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by: Meredith Notetaker

BiologyNotes3.pdf Biol 1020-001

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Principals of Biology
James Zanzot
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This 36 page Class Notes was uploaded by Meredith Notetaker on Monday November 9, 2015. The Class Notes belongs to Biol 1020-001 at Auburn University taught by James Zanzot in Fall 2015. Since its upload, it has received 13 views. For similar materials see Principals of Biology in Biology at Auburn University.


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Date Created: 11/09/15
Biology Notes 3 Chapter 12 Video Lectures • The Key Roles of Cell Division o The ability of organisms to produce more of their own kind best distinguishes living things from nonliving matter o The continuity of life is based on the reproduction of cells, or cell division § cell division can be used: • reproduction, for unicellular organisms • growth and development • tissue renewal • Concept 12.1: Most cell division results in genetically identical daughter cells o Most cell division results in daughter cells with identical genetic information, DNA o The exception is meiosis, a special type of division that can produce sperm and egg cells • Cellular Organization of the Genetic Material o All the DNA in a cell constitutes the cell’s genome o A genome can consist of a single DNA molecule (common in prokaryotic cells) or a number of DNA molecules (common in eukaryotic cells) o DNA molecules in a cell are packaged into chromosomes o Chromatin § when it is uncondensed in the nucleus, which is most of the time o Chromosomes § colored bodies o Somatic cells (nonreproductive cells) 2n § or body cells § for animals, 2 copies of every chromosome in them o Gametes (reproductive cells: sperm and eggs) n § half the number of copies as found in the somatic cells • one copy of each chromosome § In humans, n = 23 § In chimpanzees, n = 24 § In fruit flies, n = 4 § Adder’s tongue fern, n = 630 o During cell division, the two sister chromatids of each duplicated chromosome separate and move into two nuclei o Once separate, the chromatids are called chromosomes o Eukaryotic cell division consists of § Mitosis, the division of the genetic material in the nucleus § Cytokinesis, the division of the cytoplasm • cell movement o In animals, gametes are produced by a variation of cell division called meiosis o Meiosis yields nonidentical daughter cells that have half as many chromosomes as the parent cell • Phases of the Cell Cycle o The cell cycle consists of § Mitotic (M) phase (mitosis and cytokinesis) • small portion of the time is spent here § Interphase (cell growth and copying of chromosomes in preparation for cell division) • vast majority of the time is spent here • Interphase can be broken down into three subphases: o G1 o S § DNA synthesis o G2 o G2 of Interphase § DNA is already copied, but is uncondensed in the nucleus, which is still intact § Centrosomes have been replicated, so they start migrating toward opposite ends of the cell 1. Prophase § The first part of mitosis § Centrosomes are migrating apart, but are still connected to each other by the mitotic spindle • Centrosomes organize the microtubules § Chromatin is starting to condense down into chromosomes • For each chromosome, there are two sister chromatids 2. Prometaphase § Between prophase and metaphase § Nuclear envelopes fragments, or breaks into pieces § Everything gets aside for the main event to occur § Microtubules from the aster (that looks like a star) • Asters are arranged from the centrosomes, one at either end of the cell § Microtubules connect to the centromeres at the region called the kinetochore • Kinetochore is a complex of proteins that attaches by the centromere o Kinetochores are like carabineers that hook onto the chromosomes § Some microtubules, that are the non-Kinetochore microtubules, are lining up and overlapping and help push the fragments of the chromosomes away from each other, so they end up with two daughter cells where it started out with one mother cell § Everything gets arranged in place 3. Metaphase § All of the chromosome pairs (sister chromatids) are lined up right at the center of the cell at the imaginary structure called the metaphase plate § Non-Kinetochore microtubules are overlapping 4. Anaphase § Cohesins, the proteins holding the sister chromatids, separate § The daughter chromosomes start getting pulled apart from each other and they are migrating toward the centrosomes § The non-Kinetochore microtubules that are overlapping are pushing against each other • Pulling on the daughter chromosomes and pushing on the non- Kinetochore microtubules • Cell starts to elongate 5. Telophase and Cytokinesis § The chromosomes, which are now their own and each cell has the right number of copies of each chromosome § The nucleolus and nuclear envelope both start to reform § Divide up all of the organelles § Start to form this line called a cleavage furrow • Cleavage = cutting • Furrow = trough • Pinches off between and forms the boundaries of the two new daughter cells • The Mitotic Spindle: A Closer Look o The mitotic spindle is a structure made of microtubules that controls chromosome movement during mitosis o In animal cells, assembly of spindle microtubules begins in the centrosome, the microtubule organizing center o The centrosome replicates during interphase, forming two centrosomes that migrate to opposite ends of the cell during prophase and prometaphase o An aster (a radial array of short microtubules) extends from each centrosome o The spindle includes the centrosomes, the spindle microtubules, and the asters • Binary Fission ***on final*** o *pictures* o prokaryotes way of cell division instead of mitosis § binary fission is not the same thing as mitosis 1. Chromosome replication begins 2. One copy of the origin is now at each end of the cell 3. Replication finishes 4. Two daughter cells result • The Evolution of Mitosis o Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission o Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis • Concept 12.3: The eukaryotic cell cycle is regulated by a molecular control system o The frequency of cell division varies with the type of cell o These differences result from regulation at the molecular level o Cancer cells manage to escape the usual controls on the cell cycle o Endogenous and exogenous factors can control the cell cycle § Endogenous – from within § Exogenous – from outside • ex: Platelet derived growth factor (PDGF) can stimulate cell division exogenously • The Cell Cycle Control System o The cell cycle appears to be driven by specific chemical signals present in the cytoplasm (not the nucleus) o Some evidence for this hypothesis comes from experiments in which cultured mammalian cells at different phases of the cell cycle were fused to form a single cell with two nuclei • The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases o Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks) o The activity of a Cdk rises and falls with changes in concentration of its cyclin partner o MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the 2 checkpoint into the M phase o Most cell also exhibit anchorage dependence – to divide, they must be attached to a substratum o Density-dependent inhibition and anchorage dependence check the growth of cells at an optimal density o Cancer cells exhibit neither type of regulation of their division • Loss of Cell Cycle Controls in Cancer Cells o Cancer cells do not respond normally to the body’s control mechanisms o Cancer cells may not need growth factors to grow and divide § They may make their own growth factor § They may convey a growth factor’s signal without the presence of the growth factor § They may have an abnormal cell cycle control system o A normal cell is converted to a cancerous cell by a process called transformation o Cancer cells that are not eliminated by the immune system form tumors, masses of abnormal cells within otherwise normal tissue o If abnormal cells remain only at the original site, the lump is called a benign tumor o Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form additional tumors o Localized tumors may be treated with high-energy radiation, which damages the DNA in the cancer cells o To treat metastatic cancers, chemotherapies that target the cell cycle may be used § carcinogen – a chemical or physical agent that can cause cancer § mutagen – an agent that makes changes in DNA and causes mutations o Recent advances in understanding the cell cycle and cell cycle signaling have led to advances in cancer treatment o Coupled with the ability to sequence the DNA of cells in a particular tumor, treatments are becoming more “personalized” Chapter 13 Video Lectures • Overview: Variations on a Theme o Living organisms are distinguished by their ability to reproduce their own kind o Genetics is the scientific study of heredity and variation o Heredity is the transmission of traits from one generation to the next o Variation is demonstrated by the differences in appearance that offspring show from parents and siblings o Your mom gives you something your dad can’t, and that is your mitochondria • Inheritance of Genes o Genes are the units of heredity, and are made up of segments of DNA o Genes are passed to the next generation via reproductive cells called gametes (sperm and eggs) o Each gene has a specific location called a locus on a certain chromosome o Most DNA is packaged into chromosomes • Comparison of Asexual and Sexual Reproduction o In asexual reproduction, a single individual passes genes to its offspring without the fusion of gametes o A clone is a group of genetically identical individuals from the same parent o In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents • Sets of Chromosomes in Human Cells o Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes o A karyotype is an ordered display of the pairs of chromosomes from a cell o The two chromosomes in each pair are called homologous chromosomes, or homologs o Chromosomes in a homologous pair are the same length and shape and carry genes controlling the same inherited characters o Sex chromosomes § determine the sex of the individual § called X and Y o In humans § Females have a homologous pair of X chromosomes (XX) § Males have one X and one Y chromosome o The remaining 22 pairs of chromosomes are called autosomes o A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n) o For humans, the haploid number is 23 (n = 23) o Each set of 23 consists of 22 autosomes and a single sex chromosome o In an unfertilized egg (ovum), the sex chromosome is X o In a sperm cell, the sex chromosome may be either X or Y • Behavior of Chromosome Sets in the Human Life Cycle o Fertilization is the union of gametes (the sperm and the egg) o The fertilized egg is called a zygote and has one set of chromosomes from each parent o The zygote produces somatic cells by mitosis and develops into an adult o Diploid = 2n o Haploid = n • The Variety of Sexual Life Cycles o The alternation of meiosis and fertilization is common to all organisms that reproduce sexually o The three main types of sexual life cycles differ in the timing of meiosis and fertilization • Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid o Like mitosis, meiosis is preceded by the replication of chromosomes o Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II o The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis o Each daughter cell has only half as many chromosomes as the parent cell • The Stages of Meiosis o After chromosomes duplicate, two divisions follow § Meiosis I (reductional division): homologs pair up and separate, resulting in two haploid daughter cells with replicated chromosomes § Meiosis II (equational division) sister chromatids separate o The result is four haploid daughter cells with unreplicated chromosomes • ****yellow and purple mitosis/meiosis chart**** (metaphases) • A Comparison of Mitosis and Meiosis o Mitosis conserves the number of chromosome sets producing cells that are genetically identical to the parent cell o Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell § Meiosis is like Mitosis times 2, but their difference is great • Mitosis: producing daughter cells identical to the mother cell • Meiosis: producing cells that are genetically distinct from the mother cell • SUMMARY CHART: property, mitosis, meiosis o Three events are unique to meiosis, and all three occur in meiosis I § Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information § At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes § At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate • Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution o Mutations (changes in an organism’s DNA) are the original source of genetic diversity o Mutations create different versions of genes called alleles o Reshuffling of alleles during sexual reproduction produces genetic variation • Origins of Genetic Variation Among Offspring o The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation o Three mechanisms contribute to genetic variation § Independent assortment of chromosomes § Crossing over § Random fertilization • Independent Assortment of Chromosomes o Homologous pairs of chromosomes orient randomly at metaphase I of meiosis o In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs o The number of combinations possible when chromosomes assort independently into gametes is 2 , where n is the haploid number 23 o For humans (n = 23), there are more than 8 million (2 ) possible combinations of chromosomes • Crossing Over o Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent o Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene o In crossing over, homologous portions of two non-sister chromatids trade places o Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome • Random Fertilization o Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) o The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations • World Population 9 o 7 billion = 7.0 x 10 o 70 trillion = 7.0 x 10 3 o Therefore enough genetic diversity to account for 10,000 times (10 ) the earth’s current population • The Evolutionary Significance of Genetic Variation Within Populations o Natural selection results in the accumulation of genetic variations favored by the environment o Sexual reproduction contributes to the genetic variation in a population, which originates from mutations • Definitions you should know: o Genetics, gene, chromosome, locus o Gamete, sexual reproduction, asexual reproduction, clone o Somatic cell, karyotype, homologous chromosomes o Haploid vs. diploid, sex chromosome vs. autosome o Zygote, fertilization, life cycle • What should you know about meiosis o Purpose and products of meiosis o Stages of meiosis § Prophase I, metaphase I, anaphase I, telophase I, interkinesis § Prophase II, metaphase II, anaphase II, telophase II o Differences between meiosis and mitosis § be able to draw all of the stages out (and the differences) • Mechanisms of variation in meiosis o Crossing over o Independent assortment o Fertilization • Exercise: Can you draw and label all the steps in meiosis? o Prophase I, crossing over, to Telophase II Chapter 14 Video Lectures • Overview: Drawing from the Deck of Genes o What genetic principles account for the passing of traits from parents to offspring? o The “blending” hypothesis is the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green) o The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes) § contradictory to the “blending” hypothesis § this is the correct, accepted model o This hypothesis can explain the reappearance of traits after several generations o Mendel documented a particulate mechanism through his experiments with garden peas • Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance o Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments • Mendel’s Experimental, Quantitative Approach o Advantages of pea plants for genetic study § There are many varieties with distinct heritable features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits § Mating can be controlled § Each flower has sperm-producing organs (stamens) and egg- producing organ (carpel) § Cross-pollination (fertilization between different plants involves dusting one plant with pollen from another o Mendel’s approach allowed him to deduce principles that had remained elusive to others o A heritable feature that varies among individuals (such as flower color) is called a character o Each variant for a character, such as purple or white color for flowers, is called a trait o Peas were available to Mendel in many different varieties o Other advantages of using peas § Short generation time § Large numbers of offspring § Mating could be controlled; plants could be allowed to self-pollinate or could be cross pollinated § Mendel chose to track only those characters that occurred in two distinct alternative forms § He also used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate) § In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization § The true-breeding parents are the P generation § The hybrid offspring of the P generation are called the F generation 1 § When F 1ndividuals self-pollinate or cross-pollinate with other F 1 hybrids, the F 2generation is produced • The Law of Segregation o When Mendel crosses contrasting, true-breeding white- and purple-flowered pea plants, all of the F1 hybrids were purple o When Mendel crossed the F hybr1ds, many of the F plant2 had purple flowers, but some had white o Mendel discovered a ratio of about three to one, purple to white flowers, in the F 2generation o Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids o Mendel called the purple flower color a dominant trait and the white flower color a recessive trait o The factor for white flowers was not diluted or destroyed because it reappeared in the F 2eneration o Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits o What Mendel called a “heritable factor” is what we now call a gene • Mendel’s Model o Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F offspring 2 o Four related concepts make up this model o These concepts can be related to what we now know about genes and chromosomes o First: alternative versions of genes (alleles) account for variations in inherited characters o For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers o Each gene resides at a specific locus on a specific chromosome o Second: for each character, an organism inherits two alleles, one from each parent o Mendel made this deduction without knowing about chromosomes o The two alleles at a particular locus may be identical, as in the true-breeding plants of Mendel’s P generation o Alternatively, the two alleles at a locus may differ, as in the F 1hybrids o Third: if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance o In the flower-color example, the F p1ants had purple flowers because the allele for that trait is dominant o Dominant alleles may mask the presence of the recessive allele o Fourth (the law of segregation): the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes o Thus, an egg or a sperm gets only one of the two alleles that are present in the organism o This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis • Mendel’s Law of Segregation o Allele pairs separate during gamete formation, and reunite randomly at fertilization o The model accounts for the 3:1 ratio observed in the F ge2eration of Mendel’s crosses o Possible combinations of sperm and egg can be shown using a Punnett square o A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele • Useful Genetic Vocabulary o An organism with two identical alleles for a character is homozygous for the gene controlling that character o An organism that has two different alleles for a gene is heterozygous for the gene controlling that character o Unlike homozygotes, heterozygotes are not true-breeding o Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition o Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup o In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes • The Law of Independent Assortment o Mendel derived the law of segregation by following a single character o The F o1fspring produced in this cross were monohybrids, heterozygous for one character o A cross between such heterozygotes is called a monohybrid cross o Mendel identified his second law of inheritance by following two characters at the same time o Crossing two true-breeding parents differing in two characters produces dihybrids in the F 1generation, heterozygous for both characters o A dihybrid cross, a cross between F dih1brids, can determine whether two characters are transmitted to offspring as a package or independently o Using a dihybrid cross, Mendel developed the law of independent assortment o It states that each pair of alleles segregates independently of each other pair of alleles during gamete formation o This law applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome o Genes located near each other on the same chromosome tend to be inherited together • Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics o The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied o Many heritable characters are not determined by only one gene with two alleles o However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance • Extending Mendelian Genetics for a Single Gene o Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: § When alleles are not completely dominant or recessive • Incomplete dominance • Codominance § When a gene has more than two alleles • Multiple alleles § When a gene produces multiple phenotypes • Pleiotropy • Degrees of Dominance o Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical o In incomplete dominance, the phenotype of F hybri1s is somewhere between the phenotypes of the two parental varieties o In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways • Frequency of Dominant Alleles o Dominant alleles are not necessarily more common in populations than recessive alleles o For example, one baby out of 400 in the United States is born with extra fingers or toes o Polydactyly: many fingers • Multiple Alleles o Most genes exist in populations in more than two allelic forms o For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: I , I , and i. o The enzyme encoded by the I allele adds the A carbohydrate, whereas the enzyme encoded by the I allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither • Pleiotropy o Most genes have multiple phenotypic effects, a property called pleiotropy o For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease o However: § All of the symptoms are traceable to a SINGLE allele § Many phenotypic effects of a single genotype § = PLEIOTROPY • Extending Mendelian Genetics for Two or More Genes o Some traits may be determined by two or more genes • Epistasis o In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus o For example, in Labrador retrieves and many other mammals, coat colors depend on two genes o One gene determines the pigment color (with alleles B for black and b for brown) o The other gene (with alleles E for color and e for no color) determines whether the pigment with be deposited in the hair • Polygenic Inheritance o Quantitative characters are those that vary in the population along a continuum o Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype o Skin color in humans is an example of polygenic inheritance • Nature and Nurture: The Environmental Impact on Phenotype o Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype o The phenotypic range is broadest for polygenic characters o Traits that depend on multiple genes combined with environmental influences are called multifactorial • A Mendelian View of Heredity and Variation o An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior o An organism’s phenotype reflects its overall genotype and unique environmental history • Concept 14.4: Many human traits follow Mendelian patterns of inheritance o Humans are not good subjects for genetic research § Generation time is too long § Parents produce relatively few offspring § Breeding experiments are unacceptable o However, basic Mendelian genetics endures as the foundation of human genetics • Pedigree Analysis o A pedigree is a family tree that describes the interrelationships of parents and children across generations o Inheritance patterns of particular traits can be traced and described using pedigrees o Pedigrees can also be used to make predictions about future offspring o We can use the multiplication and addition rules to predict the probability of specific phenotypes • Recessively Inherited Disorders o Many genetic disorders are inherited in a recessive manner o These range from relatively mild to life-threatening • The Behavior of Recessive Alleles o Recessively inherited disorders show up only in individuals homozygous for the allele o Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal; most individuals with recessive disorders are born to carrier parents o Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair o If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low o Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele o Most societies and cultures have laws or taboos against marriages between close relatives • Sickle-Cell Disease: A Genetic Disorder with Evolutionary Implications o Sickle-cell disease affects one out of 400 African-Americans o The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells o In homozygous individuals, all hemoglobin is abnormal (sickle-cell) o Symptoms include physical weakness, pain, organ damage, and even paralysis o Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms o About one out of ten African Americans has sickle-cell trait, an unusually high frequency o Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous in regions where malaria is common • Dominantly Inherited Disorders o Some human disorders are caused by dominant alleles o Dominant alleles that cause a lethal disease are rare and arise by mutation o Achondroplasia is a form of dwarfism caused by a rare dominant allele • Huntington’s Disease: A Late-Onset Lethal Disease o Huntington’s disease is a degenerative disease of the nervous system o The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age o Once the deterioration of the nervous system begins the condition is irreversible and fatal • Marfan Syndrome o Another dominantly-inherited disease o Affects connective tissue formation o Affected individuals typically are tall and lanky o Isaiah Austin § former basketball player for Baylor University • Multifactorial Disorders o Many diseases, such as heart disease, diabetes, alcoholism, mental illnesses, and cancer have both genetic and environmental components o No matter what our genotype, our lifestyle has a tremendous effect on phenotype • Genetic Testing and Counseling o Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease • Counseling Based on Mendelian Genetics and Probability Rules o Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders o It is important to remember that each child represents an independent event in the sense that its genotype is unaffected by the genotypes of older siblings • Tests for Identifying Carriers o For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately o The tests enable people to make more informed decisions about having children o However, they raise other issues, such as whether affected individuals fully understand their genetic test results • Fetal Testing o In amniocentesis, the liquid that bathes the fetus is removed and tested o In chorionic villus sampling (CVS), the sample of the placenta is removed and tested o Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero • Newborn Screening o Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States o One common test is for phenylketonuria (PKU), a recessively inherited disorder that occurs in one of every 10,000-15,000 births in the United States • Study Guide for Chapter 14 o Gregor Mendel and garden peas o What are the laws of segregation and independent assortment? o How did Mendel deduce these laws? o What is a Punnett square? o How can Punnett squares be used to make predictions about progeny from a cross? • Deviations from Mendelian patterns o Non-Mendelian inheritance patterns due to: § Codominance § Incomplete dominance § Multiple alleles § Pleiotropy § Epistasis § Polygenic inheritance § Multifactorial conditions Chapter 15 Video Lectures • Overview: Locating Genes Along Chromosomes o Mendel’s “hereditary factors” = genes o Now we know: genes are located on chromosomes o The location of a gene can be visualized • Concept 15.1: Mendelian inheritance has its physical basis in the behavior of chromosomes o Mitosis and meiosis discovered: late 1800s o The chromosome theory of inheritance states: § Mendelian genes have specific loci § Chromosomes explain laws of segregation and independent assortment • Morgan’s experimental Evidence: Scientific Inquiry o Genes on chromosomes discovered by T.H. Morgan o Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors o Drosophila melanogaster, another great model organism § most famous fruit fly § produce many offspring § generation time: two weeks § four pairs of chromosomes § XY sex determination schema o Morgan noted wild type, or normal, phenotypes that were common in the fly populations § wild type = normal, in nature o Traits alternative to the wild type are called mutant phenotypes • Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair o Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) § F1 generation à all red-eyed § F2 generation à 3:1 red:white eye ratio • only males had white eyes o White-eyed mutant allele must be located on the X chromosome o Support for the chromosome theory of inheritance • Concept 15.2: Sex-linked genes exhibit unique patterns of inheritance o In humans and some other animals, there is a chromosomal basis of sex determination • The Chromosomal Basis of Sex o In many mammals à X Y sex determination o Only the ends of the Y chromosome have regions that are homologous with corresponding regions of the X chromosome o The SRY gene on the Y chromosome directs the development of male features o Females are XX, and males are XY o Each ovum contains an X chromosome while a sperm may contain X or Y o NOT universal system • A gene: o on any sex chromosome à sex-linked gene o on the Y chromosome à Y-linked gene (few) o on the X chromosome à X-linked gene • Inheritance of X-Linked Genes o X chromosome have genes for many characters unrelated to sex o Y chromosome mainly encodes genes related to sex determination o X-linked genes follow specific patterns of inheritance o For a recessive X-linked trait to be expressed § A female needs two copies of the allele (homozygous) § A male needs only one copy of the allele (hemizygous) • if you have only one copy of the chromosome, then you will express that trait if you have the recessive allele present o X-linked recessive disorders are much more common in males than in females o Some disorders caused by recessive alleles on the X chromosome in humans § Red-green color blindness (mostly X-linked) § Duchenne muscular dystrophy § Hemophilia § (most of these are found in males) • X Inactivation in Female Mammals o In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development o à Barr body o If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character • Concept 15.3: Linked genes tend to be inherited together because they are located near each other on the same chromosome o Each chromosome has hundreds or thousands of genes (except the Y chromosome) o Genes located on the same chromosome tend to be inherited together § Linked genes • How Linkage Affects Inheritance o Morgan did other experiments with fruit flies to see how linkage affects inheritance of two characters o Morgan crossed flies that differed in traits of body color and wing size o Not pleiotropy because you wouldn’t always see those two characteristics inherited together o “Usually, but not always” o Morgan found that body color and wing size are usually inherited together in specific combinations (parental phenotypes) o He noted that these genes do not assort independently, and reasoned that they were on the same chromosome o However, nonparental phenotypes were also produced o Understanding this result involves exploring genetic recombination, the production of offspring with combinations of traits differing from either parent • Genetic Recombination and Linkage o The genetic findings of Mendel and Morgan relate to the chromosomal basis of recombination • Recombination of Unlinked Genes: Independent Assortment of Chromosomes o Mendel: combinations of traits in some offspring differ from either parent o Offspring with a phenotype matching one of the parental phenotypes are called parental types o Offspring with nonparental phenotypes are called recombinant types, or recombinants o A 50% frequency of recombination is observed for any two genes on different chromosomes • Recombination of Linked Genes: Crossing Over o Morgan discovered that genes can be linked § linkage is incomplete • some recombinant phenotypes o Therefore: some process must occasionally break the connection between linked genes o à crossing over explains these observations • New Combinations of Alleles: Variation for Normal Selection o Recombinant chromosomes à new alleles combinations in gametes o Random fertilization increases number of variant combinations produced o Abundance of genetic variation is the raw material upon which natural selection works • Mapping the Distance Between Genes Using Recombination Data: Scientific Inquiry o Alfred Sturtevant: § constructed a genetic map, an ordered list of the genetic loci along a particular chromosome § predicted that ‘the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency’ o A linkage map is a genetic map of a chromosome based on recombination frequencies o Distances between genes can be expressed as map units; one map unit, a centimorgan, represents a 1% recombination frequency o Map units indicate relative distance and order, not precise locations of genes § the closer two genes are, the lower their recombination frequency o Genes that are far apart on the same chromosome can have a recombination frequency near 50% § which is close to what you would observe if they were on different chromosomes o Such genes are physically linked, but genetically unlinked, and behave as if found on different chromosomes o Sturtevant used recombination frequencies to make linkage maps of fruit fly genes o Using methods like chromosomal banding, geneticists can develop cytogenetic maps of chromosomes o Cytogenetic maps indicate the positions of genes with respect to chromosomal features • Concept 15.4: Alterations of chromosome number or structure cause some genetic disorders o Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders o Plants tolerate such genetic changes better than animals do • Abnormal Chromosome Number o In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis o As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy o Aneuploidy results from the fertilization of gametes in which nondisjunction occurred o Offspring with this condition have an abnormal number of a particular chromosome o A monosomic zygote has only one copy of a particular chromosome o A trisomic zygote has three copies of a particular chromosome o Polyploidy is a condition in which an organism has more than two complete sets of chromosomes § Triploidy (3n) is three sets of chromosomes § Tetraploidy (4n) is four sets of chromosomes o Polyploidy is very common in plants, but not animals § plants are a lot more tolerant of this o Polyploids are more normal in appearance than aneuploids § which has an odd number of chromosomes • Alterations of Chromosome Structure o Breakage of a chromosome can lead to four types of changes in chromosome structure § Deletion removes a chromosomal segment § Duplication repeats a segment § Inversion reverses orientation of a segment within a chromosome § Translocation moves a segment from one chromosome to another • Human Disorders Due to Chromosomal Alterations o Alterations of chromosome number and structure are associated with some serious disorders o Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond o These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy • Down Syndrome (Trisomy 21) o Down syndrome § an aneuploidy condition that results from three copies of chromosome 21 § If affects about one out of every 700 children born in the United States § The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained • Aneuploidy of Sex Chromosomes o Nondisjunction of sex chromosomes produces a variety of aneuploidy conditions o Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY individuals o Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans • Disorders Caused by Structurally Altered Chromosomes o The syndrome cri du chat (“cry of the cat”), results from a specific deletion in chromosome 5 o A child born with this syndrome is mentally retarded and has a catlike cry; individuals usually die in infancy or early childhood o Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes • Study Guide for Chapter 15 o T H Morgan’s experiments with D. melanogaster o Demonstrated the chromosomal basis of inheritance o Supported the laws of segregation and independent assortment • Questions to answer o Why fruit flies? o How did Morgan’s findings support Mendel? o How does sex determination work in fruit flies? Other organisms? o Why are men more likely to display sex-linked recessive disorders? o What is X inactivation and what are Barr bodies? o Why did PT Barnum never have to pay for a male calico cat? *video 15.1 or 2 ? • Linked genes o How do you calculate recombination frequencies? o Terms: linkage, recombinant types, parental types, centimorgan o Nondisjunction, aneuploidy, polyploidy, monosomy, trisomy, deletion, inversion, translocation, duplication • Chromosomal level genetic disorders o Down syndrome, Turner syndrome, Klinefelter syndrome o Cri du chat, CML, Philadelphia chromosome *video 15.4 ? § all have to do with errors in parts of chromosomes rather than misplacement or entire chromosomes Chapter 16 Video Lectures • Overview: Life’s Operating Instructions o 1953: Watson and Crick introduce a model for the structure of DNA, a double helix o DNA, the substance of inheritance o Hereditary information is encoded in DNA and reproduced in all cells of the body o DNA directs development of § Biochemical § Anatomical § Physiological § Behavioral* traits • *to some extent • The Search for the Genetic Material: Scientific Inquiry o T. H. Morgan’s group showed that genes are located on chromosomes o Two components of chromosomes– § DNA and protein § But which is genetic material? o The role of DNA in heredity was first discovered by studying bacteria and the viruses that infect them • Evidence that DNA Can Transform Bacteria o The discovery of the genetic role of DNA began with research by Frederick Griffith in 1928 o Griffith worked with two strains of a bacterium, one pathogenic and one harmless o 1944: Avery, McCarty, and MacLeod announce that the transforming substance is DNA o Only DNA worked in transforming harmless bacteria into pathogenic bacteria o Many biologists remained skeptical, mainly because little was known about DNA • Evidence That Viral DNA Can Program Cells o More evidence for DNA as the genetic material came from studies of viruses that infect bacteria o Such viruses, called bacteriophages (or phages), are widely used in molecular genetics research • Additional Evidence That DNA Is the Genetic Material o DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group o 1950: Erwin Chargaff reports that DNA composition varies between species o DNA: a more credible candidate for the genetic material? o Two findings became known as Chargaff’s rules § The base composition of DNA varies between species § In any species the number of A and T bases are equal and the number of G and C bases are equal o The basis for these rules was not understood until the discovery of the double helix o Watson and Crick built models of a double helix to conform to the X-rays and chemistry of DNA o Franklin had concluded that there were two outer sugar-phosphate backbones, with the nitrogenous bases paired in the molecule’s interior o Watson built a model in which the backbones were antiparallel (their subunits run in opposite directions) o Watson and Crick reasoned that the pairing was more specific, dictated by the base structures o They determined that adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C) o The Watson-Crick model explains Chargaff’s rules: in any organism the amount of A = T, and the amount of G = C • Concept 16.2: Many proteins work together in DNA replication and repair o The relationship between structure and function is evident in the double helix o Watson and Crick noted that base pairing suggested a copying mechanism for genetic material o Watson and Crick’s semiconservative model of replication predicts that when a double helix replicates, each daughter molecule will have one old strand (derived or “conserved” from the parent molecule) and one newly made strand o Competing models were the conservative model (the two parent strands rejoin) and the dispersive model (each strand is a mix of old and new) o Experiments by Meselson and Stahl supported the semiconservative model o They labeled the nucleotides of the old strands with a heavy isotope of nitrogen, while any new nucleotides were labeled with a lighter isotope • DNA Replication: A Closer Look o The copying of DNA is remarkable in its speed and accuracy o More than a dozen enzymes and other proteins participate in DNA replication • Getting Started o Replication begins at particular sites called origins of replication, where the two DNA strands are separated, opening up a replication “bubble” o A eukaryotic chromosome may have hundreds or even thousands of origins of replication o Replication proceeds in both directions from each origin, until the entire molecule is copied o At the end of each replication bubble is a replication fork, a Y-shaped region where new DNA strands are elongating o Helicases are enzymes that untwist the double helix at the replication forks o Single-strand binding proteins bind to and stabilize single-stranded DNA o Topoisomerase corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands o DNA polymerases cannot initiate synthesis of a polynucleotide; they can only add nucleotides to the 3’ end o The initial nucleotide strand is a short RNA primer o An enzyme called primase can start an RNA chain from scratch and adds RNA nucleotides one at a time using the parental DNA as a template o The primer is short (5-10 nucleotides long), and the 3’ end serves as the starting point for the new DNA strand • Synthesizing a New DNA Strand o Enzymes called DNA polymerases catalyze the elongation of new DNA at a replication fork o Most DNA polymerase require a primer and a DNA template strand o The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells o Each nucleotide that is added to a growing DNA strand is a nucleotide triphosphate o dATP supplies adenine to DNA and is similar to the ATP of energy metabolism o dATP has deoxyribose while ATP has ribose o As each monomer of dATP joins the DNA strand, it loses two phosphate groups as a molecule of pyrophosphate • Antiparallel Elongation o The antiparallel structure of the double helix affects replication o DNA polymerases add nucleotides only to the free 3’ end of a growing strand; therefore, a new DNA strand can elongate only in the 5’ to 3’ direction o Along one template strand of DNA, the DNA polymerase synthesizes a leading strand continuously, moving toward the replication fork o To elongate the other new strand, called the lagging strand, DNA polymerase must work in the direction away from the replication fork o The lagging strand is synthesized as a series of segments called Okazaki fragments, which are joined together by DNA ligase • The DNA Replication Complex o The proteins in DNA replication form a large complex, a “DNA replication machine” o The DNA replication machine may be stationary during the replication process o Recent studies support a model in which DNA polymerase molecules “reel in” parental DNA and “extrude” newly made daughter DNA molecules • Proofreading Repairing DNA o DNA polymerases “proofread” newly made DNA, replacing any incorrect nucleotides o In mismatch repair of DNA, repair enzymes correct errors in base pairing o DNA can be damaged by exposure to harmful chemical or physical agents such as cigarette smoke and X-rays; it can also undergo spontaneous changes o In nucleotide excision repair, a nuclease cuts out and replaces damaged stretches of DNA • Evolutionary Significance of Altered DNA Nucleotides o Error rate after proofreading repair is low but not zero o Sequence changes may become permanent and can be passed on to the next generation o These changes (mutations) are the source of the genetic variation upon which natural selection operates • Replicating the Ends of DNA Molecules o Limitations of DNA polymerase create problems for the linear DNA of eukaryotic chromosomes o The usually replication machinery provides no way to complete the 5’ ends, so repeated rounds of replication produce shorter DNA molecules with uneven ends o This is not a problem for prokaryotes, most of which have circular chromosomes o Eukaryotic DNA molecules have special nucleotide sequences at the ends, telomeres o Telomeres do not prevent the shortening of DNA molecules, but they do postpone the erosion of genes near the ends of DNA molecules o It has been proposed that the shortening of telomeres is connected to aging o Telomeres are series of repeats of a short (6-8 bp) DNA sequence o May have 100s to 1000s of repeats o If chromosomes of germ cells became shorter in every cell cycle, essential genes would eventually be missing from the gametes they produce o An enzyme called telomerase catalyzes the lengthening of telomeres in germ cells o The shortening of telomeres might protect cells from cancerous growth by limiting the number of cell divisions o There is evidence of telomerase activity in cancer cells, which may allow cancer cells to persist o Chromatin undergoes changes in packing during the cell cycle o At interphase, some chromatin is organized into a 10-nm fiber, but much is compacted into a 30-nm fiber, through folding and looping o Though interphase chromosomes are not highly condensed, they still occupy specific restricted regions in the nucleus o Most chromatin is loosely packed in the nucleus during interphase and condenses prior to mitosis o Loosely packed chromatin is called euchromatin o During interphase a few regions of chromatin (centromeres and telomeres) are highly condensed into heterochromatin o Dense packing of the heterochromatin makes it difficult for the cell to express genetic information coded in these regions Chapter 17 Video Lectures • The Flow of Genetic Information o The information content of genes is in the specific sequences of nucleotides o The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins o Proteins are the links between genotype and phenotype o Gene expression the process by which DNA directs protein synthesis, includes two stages: transcription and translation • Concept 17.1: Genes specify proteins via transcription and translation o How was the fundamental relationship between genes and proteins discovered? • Nutritional Mutants in Neurospora: Scientific Inquiry o George Beadle and Edward Tatum exposed bread mold to X-rays, creating mutants that were unable to survive on minimal media o Using crosses, they and their coworkers identified three classes of arginine- deficient mutants, each lacking a different enzyme necessary for synthesizing arginine o They developed a one gene–one enzyme hypothesis, which states that each gene dictates production of a specific enzyme • The Products of Gene Expression: A Developing Story o Some proteins aren’t enzymes, so researchers later revised the hypothesis: one gene–one protein o Many proteins are composed of several polypeptides, each of which has its


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