Chapter 19 & 20

1. Chapter 19 & 20 Biology 25: Human Biology Prof. Gonsalves Los Angeles City College Based on Mader’s Human Biology,7th edition and Fox’s 8th ed Powerpoints

2. Heredity: The transmission of traits from one generation to another. Variation: Offspring are different from their parents and siblings. Genetics: The scientific study of heredity and hereditary variation. Involves study of cells, individuals, their offspring, and populations.

3. I. History of Genetics • Blending Hypothesis: In 1800s biologists and plant breeders suggested that traits of parents mix to form intermediate traits in offspring. Parents Offspring Red flower x White flower Pink flower Tall height x Short height Medium height Blue bird x Yellow bird Green birds Fair skin x dark skin Medium skin color If blending always occurred, eventually all extremecharacteristics would disappear from the population. • Gregor Mendel: Established genetics as a science in 1860s. Considered the founder of modern genetics.

4. II. Modern Genetics Began as a science in 1860s. • Gregor Mendel:An Austrian monk, who was a farmer’s son. He was trained in mathematics, chemistry, and physics. • Studied the breeding patterns of plants for over 10 years. • Artificially crossed peas, watermelons, and other plants. • Kept meticulous records of thousands of breedings and resulting offspring. • Rejected blending hypothesis, and stressed that heritable factors (genes) retain their individuality generation after generation.

5. II. Modern Genetics Gregor Mendel: • Calculated the mathematical probabilities of inheriting many genetic traits. • Published results in 1866. They were largely ignored due to fervor surrounding Darwin’s publications on evolution. • Discouraged by the lack of attention from the scientific community, he quit his work and died a few years later. • Importance of Mendel’s work was not appreciated until early 1900s when his paper was rediscovered.

6. III. Mendel’s Experiments • Used “true-breeding” or purebred plant varieties for seven pea characteristics. Self-pollination produces all identical offspring. • Using artificial pollination, he crossed true-bred varieties. Trait Varieties Flower color Purple or white Seed color Yellow or green Seed shape Round or wrinkled Pod color Green or Yellow Pod shape Smooth or constricted Flower position Axial or terminal Plant height Tall or short

7. III. Summary of Mendel’s Results All plants displayed one trait only. Trait Varieties Offspring Flower color Purple or white 100% Purple Seed color Yellow or green 100% Yellow Seed shape Round or wrinkled 100% Round Pod color Green or Yellow 100% Green Pod shape Smooth or constricted 100% Smooth Flower position Axial or terminal 100% Axial Plant height Tall or short 100% Tall The trait that prevailed was dominant, the other recessive.

8. IV. Mendel’s Conclusions 1. Results indicate that blending hypothesis is not true. 2. Only one of the two traits appeared in the first generation. He called this the dominant trait. He called the trait that disappeared the recessive trait.

9. IV. Mendel’s Conclusions 1. Results indicate that the recessive trait is intact. 2. The crossbred plants with purple flowers must be carrying the genetic information to produce white flowers. 3. The crossbred plants with purple flowers are genetically different from the purebred plants, even though they look the same.

10. IV. Mendel’s Conclusions 4. Must distinguish between: Phenotype:Physical appearance of individual. Example:Two phenotypes for flower color. • Purple flowers • White flowers. Genotype:Genetic makeup of an individual. Not all purple flowers are genetically identical.

11. IV. Mendel’s Conclusions 5. Each individual carries two genes for a given genetic trait. One gene comes from the individual’s mother, the other from the father. There are two alternative forms of genes or hereditary units. The alternative forms of these hereditary units are called alleles. P: Allele for purple flowers p: Allele for white flowers

12. IV. Mendel’s Conclusions 6. In a given individual, the two genes for a given trait may be the same allele (form of a gene) or different. PhenotypeGenotype: Purple PP (Homozygous dominant) Purple Pp (Heterozygous dominant) White pp (Homozygous recessive)

13. Homologous Chromosomes Bear the Two Alleles for Each Characteristic

14. Phenotype and Genotype of Mendel’s Pea Plants

15. Punnet Square: Used to determine the outcome of a cross between two individuals. Heterozygotes make 1/2 P and 1/2 p gametes. P p P PP Pp pPp pp Offspring: Genotype: 1/4 PP, 1/2 Pp, and 1/4 pp Phenotype: 3/4 Purple and 1/4 white

16. Genotypic and Phenotypic Ratios of F2 Generation

17. VI. Principles of Mendelian Genetics 1. There are alternative forms of genes, the units that determine heritable traits. These alternative forms are called alleles. Example: Pea plants have one allele for purple flower color, and another for white color.

18. VI. Principles of Mendelian Genetics 2. For each inherited characteristic, an individual has two genes: one from each parent. In a given individual, the genes may be the same allele (homozygous) or they may be different alleles (heterozygous).

19. VI. Principles of Mendelian Genetics 3. When two genes of a pair are different alleles, only one is fully expressed (dominant allele). The other allele has no noticeable effect on the organism’s appearance (recessive allele). Example: Purple allele for flower color is dominant White allele for flower color is recessive

20. VI. Principles of Mendelian Genetics 4. A sperm or egg cell (gamete) only contains one allele or gene for each inherited trait. Principle of Segregation: Alleles segregate (separate) during gamete formation. (When? During meiosis I) During fertilization, sperm and egg each contribute one allele to the new organism, restoring the allele pair.

21. VI. Principles of Mendelian Genetics 5. Principle of Independent Assortment: Two different genetic characteristics are inherited independently of each other.* *As long as they are on different chromosomes. Mendel did not know about meiosis, but meiosis explains this observation. Why? How are chromosomes shuffled during meiosis I?

22. VII. Human Genetics Inheritance of human traits. Most genetic diseases are recessive. Dominant Traits Recessive Traits Widow’s peak Straight hairline Freckles No freckles Free earlobe Attached earlobe Normal Cystic fibrosis Normal Phenylketonuria Normal Tay-Sachs disease Normal Albinism Normal hearing Inherited deafness Huntington’s Disease Normal Dwarfism Normal height

24. Eucaryotic cell division is a more complex and time consuming process than binary fission Features of Eucaryotic DNA 1. DNA is in multiple linear chromosomes. • Unique number for each species: • Humans have 46 chromosomes. • Cabbage has 20, mosquito 6, and fern over 1000. 2. Large Genome: Up to 3 billion base pairs (humans) • Contains up to 50,000-150,000 genes • Human genome project is determining the sequence of entire human DNA. 3. DNA is enclosed by nuclear membrane. Correct distribution of multiple chromosomes in each daughter cell requires a much more elaborate process than binary fission.

25. DNA: Found as Chromosomes or Chromatin Chromosomes Chromatin Tightly packaged DNA Unwound DNA Found only during cell Found throughout cell divisioncycle DNA is not being used DNA is being used for macromolecule for macromolecule synthesis. synthesis.

26. Eucaryotic Chromosomes Duplicate Before Each Cell Division

28. Cell Cycle of Eucaryotic Cells • Sequence of events from the time a cell is formed, until the cell divides once again. • Before cell division, the cell must: • Precisely copy genetic material (DNA) • Roughly double its cytoplasm • Synthesize organelles, membranes, proteins, and other molecules. • Cell cycle is divided into two main phases: • Interphase: Stage between cell divisions • Mitotic Phase: Stage when cell is dividing

29. Eucaryotic Cell Cycle: Interphase + Mitotic Phase

34. Mitosis: The Stages of Cell Division 1. Prophase • Chromatin condenses into chromosomes, which appear as two sister chromatids joined by a centromere. • Nucleoli disappear. • Nuclear envelope breaks apart. • In animal cells, mitotic spindle begins to form as mictotubules grow out of two centrosomes or microtubuleorganizingcenters (MTOCs). • Each centrosome is made up of a pair of centrioles. • Microtubules attach to kinetochores on chromatids and begin to move chromosomes towards center of cell. • Centrosomes begin migrating to opposite poles of cell.

35. Interphase and Prophase of Mitosis in Animal Cell

36. Mitosis: The Stages of Cell Division 2. Metaphase • Short period in which chromosomes line up along equatorial plane of cell (metaphase plate). • Chromosomes are completely condensed and easy to visualize. • Mitotic spindle is fully formed. • Kinetochores of sister chromatids face opposite sides and are attached to spindle microtubules at opposite ends of the cell.

37. Metaphase, Anaphase, and Telophase of Mitosis in an Animal Cell

38. Mitosis: The Stages of Cell Division 3.Anaphase • Centromeres of sister chromatids begin to separate. • Each chromatid is now an independent daughter chromosome. • The separate chromosomes are pulled toward opposite ends by spindle microtubules, attached to the kinetochores. • Cell elongates as poles move farther apart. • Anaphase ends when a complete set of chromosomes reaches each pole.

40. Mitosis: The Stages of Cell Division 4. Telophase • Cell continues to elongate. • Cell returns to interphase conditions: • A nuclear envelope forms around each set of chromosomes. • Chromosomes uncoil, becoming chromatin threads. • Nucleoli reappear. • Spindle microtubules disappear. • Cytokinesis usually occurs at the end of this stage

41. Mitotic Phase: Mitosis + Cytokinesis • Cytokinesis • The division of cytoplasm to produce two daughter cells. Usually begins during telophase. • In animal cells: Division is accomplished by a cleavage furrow that encircles the cell like a ring in the equator region. • In plant cells: Division is accomplished by the formation of a cell plate between the daughter cells. Each cell produces a plasma membrane and a cell wall on its side of the plate.

42. Cytokinesis in Animal and Plant Cells Animal Cell Plant Cell

47. External Factors Control Mitosis 1. Anchorage • Most cells cannot divide unless they are attached to a solid surface. • May prevent inappropriate growth of detached cells 2. Nutrients and growth factors • Lack of nutrients can limit mitosis • Growth factors: Proteins that stimulate cell division. 3. Cell density • Density-dependent inhibition: Cultured cells will stop dividing after a single layer covers the petri dish. Mitosis is inhibited by high cell density. • Cancer cells do not demonstrate density inhibition

48. Density Dependent Inhibition of Mitosis Normal Cells Stop Dividing at High Cell Density Cancer Cells are Not Inhibited by High Cell Density

49. Cell-Cycle Control System There are three critical points at which the cell cycle is controlled*: 1. G1 Checkpoint: Prevents cell from entering S phase and duplicating DNA. • Most important checkpoint. • Amitotic cells (muscle and nerve cells) are frozen here. 2. G2 Checkpoint: Prevents cell from entering mitosis. 3. M Checkpoint: Prevents cell from entering cytokinesis. *Cells must have proper growth factors to get through each checkpoint.

50. Cell Division is Controlled at Three Key Stages Growth factors are required to pass each checkpoint