Bài giảng Biology 25: Human Biology - Chapter 19 & 20

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.

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Chapter 19 & 20Biology 25: Human BiologyProf. GonsalvesLos Angeles City CollegeBased on Mader’s Human Biology,7th edition and Fox’s 8th ed PowerpointsHeredity: 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. I. History of GeneticsBlending 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 colorIf blending always occurred, eventually all extreme characteristics would disappear from the population.Gregor Mendel: Established genetics as a science in 1860s. Considered the founder of modern genetics. 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. 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. III. Mendel’s ExperimentsUsed “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 VarietiesFlower color Purple or whiteSeed color Yellow or greenSeed shape Round or wrinkledPod color Green or YellowPod shape Smooth or constrictedFlower position Axial or terminalPlant height Tall or short III. Summary of Mendel’s Results All plants displayed one trait only. Trait Varieties OffspringFlower color Purple or white 100% PurpleSeed color Yellow or green 100% YellowSeed shape Round or wrinkled 100% RoundPod color Green or Yellow 100% GreenPod shape Smooth or constricted 100% SmoothFlower position Axial or terminal 100% AxialPlant height Tall or short 100% TallThe trait that prevailed was dominant, the other recessive. IV. Mendel’s Conclusions1. 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. IV. Mendel’s Conclusions1. 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. IV. Mendel’s Conclusions4. Must distinguish between:Phenotype: Physical appearance of individual. Example: Two phenotypes for flower color. Purple flowersWhite flowers.Genotype: Genetic makeup of an individual.Not all purple flowers are genetically identical. IV. Mendel’s Conclusions5. 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 IV. Mendel’s Conclusions6. In a given individual, the two genes for a given trait may be the same allele (form of a gene) or different.Phenotype Genotype: Purple PP (Homozygous dominant)Purple Pp (Heterozygous dominant)White pp (Homozygous recessive)Homologous Chromosomes Bear the Two Alleles for Each CharacteristicPhenotype and Genotype of Mendel’s Pea Plants 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 p Pp ppOffspring:Genotype: 1/4 PP, 1/2 Pp, and 1/4 pp Phenotype: 3/4 Purple and 1/4 whiteGenotypic and Phenotypic Ratios of F2 GenerationVI. Principles of Mendelian Genetics1. 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.VI. Principles of Mendelian Genetics2. 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).VI. Principles of Mendelian Genetics3. 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 recessiveVI. Principles of Mendelian Genetics4. 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. VI. Principles of Mendelian Genetics5. 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? VII. Human GeneticsInheritance of human traits. Most genetic diseases are recessive.Dominant Traits Recessive TraitsWidow’s peak Straight hairlineFreckles No frecklesFree earlobe Attached earlobeNormal Cystic fibrosisNormal PhenylketonuriaNormal Tay-Sachs diseaseNormal AlbinismNormal hearing Inherited deafnessHuntington’s Disease NormalDwarfism Normal heightEucaryotic cell division is a more complex and time consuming process than binary fissionFeatures 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 genesHuman 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.Chromosomes ChromatinTightly packaged DNA Unwound DNAFound only during cell Found throughout cell division cycleDNA is not being used DNA is being used for macromolecule for macromolecule synthesis. synthesis.DNA: Found as Chromosomes or ChromatinEucaryotic Chromosomes Duplicate Before Each Cell DivisionCell 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 cytoplasmSynthesize organelles, membranes, proteins, and other molecules.Cell cycle is divided into two main phases:Interphase: Stage between cell divisionsMitotic Phase: Stage when cell is dividingEucaryotic Cell Cycle: Interphase + Mitotic Phase Mitosis: The Stages of Cell Division1. ProphaseChromatin 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 microtubule organizing centers (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.Interphase and Prophase of Mitosis in Animal Cell Mitosis: The Stages of Cell Division2. MetaphaseShort 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.Metaphase, Anaphase, and Telophase of Mitosis in an Animal Cell Mitosis: The Stages of Cell Division3.AnaphaseCentromeres 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. Mitosis: The Stages of Cell Division4. TelophaseCell 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 Mitotic Phase: Mitosis + CytokinesisCytokinesisThe 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.Cytokinesis in Animal and Plant CellsAnimal Cell Plant CellExternal Factors Control Mitosis1. AnchorageMost cells cannot divide unless they are attached to a solid surface.May prevent inappropriate growth of detached cells2. Nutrients and growth factors Lack of nutrients can limit mitosisGrowth factors: Proteins that stimulate cell division.3. Cell densityDensity-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 inhibitionDensity Dependent Inhibition of Mitosis Normal Cells Stop Dividing at High Cell DensityCancer Cells are Not Inhibited by High Cell DensityCell-Cycle Control SystemThere 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.Cell Division is Controlled at Three Key StagesGrowth factors arerequired to passeach checkpointCancer is a Disease of the Cell CycleCancer kills 1 in 5 people in the United States.Cancer cells divide excessively and invade other body tissues.Tumor: Abnormal mass of cells that originates from uncontrolled mitosis of a single cell.Benign tumor: Cancer cells remain in original site. Can easily be removed or treatedMalignant tumor: Cancer cells have ability to “detach” from tumor and spread to other organs or tissuesMetastasis: Spread of cancer cells form site of origin to another organ or tissue. Tumor cells travel through blood vessels or lymph nodes.Functions of Mitosis in Eucaryotes: 1. Growth: All somatic cells that originate after a new individual is created are made by mitosis.2. Cell replacement: Cells that are damaged or destroyed due to disease or injury are replaced through mitosis.3. Asexual Reproduction: Mitosis is used by organisms that reproduce asexually to make offspring. Meiosis: Generates haploid gametesReduces the number of chromosomes by half, producing haploid cells from diploid cells.Also produces genetic variability, each gamete is different, ensuring that two offspring from the same parents are never identical.Two divisions: Meiosis I and meiosis II. Chromosomes are duplicated in interphase prior to Meiosis I. Meiosis I: Separates the members of each homologous pair of chromosomes. Reductive division.Meiosis II: Separates chromatids into individual chromosomes.Interphase:ChromosomesreplicateMeiosis I: Reductive division. Homologouschromosomes separateMeiosis II: Sister chromatidsseparateSTAGES OF MEIOSIS Meiosis I: Separation of Homologous Chromosomes1. Prophase I: Most complex phase of meiosis (90% of time)Chromatin condenses into chromosomes. Nuclear membrane and nucleoli disappear.Centrosomes move to opposite poles of cell and microtubules attach to chromatids.Synapsis: Homologous chromosomes pair up and form a tetrad of 4 sister chromatids.Crossing over: DNA is exchanged between homologous chromosomes, resulting in genetic recombination. Unique to meiosis. Chiasmata: Sites of DNA exchange.Prophase I: Crossing Over Between Homologous Chromosomes Meiosis I: Separation of Homologous Chromosomes2. Metaphase I: Chromosome tetrads (homologous chromosomes) line up in the middle of the cell.Each homologous chromosome faces opposite poles of the cell.Meiosis I: Homologous Chromosomes Separate Stages of Meiosis: Meiosis I 3. Anaphase I: Chromosome tetrads split up.Homologous chromosomes of each pair separate, moving towards opposite poles.Random assortment: One chromosome from each homologous pair is shuffled into the two daughter cells, randomly and independently of the other pairs. Random assortment increases genetic diversity of offspring. Possible combinations: 2n.One human cell can generate 223 or over 8.3 million different gametes by random assortment alone.Random Assortment of Homologous Chromosomes During Meiosis I Generates Many Possible Gametes Meiosis I: Separation of Homologous Chromosomes4. Telophase I and Cytokinesis: Chromosomes reach opposite poles of the cell.Nucleoli reorganize, chromosomes uncoil, and cytokinesis occurs.New cells are haploid. Meiosis II: Separation of Sister Chromatids During interphase that follows meiosis I, no DNA replication occurs.Interphase may be very brief or absent.Meiosis II is very similar to mitosis.1. Prophase II: Very brief, chromosomes reform.No crossing over or synapsis.Spindle forms and starts to move chromosomes towards center of the cell. Meiosis II: Separation of Sister Chromatids 2. Metaphase II: Very brief, individual chromosomes line up in the middle of the cell.Kinetochores of chromatids face opposite poles.3. Anaphase II: Chromatids separate and move towards opposite ends of the cell.Meiosis II: Separation of Sister Chromatids Meiosis II: Separation of Sister Chromatids 4. Telophase II: Nuclei form at opposite ends of the cell.Cytokinesis occurs.Product of meiosis: Four (4) haploid gametes, each genetically different from the other.Meiosis Produces Four Genetically Different Gametes Mitosis versus Meiosis (Review)Mitosis MeiosisOne cell division Two successive cell divisionsProduces two (2) cells Produces four (4) cellsProduces diploid cells Produces haploid gametesDaughter cells are genetically Cells are genetically different fromidentical to mother cell mother cell and each otherNo crossing over Crossing over* Functions: Growth, Functions: Sexual reproductioncell replacement, andasexual reproduction*Crossing over: Exchange of DNA between homologous chromosomes. Meiosis in Males and Females Spermatogenesis: Four sperm cells are made. Starts in puberty and occurs continuously. Males produce millions of sperm cells a month. Oogenesis:Only one large egg is produced. The other three cells are small polar bodies. Oogenesis starts before birth in females, stops at Prophase I, and resumes during puberty.Meiosis is completed only after fertilization.Females make one mature egg/month. Sources of Genetic Variability in Sexual Reproduction1. Crossing Over: After crossing over and synapsis, sister chromatids are no longer identical.2. Independent Assortment: Each human can produce over 8.3 million different gametes by random shuffling of chromosomes in meiosis I.3. Fertilization: A couple can produce over 64 trillion (8.3 million x 8.3 million) different zygotes during fertilization. This figure does not take into account diversity created by crossing over. Accidents During Meiosis Can Cause Chromosomal Abnormalities Nondisjunction: Chromosomes fail to separate.Members of a pair of homologous chromosomes fail to separate during meiosis I or:Sister chromatids fail to separate during meiosis II. Nondisjunction increases with age. Gametes (and zygotes) will have an extra chromosome, others will be missing a chromosome.Trisomy: Individuals with one extra chromosome, three instead of pair. Have 47 chromosomes in cells.Monosomy: Missing a chromosome, one instead of pair. Have 45 chromosomes in cells.Nondisjunction of Chromosomes During Meiosis Produces Abnormal Gametes Accidents During Meiosis Can Result in a Trisomy or MonosomyMost abnormalities in numbers of autosomes are very serious or fatal.Down’s syndrome: Caused by a trisomy of chromosome number 21 (1 in 700 births). Mental retardation, mongoloid features, and heart defects. Most abnormalities of sex chromosomes do not affect survival.Klinefelter Syndrome: Males with an extra sex chromosome (XXY) (1 in 1000 male births).Turner Syndrome: Females missing one sex chromosome (XO) (1 in 2500 female births).Down’s Syndrome is More Common in Children Born to Older MothersAbnormal Numbers of Sex Chromosomes Usually Do Not Affect SurvivalKlinefelter Syndrome (XXY) Turner Syndrome (XO)Incidence: 1:1000 male births Incidence: 1 in 2500 female births
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