Syndrome (p. 382, Fig. 18.4)
			i.	Fragile X syndrome (1/1,000 male births and 1/2,500 female births) involves a nearly broken X chromosome. Children with this trait may be autistic or hyperactive but appear normal. A prominent jaw and large ears develop in adulthood.
			ii.	The cause is a permutation of triplet CGC repeats in excess of the number normally seen on an X chromosome.
		c.	Too Many/Too Few Sex Chromosomes (p. 383, Fig. 18.5)
			i.	A number of genetic abnormalities can occur when an individual inherits too many or too few of the sex chromosomes.
			ii.	Klinefelter syndrome (1/1,500 births) involves a male with two or more X chromosomes. Gonads are underdeveloped, and breasts may develop. These individuals are usually slow to learn.
			iii.	A triplo-X female occurs in 1/1,500 births. Most are completely normal individuals but may have menstrual irregularities.
			iv.	Jacob syndrome (1/1,000 births) are XYY males that are tall, have persistent acne, and have speech and reading problems.
18.2 Human Life Cycle (p. 384, Fig. 18.6, Table 18.1)
	1.	The human life cycle involves two types of cell division. Cells divide by mitosis for normal growth and development or for repair of tissues. Mitosis produces cells identical to the mother cell, with the 2n number of chromosomes. Meiosis is a division used only in gonads to produce gametes (n).
	2.	The human life cycle begins as a 2n zygote, which grows into a baby, a child, and an adult. Two adults reproduce n gametes, which join to make a 2n zygote, and the human life cycle repeats.
18.3 Mitosis (p. 385)
	1.	Mitosis produces two daughter cells, each with the same set of chromosomes as the parent cell. 
	2.	Cell Cycle (p. 385)
		The cell cycle consists of interphase, a period of growth and differentiation, and mitosis, when the nucleus and cell divide.
	3.	Overview of Mitosis: 2n -> 2n (p. 385, Fig. 18.7)
		Before mitosis, the chromatin duplicates and shortens into chromosomes, the sister chromatids of which are held together at the centromere. Mitosis is used for repair or growth.
	4.	Stages of Mitosis (p. 386, Fig. 18.8)
		a.	Prophase (p. 386)
			i.	Spindle fibers appear, the chromosomes condense, the nuclear envelope fragments, and the nucleolus disappears.
			ii.	Spindle fibers are made from reorganized microtubules, once a portion of the cytoskeleton. Centrioles position themselves at the poles of the cell, and microtubule organizing centers (MTOC) are associated with them. The centrioles have duplicated by this point, ensuring that each daughter cell will receive a pair.
		b.	Metaphase (p. 387, Fig. 18.9)
		Metaphase involves a lining up of chromosomes along the cell equator.
		c.	Anaphase (p. 387)
			i.	At the start of anaphase, sister chromatids split, and then are pulled toward respective poles of the cells. The chromatids are now chromosomes.
			ii.	The spindle moves chromosomes during this process.
		d.	Telophase (p. 387)
			i.	When chromosomes arrive at each pole, telophase begins. The spindle disappears, the nucleolis reappear, and nuclear envelopes form.
			ii.	At the end of mitosis, a cleavage furrow forms, splitting the one large cell into two.
18.4 Meiosis (p. 388)
	1.	Overview of Meiosis: 2n -> n (p. 388, Figs. 18.10, 18.11)
		a.	Meiosis results in the production of four daughter cells, each with n number of chromosomes, from one 2n mother cell. Meiosis has two divisions, and DNA duplicates before the beginning of the first division.
		b.	During meiosis I, homologues pair and synapse, during which time nonsister chromatids exchange genetic material in crossing-over. Next, the homologous chromosomes of each pair separate so that one chromosome from each pair will be in the daughter cell. This reduces the number of chromosomes to half.
		c.	During meiosis II, the haploid number of chromosomes per cell are still in duplicated condition. This division separates the sister chromatids.
	2.	The Importance of Meiosis (p. 389)
		a.	Due to meiosis, the number of chromosomes stays the same in each successive generation. Also, the process assures that each new individual will have a slightly different genetic combination than either parent in three ways:
			i.	Crossing-over recombines genes located on homologous chromosomes derived from both parents.
			ii.	Each gamete has a different combination of chromosomes.
			iii.	Fertilization recombines chromosomes.
	3.	Stages of Meiosis (p. 390)
		Prophase, metaphase, anaphase, and telophase all occur during both meiosis I and meiosis II.
	4.	The First Division (p. 390, Fig. 18.12)
		a.	In prophase I, the spindle appears, nuclear envelopes disappear, homologous chromosomes pair and synapse to form tetrads, and crossing-over occurs.
		b.	In metaphase I, the tetrads line up along the equator.
		c.	Anaphase I results in the separation of homologous pairs. Cells are haploid at this point.
		d.	Telophase I results in a brief reappearance of nuclear envelopes, and the spindle disappears. The cell waits momentarily during interkinesis.
	5.	The Second Division (p. 390, Fig. 18.12)
		a.	In prophase II, the spindle reappears, and the nuclear membrane fragments.
		b.	In metaphase II, the chromosomes align at the equator.
		c.	In anaphase II, sister chromatids separate.
		d.	In telophase II, the nuclear envelopes reappear, and four haploid cells are the result. 
	6.	Nondisjunction (p. 392, Fig. 18.13)
		a.	Nondisjunction is the failure of either homologous pairs of chromosomes or sister chromatids to separate properly. It results in Down syndrome or any of the anomalies in sex chromosomes discussed earlier in the chapter. 
	7.	Spermatogenesis and Oogenesis (p. 393, Fig. 18.14)
	During spermatogenesis, haploid sperm are produced in the testes. During oogenesis, haploid egg cells are produced in the ovaries. Both sperm and egg contribute half of the chromosomes that will comprise the new individual.