Bio 240 Exam 2 Study Guide!
Bio 240 Exam 2 Study Guide! Bio 240
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This 24 page Study Guide was uploaded by Izabella Nill Gomez on Saturday October 10, 2015. The Study Guide belongs to Bio 240 at University of Tennessee - Knoxville taught by Dr. Hughes in Summer 2015. Since its upload, it has received 182 views. For similar materials see General Genetics (Bio 240) in Biology at University of Tennessee - Knoxville.
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Date Created: 10/10/15
Biology 240 Exam 2 Study Guide Biology 240 Chapter 25 notes Evoution is a consequence of changes in genetic material through mutation and changes in allele frequencies in populations over time Union of population genetics with the theory of natural selection is neoDarwinism Speciation formation of new species It is facilitated by environmental diversity If a population is spread over a geographic range encompassing a number of ecologically distinct subenvironments with different selection pressures populations occupying these areas may gradually adapt and become distinct from one another These may remain in existence continue to diverge or become extinct or reunite with other populations Populations that are reproductively isolated are different speCIes Microevolution evolutionary change within populations of a species Macroevolution evolutionary events leading to the emergence of new species and new taxonomic groups Variation in Homo sapiens is low only 9 because of race 6 because of geographic location Population group of individuals belonging to the same species that live in a de ned geographic area and actually or potentially of a population Most populations contain a high degree of heterozygosity One way to determine if genetic variation exists in a population is arti cial selection if little to no variation arti cial selection will have no effect on the phenotype if genetic variation present phenotype will change over a few generations Ex domestic dog Gene pool all alleles in a population and the genetic information that the members carry There is an enormous reservoir of genetic variability in populations alleles representing these variations are distributed among members of a population Variations between populations re ect the product of local adaptation or geographic isolation Variation among homologous DNA sequences ingle nucleotide polymorphisms There are approximately 1 million human SNPssingle base pair mutationsthese are detected by microarraydots are the binding of SNPs and hybridized Some SNPs are inherited from crossing over in meiosis Microsatellites are areas where there a short repeated segments can be used as DNA markers Ex CACACA It the DNA sequence is known on either end of the repeat the repeat can be PCR ampli ed Microsatellites can expand and contract Mutations are inherited Neutral theory of molecular evolution proposes that mutations leading to amino acid substitutions are usually detrimental with only a very small fraction being favorable Some mutations are neutralfunctionaly equivalent to replaced allele Favorabledetrimental ones are preservedeliminated by natural selection The frequency of neutral alleles in a population is determined by mutation rates and random genetic drif not selection Some neutral mutations will drift to xation others will be lost Diversity of alleles at most loci doesn39t re ect action of natural selection but is a function of population size largermore variation Other explanation for high genetic variation is natural selection Some enzyme or protein variations are maintained by adaptation Ex sicklecell anemia heterozygotes to malaria Neutra theorv points out that some genetic variation is expected simply because of mutation and drift and studies reason for molecular evolution Changes in allele frequencies in a population that do not result in reproductive isolation are examples of microevolution HardyWeinberg Law describes what happens to allele and genotype frequencies in an quotidealquot population that is in nitely large and randomly mating not subject to evolutionary forces such as mutation migration or selection 1 Frequencies in alleles in gene pool do not change over time 2 If two alleles at a locus A and a are considered then after one generation of random mating frequencies of AAAaaa in populations can be calculated as 2 2 P 2Pqq 1 where pthe frequency of allele A and qthe frequency for a It is rare for any population to be in HardyWeinberg ln HardyWeinberg all genotypes have equal rates of survival and reproduction ln next generation all genotypes contribute equally to the new gene pool Chisquare can be used to check whether deviations from the expected frequencies are larger than expected If the deviation is larger than expected one or more HardyWeinberg assumptions are being violated Assumptions of HardyWeinberg 1 Individuals of all genotypes have equal rates of survivalno selection 2 No new alleles are created by mutation 3 No individuals migrate into or out of the population 4 Population is in nitely large no sampling error 5 Mating is random By specifying conditions under which the population does not evolve Hardy Weinberg can be used to identify realworld forces that cause allele frequencies to change Application of this model can reveal quotneutral genesquot in a population gene pool not operated by forces of evolution HardyWeinberg shoes that dominant traits do not necessarily increase from one generation to the next Second it demonstrates genetic variability can be maintained in a population since once established in an ideal population allele frequencies remain unchanged Third knowing the frequency if one genotype allows to calculate all other genotypes at that locus It is useful to calculate the frequency of heterozygote carriers for recessive disorders According to HardyWeinberg 2 2 genotype frequencies are predicted to t P 2Pqq 1 relationship If they do not one of the assumptions or more have been violated HardyWeinberg can be used to calculate allele frequencies for Xlinked traits The frequency of Xlinked alleles in the gene pool and the frequency of males expressing the Xlinked trait is the same Natural selection individuals of a species exhibit variations in phenotypedifference in size agility coloration etc Many of these variations are passed on to offspring Organisms tend to reproduce in an exponential fashion more are reproduced than can survive This causes a struggle for survival in this struggle individuals with particular phenotypes will be more successful than others allowing the former to survive and reproduce at higher rates Weak selection may just involve a fraction of a percent difference in survival rates Fitness individual organisms39 genetic contribution to future generations Genotypes with high rates of reproductive success have high tness A homozygous recessive individual that has died before reproduction has a tness W0 Selection against deleterious alleles show frequency of allele decline with qg6 frequency of allele in generation g 18Q0 qozb starting frequency of allele qg 32L number of generations passed There is a rapid decline of the allele then a plateau to be expected because the homozygotes have been eliminated and heterozygotes are present not selected against As more time passes the elimination is slower As long as heterozygotes mate it39s dif cult to eliminate the deleterious recessive allele For selection to produce rapid changes in allele frequencies differences in tness among genotypes must be large Phenotype is the result of combined in uence of the individual39s genotype at many different loci and the effects of the environment Directional selection phenotypes at one end of the spectrum present in the population become selected for or against usually as a result of changes in the environment Ex Galapagos finches beak size bacterial resistance Stabilizing selection tends to favor intermediate phenotypes with those at both extremes being selected against Over time it reduces the phenotypic variance Ex human birth weight Disruptive selection selection against intermediates and for both phenotypic extremes Opposite to stabilizing selection Within a population the gene pool is reshuf ed in each generation to produce new genotypes in offspring Allows for new genotypic combinations Assortment and recombination do not produce new alleles Mutation alone acts to create new alleles Mutational events occur at random For dominant mutations 1 The allele must produce a distinctive phenotype that can be distinguished from similar phenotypes produced by crossover alleles 2 The trait must be fully expressedcompletely penetrant 3 Identical phenotypes must not be produced by nongenetic agents drugs chemicals Mutation rates can be stated as the number of new mutant alleles per given number of gametes Migration occurs when individuals move between populations Migration can change the frequency of an allele by breeding with another population from the same locus p1 how migration a ects frequency of the allele m migrants from mainland to island and migration is random pm frequency of allele on mainland 916 frequency on island change in allele frequency is attributable to migration is proportional to the differences in allele frequency between the donor and recipient populations and to the rate of migration If m is large or pm is different from pi a large change in frequency of that allele can occur in a single generation If migration is the only acting force equilibrium will be attained when pizpm Genetic drift in small populations signi cant random uctuations in allele frequencies are possible by change alone The degree of uctuation increases as population size decreases Drift can also arise through the founder effect which occurs when a population originates from a small number of individuals Genes carried by all members derive from the founders like homozygous cheetahs Drift can also arise via genetic bottleneck Develops when a large population undergoes a drastic but temporary reduction in numbers Even though the population recovers genetic diversity is greatly reduced Ex Navajo Indians with albinism Nonrandom mating can change the frequency of genotypes in a population Subsequent selection for or against certain phenotypes has the potential to affect the overall frequencies of the alleles they contain but it is important not that nonrandom mating does not itself directly change allele frequencies Positive assortive mating similar genotypes are more likely to mate than dissimilar ones Ex humans are attracted to people that resemble themselves not selected for ABO blood groups MN etcthey foIIow HardyWeinberg Negative assortive mating dissimilar genotypes are more likely to mate Ex plants that cannot selffertilize so they crosspollinate Nonrandom mating changes genotype frequency but not allele frequency Changes phenotype of individuals Inbreeding has most commonly found to affect genotype frequencies Occurs when mating individuals are more closely related than any two individuals drawn from the population at random Inbreeding increases the proportion of homozygotes in a population Coefficient of inbreeding F quanti es the probability that the alleles of a given gene in an individual are identical because they are descended from the same single copy of the allele in an ancestor If F1 homozygous F0 no individual has two alleles derived from a common ancestral copy Species a group of actually or potentially interbreeding organisms that is reproductively isolated in nature from all other such groups In sexually reproducing organisms speciation transforms the parental species into another species or divides a single species into two or more Genetic divergence of populations with considerable genetic variation present as differences in allelesallele frequencies can re ect the action of natural selection drift or both Sympatric speciation process through which new species evolve from a single ancestral species while inhabiting the same geographic region Allopatric speciation occurs when populations of the same species become isolated from each other to an extent that prevents or interferes with genetic exchange Different geographical ranges are occupied The most common form of speciation Once migration stops species form Reproductive isolating mechanisms are biological barriers that prevent or reduce interbreeding between populations Prezygotic isolating mechanisms prevent individuals from mating unable to by some reason Prezygotic isolating mechanisms create reproductive isolation even when two members are willing to mate Viabilityfertility may be reduced or hybrids are sterile Others Ecoogica Behaviora Mechanical Physioogica Seasonal Average time for speciation is 100000 years but can occur in a much shorter time span Ex cichlids in in Nicaragua Phylogeny can be used to determine the evolutionary history of a species by establishing the relationships between them Can be constructed by amino acid sequences through the sequence of cytochrome C a mitochondrial protein that has changed slowly over time Genetic equidistance differences in amino acid sequence between species and major groups is proportional to evolutionary distance Minimal mutational distance nucleotide changes necessary for all amino acids Differences in a protein are totaled Molecular clocks measures the rate of change in amino acid or nucleotide sequences in species Ex Africans provide the highest levels of diversity from original Homo sapiens Nonafricans originated 50000 years ago Neanderthals 30000 years ago coexisted with sapiens are 997 identical Possible test questions The parental frequency of a generation is M5 If the population is in Hardy Weinberg equilibrium the frequency of the next generation will be M5 Is the population evolving No Given allele that results in disease state in humans may nevertheless be propagated in populations because the disease has a late onset the individual is heterozygous for the disease and the wild type has a selective advantage In assertive mating there is a de ciency of heterozygotes Biology 240 Chapter 6 notes Bacteria and their viruses have extremely short reproductive cycles They can also be studied in pure culturesinge species or mutant strain of bacteria or one type of virus can be isolated and investigated independently of other similar organisms Bacteria are essential to studies due to their easy manipulation and generation of solid results for researchers Genetically homogeneous bacteria cultures can give rise to cells with inheritable variation under unique environmental conditions Adaptation hypothesis implies that the interaction fo the phage and bacterium is essential to acquisition of immunity Exposure quotinducesquot resistance Spontaneous mutations occur regardless of the presence or absence of bacteria phage T1 is another alternative to the resistance of E coli Fluctuation tests mark initiations of modern bacterial genetic studies Mutant cells that arise can be isolated by selection culturing organisms under conditions where only the desired mutants grow well Bacteria are usually haploid so all mutations are directly expressed in the descendants Bacteria are grown either in liquid or a semisolid agar surface Minimal mediums contain only one organic carbon source Z zK glucoselactose various inorganic ions such as ZNa etc can be added as Clquot well To grow on the medium bacteria must be able to synthesize all nutrients being prototrophic wild type The mutant that cannot synthesize one or more nutrients is an auxotroph A complete medium has been extensively supplemented with nutrients an auxotroph might not be able to synthesize If indicated by a minus sign after a nutrientamino acid the auxotroph cannot metabolizesynthesize the chemical If an antibiotic is present with an r the bacteria is resistant if with an s the bacteria is susceptible and cannot resist the antibiotic and survive The growth pattern of bacteria Lag phase slow Log phase period of rapid exponential growth cells divide continually 9 Stationary phase reached when the cell density is maximized to about 10 cellsmL in a petri dish Genetic recombination here refers to the replacement of one or more genes present in a chromosome of one cell with those from the chromosome of a genetically distinct cell Vertical gene transfer transfer of genetic information within a species Horizontal gene transfer transfer of genetic information between species Plays a role in the evolution of bacteria Those transferred horizontally usually convey a selective advantage Ex antibiotic resistance Conjugation process by which genetic information from one bacterium is transferred to and recombined with that of another bacterium Ex two auxotrophs are mated to create a colony of prototrophs 4 Fquot cells cell donors that give part of their chromosomes F for fertility Fquot cells receive donor chromosome material DNA and recombine with their own chromosome Cel to cell contact is essential for chromosome transfer and genetic recombination This is the initial stage for conjugation and is mediated by the F pilus sex pilus a 69 mm tubular extension of the cell After contact is initiated chromosome transfer is possible The pilus carries DNa from one bacteria to another and is easily broken affecting the transfer of DNA partial transfer occurs 4 Fquot cells have a Fertility factor that confers the ability to donate a part of their chromosome during conjugation Certain environmental factors eliminate the F factor from fertile cells However if quotinfertilequot cells are grown with fertile donor 2 4 cells the F factor is regained The F factor is mobile Fquot becomes Fquot and the F factor is passed to all recipient cells The F factor consists of a circular double stranded DNA molecule equivalent to 2 of the chromosome and has 40 genes tra genes transfer genetic information involving genes for formation of the pilus since Fquot cannot form a pilus on its own The transfer of the F factor involves the separation of one strand to move into the recipient Both are replicated F factor is really an autonomous genetic unit called the plasmid DNA that is transferred to the receiving bacterial cells are nonfunctional unless they are incorporated into the bacterial chromosome Proteins mediate this recombination Hfr high frequency recombination cells undergo recombination 1000x more than 4 the F factor cells alone and constitute a special class of Fquot cells If the donor 4 cell is Hfr recipients never become Hfr and remain Fquot Hfr bacteria are produced by the spontaneous integration of a plasmid with transposable elements DNA oaters The plasmid recognizes the site can integrate into the bacterial chromosome and still exchange information to Hfr and bacterial genes Conjugation 4 is usually interrupted between Fquot and Hfr before the chromosome transfer is complete Z ZFquot Hg FL reCIpent turns F low recombination rate 6 4 HfrgtIltFquot recipient remains Fquot high recombination rate Hfr contains a nonrandom pattern of gene transfer but varies Hfr is helpful for chromosome mapping of E coli Interrupted mating technique sets two strains in a blender and interrupts mating at certain intervals Demonstrated that depending on the Hfr strain certain genes are transferred and recombine sooner than others Chromosome of Hfr bacterium is transferred linearly so gene order and distance between genes can be predicted through time mapping which uses time to measure recombination Order in which Hfr genes are donated to a recipient vary The E coli chromosome is circular because the point of origin transfer varied In various Hfr strains the F factor integrated into the chromosome at different points and the position determined the origin 0 site Conjugation rarely if ever lasts long enough to allow the chromosome to pass through conjugation tube leaving recipient cells from Hfr to 2 F6 Z ZgtIltFquot F1 receives F factor but no recombination occurs the F factor can lose its integrated status When this occurs the F factor frequently Z carries several adjacent genes with itdenoted F a special case of Fquot F 4 4 behaves like Fquot by initiating conjugation with Fquot when this occurs the F 4 factor is transferred to Fquot and replicated making duplicates of genes because the recipient still has a whole chromosomethis creates a merozygote Rec genes play an essential role in recombination mutants diminish The gene product Rec A protein play an important role in recombination with single stranded DNA or the linear end of a double stranded DNA that has unwound Singlestrand displacement is a common form of recombination in bacterial species Rec A facilitates recombination of a homologous region of the host chromosome When double stranded DNA enters the recipient cell one stand is often degraded leaving the other complementary as a source of recombination Must nd homologous region along the host and the Rec A facilitates recombination Rec BCD protein is an enzyme consisting of polypeptide subunits encoded by 3 other rec genes Important for when double stranded DNA is a source of genetic recombination Rec BCD unwinds the helix facilitating recombination with Rec A Rec A and BCDunwind DNA to make it single stranded degrade and knick the pieces Rec A is loaded onto the molecule BCD disassembles and allows rec A to create a crossing over double stranded heteroduplex for repair and integration of genes Plasmids Ex F factors Often exist in multiple copies in the cytoplasm each may contain one or more genes and often have a few Replication depends on the same enzymes that replicate the chromosome of the host cell and are distributed to daughter cells along with the host chromosome during cell division Many plasmids are con ned to the cytoplasm Others such as the F factor can integrate into the host plasmids that can exist autonomously or integrate into host episomes Plasmids can infect different bacteria and pick up pieces of DNA and send to other bacteria this is how antibiotic genes are spread Some genes are born by plasmids ex F factor toxin gene R plasmids consist of resistant transfer factors RTF and one or more determinants RTF encoded genetic information essential to transferring the plasmid between bacteria R determinants are genes conferring resistance to antibiotics or heavy metals such as mercury There are many similar RTFs but not rdeterminants each speci c to one antibiotic There can be resistance to many or several antibiotics However there can be many rdeterminants but not RTFs information cannot be transferred Col Plasmid ColEl from E Coli Encodes 1 or more porteints highly toxic to bacterial strains that do not have the same plasmid Colicin protein can kill neighboring bacteria and bacteria carrying the plasmid are coicinogenic Col plasmids also have gene encoding immunity proteins that protect host cells from the toxin Col plasmids are usually not transmissible Transformation another mechanism for recombining genetic information in some bacteria Small pieces of extracellular DNA is taken up by living bacterium potentially leaving to stable genetic change in the recipient cell Also allows for incorporation of antibiotic resistance Important to bacterial survival special receptors allow for DNA to enter cell Double stranded DNA crosses the complex and one strand is degraded by nucleases DNA entering the cell is single stranded integrated with chromosomal DNA through rec A Can be used to map genes although more limitedly than conjugation PFOCESSE 1 Entry of foreign DNA into recipient cell 2 Recombination between foreign DNA and homologous region in recipient chromosome The rst step can occur without the second step although it will not lead to recombination results in addition of foreign DNA to bacterial cytoplasm but not chromosome Competence physiological state necessary to take up DNA Various kinds of bacteria undergo transformation naturally and others can be induced Entry of DNA occurs at limited number of receptor sites on the surface of competent cells Passage through cell is an active process that requires energyspeci c transport molecules Substances that inhibit energy production or protein synthesis inhibit transformationfor recombination to be detected the transforming DNA must be derived from a different strain of bacteria that bears variation mutation Once integrated the recombinant region contains one host present originally mutant start Heteroduplex is the region named after this Mismatch of base sequences activates the repair process following repair and DNA replication once chromosome has the original recipient sequence and one with the mutant Following division one nonmutant cell and mutant cell is produced Cotransformation of several genes can occur simultaneously Genes close enough to each other are linked linkage here is the proximity of genes to permit co transformation If not linked the simultaneous transformation occurs only as a result of two independent events involving two distinct segments of DNA chances are low Bacteriophagesphages viruses that have bacteria as a host Bacterial reproduction of phages can lead to another mode of recombination transduction Bacteriophage T4 is a group of relates viruses referred as Teven phages lts DNA is contained by an icosahedral 20 faced protein coat and can encode more than 150 average genes The tail bers contain binding sites in tips that recognize unique areas of the outer surface of the cell wall of most E coli The life cycle of page T4 begins when the virus binds to the bacterial host cell by adsorption Then ATPdriven contraction of the tail causes the core to penetrate the cell wall and the DNA enters the host DNA RNA and protein synthesis is inhibited from the host and sunthesis of viral molecules begins At the same time degradation of the host DNA occurs Infection period is intense viral gene activity Initially the phage DNA replication occurs leading to a pool of viral DNA then body is synthesized the host cell lysed and phages released 1 DNA packaging as viral heads assembled 2 Tail assembly 3 Tail ber assembly Approximately 200 new viruses are constructed Phage gene product lysozyme ruptures the cell through lysis and completes the lytic cycle Plaque assay determines the number of bacteriophages that are produced during infection Plaque occurs wherever a single virus initially infected one bacterium in the culture that has grown during recombination Represents clones of single infecting bacteriophages If the dilution factor is low plaques are plentiful lysing a whole lawn of bacteria lf dilution is increased plaques can be counted The initial phage densityplaque mL x dilution factor Essential in mutational and recombinational studies of bacteriophages lnfection of a phage does not always result in viral reproductions and lysisviruses can coexist with bacteria Upon entry viral DNA is integrated into the host chromosome instead of replicating in the bacterial cytoplasmysogeny So for each of the bacterial chromosomes replicated viral DNA is also replicated and passed on No new viruses are produced no lysis Under certain circumstance however it can occur ex ultraviolet radiation or light treatment Prophage viral DNA is integrated into the bacterial chromosome Temperate phages viruses that lyse the cell or behave as prophages Virulent phages only lyse bacteria The number of bacterium that harbors a prophage is lysogenized and lysogenic capable of being lysed as a result of induced viral reproduction Viral DNA is an episome that can replicate either in the cytoplasm of the cell or is part of its chromosome Transduction the process of bacterial recombination mediated by bacteriophages Sometimes a small piece of bacterial DNA can be packaged along with viral chromosomes or instead of it Only a few bacterial genes are present in the transducing phage the ability to transduceinfect is unrelated to the type of DNA in the phage head making the transduction possible When bacterial DNA is injected into the host it either remains in the cytoplasm or recombines with homologous region of the bacterial chromosome If the bacterial DNA remains in the cytoplasm it is transmitted to the progeny but not replicated Only a single cell is partially diploid and is producedabortive transduction lf bacterial DNA recombines with the host complete transduction occurs where transduced genes become a permanent part of the chromosome which is passed on to all daughters Both are subclasses of generalized transduction characterized by the random nature of DNA fragments and genes that are transduced most are the abortive type 1020x less frequent than complete transduction Specialized transduction occurs when transfer of bacterial DNA is not random but only the strainspeci c genes are transduced Occurs when DNA representing temperate phages break out of the host chromosome bringing with it bacterial DNA on either of its ends Transduction is also used for linkage and mapping studies of bacterial chromosomes Cotransduction can occur when the fragment of bacterial DNA involved is large enough to include several genes Transduction studies by studying 2 or 3 linked genes can determine the precise order of those genes Phage mutations often affect the morphology of plaques formed following lysis of bacterial cells In wildtype phages reproduction is inhibited once a particular sized plaque is formed Rlysis r mutants of T2 phages overcome this producing larger plaques The host range h mutants extends the range of bacterial hosts a phage can haveconfers with the ability to adsorb E coli B2 unlike its wild type counterpart Genetic recombination of bacteriophages was discovered during mixed infection experiments in which 2 nutrient strains simultaneously infect the same bacterial culture Mixed infection experiments are designed so that the number of viral particles sufficiently exceeds the number of bacterial cells to ensure simultaneous infection of most cells nu both viral strains If 2 loci are involved recombination is intergenic Ex study of E coli with parental viruses hr wild type locus range rapid lysis or hr extended host range normal lysis genotype If no recombination occurs the parental would be the offspring genotype Recombinants hr and hr are detected in addition to parental genotypes Recombination frequencyhrhrtotal plaques x 100 Similar studies to eukaryotes have been performed Recombination is not restricted to 2 chromosomes As page development progresses chromosomes are randomly removed from the pool and packed into the phage head forming mature particles Thus a variety of parental and recombinant genotypes are represented in the progeny Possible exam questions 1 What are the possible mediums prototrophs can inhabit minimal and complete 2 Recombination in bacteria is better with 2 crossovers Biology 240 Chapter 7 notes Meiosis produces haploid gametes after fertilization a diploid zygote Meiosis ensures genetic constancy within members of the same species These events depend ultimately on an ef cient union of gametes during fertilization In turn successful fertilization depends on some form of sexual differentiation Heteromorphic chromosomes dissimilar chromosomes such as the XY pair in mammals characterize one sex or the other in a wide range of species resulting in sex chromosomes Genes rather than chromosomes ultimately serve as the base for sex determination Some of these genes are present on sex chromosomes but others are autosomal Primary sexual differentiation involves only gonads where gametes are produced Secondary sexual differentiation involves the overall appearance of the organism including clear differences in mammary glands external genitalia and other non reproductive organs Differentiation of sexes is evident via phenotypic dimorphism Homogametes produce like chromosomes ex zygotes with two X sresults in female Heterogametes produce two different chromosomes ex zygotes with X and Y results in male Humans are a heterogametic system Unisexual diecious gonochoic refer to an individual containing only male or only female reproductive organs Bisexual monoecious hermaphrodite individuals with both male and female reproductive organs common in plants lntersex refer to those with intermediate conditions mostly sterile Chlamydomonas green algae with infrequent periods of sexual reproduction Stays mostly in the haploid phase asexually reproducing daughter cells by mitosis Under unfavorable nutrient conditions certain daughter cells function as gametes joining in fertilization and a diploid zygote is formed to withstand unfavorable conditions There is little difference between haploids and gametes lsogametes are two gametes that fuse and are not distinguishable from one anotherthe species producing them are isogamous Gametes in a Chlamydomenac has two mating typespus and minusthese form zoospores They differ chemically to tell their sexuality Zea mays these plants undergo both haploid gametophyte stage and diploid sporophyte stage Meiosis and fertilization link the processes together Maize is a monoecious plant sporophyte phase dominate the life cycle Both male and female structures are present Sexual differentiation occurs in the tissues of the plant The stamen produces the mother cells which produce the male microspores pollen grain The pistil produces female cells Pollination occurs in the pistil and double fertilization takes placeproducing kernels C elegans nematode is a popular organism in genetic studies particularly for investigating genetic control of development Males only have testes hermaphrodites during larval development form two gonads which produce sperm and eggsthey selffertilize Less than 1 of offspring become just male Heterogametic sex malesXY Homogametic sex femalesXX The presence or absence of the X chromosome in male gametes provides an efficient mechanism for sex determination The male is not always the heterogametic sex this depends on the species Ex the female chicken is ZW and males are 22 The Y chromosome determines maleness in humans Klinefelter syndrome 47 XXY individuals with this abnormality are generally tall with long arms and legs long handsfeet Usually have internal male genitaliaducts but cannot produce sperm The feminine sexual development of the individual is not entirely suppressed rounded hips slight breasts are produced The IQ level is lower 1660 male births have this syndrome Turner syndrome 45 X the individual has female externalinternal genitaliaducts but the ovaries are rudimentary Marked by short stature skin folds on the neck lower IQ and a childish development This occurs in 12000 female births Both conditions result from nondisjunction or the failure of X chromosome to segregate during meiosis Even with two X39s one Y determines maleness A lack of Y de nes female development lndividuas with more XXY39s leads to more severe conditions of Klinefelter39s Turner39s syndrome can also result from karyotypes other than 45 X including individuals called mosaics whose somatic cells display two different genetic cell lines each with a different karyotype due to mitotic error in early development Ex 45 X 46XY 45X 46 XX A majority of 45 X die in utero and are aborted spontaneously 47 XXX syndrome results in female differentiation triploX occurs in 11000 female births Some have perfectly normal development In 48 XXXX this results in underdeveloped sex characteristics sterility and lower IQ The same occurs in 49 XXXXX only the conditions are more pronounced Additional X s disrupt the delicate balance of genetic information essential to normal female development 47 XYY results in above average height used to be associated with huge antisocial criminal acts Subnormal intelligence and personality disorders are other possible characteristics No real case study has ever been performed on this condition due to ethical reasons Mode of sex determination ProtenorXXXO mode of sex determination depends on the random distribution of X chromosome into half of male gametes The presence of two X39s results in female offspring and X0 determines male common in butter ies Lygaeus milkweed bug XXXY mode of sex determination female gametes have one X chromosome male gametes have either an X or Y chromosome humans follow this mode of determination During early development every human embryo undergoes a hermaphroditic stage By the 5th week of gestation period gonadal primordia arise as a pair of gonadal genital ridges associated with the embryonic kidney As development progresses germ cells migrate to the right where the outer cortex and inner medulla form The cortex can develop into an ovary and the medulla into testes bi potential gonads Presence or absence of the Y chromosome is the key to gonad development By the 7th week XY39s develop into testes The presence of the Y also inhibits the formation of female reproductive organs The Y chromosome has about 50 genes compared to 1000 on the X Both are believed to have originated from two homologous autosomal chromosomes 200 million years ago At both of the Y are pseudoautosomal regions PARS that share homology with the X chromosome and synapse and recombine during meiosis Pairing is critical for segregation during male gametogenesis X and Y The rest of the chromosome 95 does not synapseit is a nonrecombining region of the Y NRYor malespeci c region of the Y MSY Some proteins share homology with genes on the X chromosome and are divided into euchromatic with functional genes and heterochromatic lacking genes regions The sex determining region Y SRY within euchromatin are adjacent to PAK of the short arm Absence of the Y almost always leads to female development lack of SRY At 68 weeks of development the SRY gene is active in XY it encodes a protein that triggers testes developmentTestisdetermining factor TDF SRY is present in all mammals TDF is a transcription factor a DNA binding protein that interacts directly with regulatory sequences of other genes to stimulate expression TDF acts as a maser switch that controls other genes Xtransposed region comprises about 15 of MSY and is originally derived from the X chromosome in the course of human evolution 99 of it is identical to Xq21 of the modern chromosometow genes both with X chromosome homologs are present in this region Palindrones are sequences of base pairs that read the same but in the opposite direction on the complimentary strands and are present through MSY Recombination between palindrones occurs on sister chromatid is the Y during replication for the purpose of mutation repair The second area of the MSY is the Xdegenerative region which comprises 20 of the MSY and contains DNA sequences that are more distantly related to those present on the X chromosome It contains 27 single copy genes and a number of pseudogenes nonfunctional All share some homology on the X chromosome One is the SRY gene The third area is the ampliconic region which comprises 30 of the MSY and most genes are related to development of the testes They lack counterparts on the X chromosome 60 transcription units are divided among 9 families Almost all are identical Each repeat unit is an amplicon They encode proteins speci c to the development and function of the testes lncreased paternal age is now associated with an increased risk in offspring of congenital disorders with a genetic basiscancer schizophrenia autism etc paternal age effects PAE Sex ratio actual ratio of male to females there are more males than femalesthe reason is unknown Primary sex ratio re ects the proportion of males to females conceived in a population Secondary sex ratio the proportion of each sex that is born This is much easier to determine but has the disadvantage of not accounting for any disproportionate embryonic or fetal mortality Theoretical assumptions of the sex ratio 1 Because of segregation maes produce an equal number of X and Y bearing sperm 2 Each type of sperm has equivalent viability and motility in the female reproductive tract 3 The egg surface is equally receptive to both X and Y bearing sperm The disparity between the X and Y chromosomes creates a quotgenetic dosagequot difference between males and females for all Xinked genes Dosage compensation balances the dose of the X chromosome gene expression in males and females Barr bodysex chromatin body A highly condensed structure that lies against the nuclear envelope of interphase cells It is an inactivated X chromosome It suggests the female dosage of X is equivalent to males Evidence lies in the Klinefelter maes XXY that have one none in Turner X two in XXX females 3 in XXXX females and so on Follows the Nl rue where N is the number of X chromosomes present However the presence of Barr bodies does not stop the effects of each syndrome The explanation could be that chromosome inactivation does not occur in early stages of development of cells destined to be gonadal tissues The other explanation is that not all of each X chromosome forming a Barr body is inactivated 15 of human X genes actuay escape X inactivation Lyon hypothesis the inactivation of the X chromosomes occurs randomly in somatic cells at a point early in embryonic development most likely sometime during the bastocyst stage of development Once inactivation has occurs a descendant cells have the same X chromosome inactivated as their initial progenitor cell Based on observations of female mice heterozygous for Xinked coat coor genes Pigmentation is mottled Other observations lie in female Calico catsmosaic pattern Redgreen coorbindness Xinked recessive disorder In humans hemizygous males are fully coorbind in all retina cells However heterozygous females have mosaic retinas with patches of defective color vision and surrounding areas with normal perception Random inactivation of the X chromosomes led to this phenotype Imprinting process in which one homolog but not the other it affected DNA histones or both silence the chromosome and its genes Xic X inactivation center region of the mammalian X chromosome at the proximal end of the p arm in humans Its genetic composition occurs only on the X chromosome that is inactivated Contains severa regulatory units and 4 genes One is the Xinactive speci c transcript XIST and is a critical gene for Xinactivation The RNA product is large and does not encode a protein and not translated RNA products of XIST spread and coat the chromosome that produced them Two other noncoding genes at the Xic locus Tsix antisense partner of XIST and Xite also play a role Transcription of XIST begins at low levels on all X chromosomes and as inactivation begins it is enhanced on the X that is inactivated In cells with more than two X chromosomes the paternal and maternal X pair brie y and align at Xic loci to count the number of X chromosomes prior to inactivation In Drosophila the Y chromosome does not determine maleness X chromosomes and autosomes together play a role XXY ies are normal females and X0 ies are sterile males Y does not determine maleness but plays a part in male fertility A critical factor in determining sex is the ratio of X chromosomes to the number of haploid sets of autosomes A present Normal 2x2A and triploid 3X3A females have a ratio of 1 and are both fertile As the ratio exceeds unity a metafemale is formed Normal XY2A and sterile XO2A males have a 5 ratio When the ratio decreases to 33 infertile metamales are produced Between 5 and 10 are intersexes The genic balance theory results and a ratio of 12 is maleness X2A but XX2A results in females Dosage compensation complex DCC cluster of gene activating proteins that enhances genetic expression along sites of the X chromosome related to maleness in Drosophila The Master switch le gene plays are role in dosage compensation ln XY ies le is inactive and autosomal genes are active causing enhanced X chromosome activity In females le is active and functions to inactivate one or more male speci c autosomal genes like ml for maleness Ensures no double expression of the Xlinked genes in females le acts as a sensor for several other X linked genes by counting X chromosomes Genotypic sex determination GSD where genetically sex is determined Temperature dependent sex determination TSD sex is determined by temperature In many species of reptiles GSD is involved at conception based on sexchromosome composition as is the case in most organisms However in other reptiles TSD is the norm where sex determination is achieved according to incubation temperature of eggs during a critical point in embryonic development In some cases high temperatures yield males in others both high and low temperatures yield females and intermediates yield different proportions of males Possible exam questions 1 A male with two X chromosomes is often the result of SRY relocating to an X 2 A Drosophila with XXYAAA is what sex intersex 3 An individual with Turner syndrome has no Barr bodies True Biology 240 Chapter 8 notes Chromosome mutationsaberrations modi cations in the total number of chromosome sets deletions duplication of genes or segment 5 of a chromosome and rearrangements of genetic material within or among chromosomes These are passed on to offspring in a predictable manner Aneuploidy an organism gains or loses one or more chromosomes but not a complete set Loss of one chromosome from a diploid genome is monosomy Gain of a chromosome is triploidy Euploidy complete haploid sets of chromosomes are present Polyploidy refers to the addition of more than 2 sets With three sets are triploid four are tetraploid Loss of one chromosome results in 2n1 or monosomy Ex 45 X Turner syndrome monosomy of autosomes is generally not tolerated in mammals Monosomy unmasks recessive lethals that are otherwise tolerated in heterozygotes carrying wild type alleles Haploinsuf ciency may also occur Aneuploidy is better tolerated in plants although they are less viable Trisomy 2n1 addition of one chromosome produces somewhat more viable individuals in both animal and plant species than does the loss of a chromosome In plants trisomy is viable but the phenotype may be alteredresults in slower growth Down syndrome Trisomy 21 found in approximately 1800 live births Causes speci c characteristics such as the simian fold on the hands epicanthic fold of the eyes and at face and round head Exhibits mental retardation and poor muscle tone It is suggested that a critical region of the chromosome 21 contains dosage sensitive genes that are responsible for altered phenotypes the region portion is Down Syndrome critical region DSCR Findings related to the region developed that these individuals are at a lower risk for cancerous tumors Correlated to an extra copy of the DSCR1 gene which encodes protein for suppressing of vascular endothelial growth factor VEGF In turn it blocks angiogenesis Ovum is the source of 95 of trisomy cases Passing 45 the woman is more likely to develop errors in meiosis related to the ovum which leads to Down39s Amniocentesis or CVS allows women to culture eggs to test if they are at risk for a Down39s child In a new approach noninvasive prenatal genetic diagnosis NIPGD is used and DNA and fetal cells are tested Down syndrome is a random accident of nondisjunction of chromosome 21 but can be inherited in some cases familial Down Syndrome involves the translocation of chromosome 21 Patau and Edwards syndrome 47 13 47 18 lethal diseases in which individuals live only for a few years 20 of all conceptions terminate in spontaneous abortion 30 of aborted fetuses have a chromosomal abnormality These observations support the hypothesis that normal embryonic development requires a precise diploid complement of chromosomes to maintain the delicate balance in the expression of genetic information Polyploidy originates in two ways 1 Addition of one or more extra sets of chromosomes identical to normal haploid complement of the same species which results in autoploidy or 2 The combination of chromosome sets from different species occurring as a consequence of hybridization resulting in alloploidy A normal organism is represented simply as AA A is the haploid set of chromosomes Autoploids arises either by failure of the chromosomes to segregate during meiotic divisions and is fertilized by haploid gametes or two sperm fertilize an ovum Autotetraploids more easily found in nature due to the even number of chromosomes ln crops colchicine is added to plants to inhibit spindle formation and create replicated and triploid fruit generally sterile due to the imbalanced number of chromosomes In general autopolyploids are larger due to larger cell size Ex triploid strawberries bananas watermelons Amphidiploid hybridization of two species results in imbalanced numbers and inviable zygotes normally but some species can create balanced chromosomes to produce a zygote Ex cotton Amphidiploids often express traits of both parents Endopolyploidy condition in which only certain cells in an otherwise diploid organism are polyploid In such cells the set of chromosomes replicates repeatedly without nuclear division Ex human liver cells Although their role is not clear proliferation of chromosome copies often occurs in cells where high levels of certain gene products are required If an aberration is found in one homolog of a chromosome but not the other an individual is said to be heterozygous for the aberration Unusual pairing of the arrangement of chromosomes often leads to gametes that are duplicated or de cient for some chromosomal regions Offspring of quotcarriersquot are more likely to demonstrate phenotype changes Deletion when a chromosome breaks in one or more places and a portion of it is lost Can occur near one end terminal deletion or within the interior of the chromosome intercalary deletion The portion that contains the centromere region is usually maintained when the cell divides whereas the segment without it is eventually lost in the progeny cells following meiosismitosis For synapsis to occur between a chromosome with a large intercalary deletion and a normal homolog the unpaired region must buckle out into a compensation loop Cri du chat syndrome results from deletion of small terminal portion of chromosome 5 Considered to be a segmental deletion Associated with the loss of a small variable part of the short arm of chromosome 546 5p lnfants exhibit malformations and often mentally retarded Abnormal development of the larynx leads to the characteristic cry 12550000 births Results from sporadic loss of the small piece of chromosomal material in gametes Missing piece encodes for telomerase reverse transcriptase essential for maintaining of telomeres during DNA replication Duplication any part of the genetic material is present more than once in the genome As in deletions pairing loop can be used to compensate Duplications can arise because of unequal crossing over between chromosomes during meiosis or replication error before meiosis rDNA codes for rRNA and multiple genes code for itgene redundancy Sometimes even having multiple genes to code for rRNA is not enough to supply for construction of ribosomes needed for oocytes of the ovum and essential in early development of embryo Gene ampli cation increases the amount of rRNA that can be encoded Genes that code rRNA are located in the nucleolar organizer region NOR Associated with nucleolus To amplify rDNA is selectively replicated and form new NORs Duplications can cause phenotypic variation that may appear to be caused by a simple gene mutation Ex bareye phenotype in Drosophila Process of gene duplication seems to be the major source of new genes based on supposition that products of many genes are indispensable to survival of members of any species in evolution Therefore unique genes are not free to accumulate mutations to alter their primary function and produce new genes However if duplication occurs of the unique genes mutations will be tolerated because they are essential for survival The Duplication can receive many mutational changes However over time it may change enough to diverge and produce a new role in a cell New function may be an adaptive advantage Gene families groups of contiguous genes whose products perform the same or similar functions Share a common origin and arose through gene duplication Ex different human hemoglobin polypeptide chains Dupications of portions of genes occur on a regular basis Copy number variants CNVs variations in copies of any duplicated gene that represent differences in the number of large DNA sequences found both in coding and noncoding regions of the genome Play crucial role in many of individual traits Have both positive and negative associations with diseases Inversion type of chromosomal aberration in which a segment of a chromosome is turned around 180 degrees within a chromosome Rearranges linear gene sequence Requires two break points and reinsertion of the inverted segment May or may not include the centromere if the centromere is not part of the rearranged chromosome it is a paracentric inversion It the centromere is part of it it is a pericentric inversion If only one member of a homologous pair of chromosomes has an inverted segment normal linear synapsis not possible Inversion heterozygotes are organisms with one inverted chromosome and a noninverted homolog Two chromosomes in meiosis can only pair with an inversion loop If crossing over does not occur within inverted segments of the inversion loop the homologs will segregate which results in two normal and two inverted chromatids distributed into gametes However if crossing over occurs within inversion loop abnormal chromatids are produced When crossover occurs within a paracentric inversion one recombinant dicentric chromatid with 2 centromeres and one recombinant acentric chromatid without a centromere are produced Both have duplications and deletions of chromosome segments During anaphase acentric may be lost or randomly distributed Dicentric is pulled in both directionsproduces dicentric bridges and the dicentric chromatid usually breaks at some point Usually zygote produced is abnormal if it even develops Similar chromosomal imbalance occurs in pericentric inversion Recombinant chromatids have duplications and deletions lnviable embryos usually produced In inversion heterozygotes the inversion has the effect of suppressing the recovery of crossover products when chromosome exchange occurs Viability therefore greatly diminished Because recovery of crossover is suppressed groups of speci c alleles in adjacent loci within inversions may be preserved from generation to generation If the alleles confer a survival advantage the inversion is bene ciary Translocation movement of a chromosomal segment to a new location in the genome Reciprocal translocation involves exchange of segments between two non homologous chromosomes Occurs if the arms come close to each other so that an exchange is facilitated Genetic consequences similar to inversions Genetic information not lost or gained only rearrangement Homologs heterozygous for reciprocal translocation undergo unorthodox synapsis in crosslike con guration Some gametes produced are genetically imbalanced Alternate segregation at the rst meiotic division can occur Second pattern is adjacent segregation where homologous centromeres migrate to the same pole When genetically imbalanced gametes participate in fertilization in animals resultant offspring generally do not survive Result in parent having semisterility where only few offspring are produced if fortunate Robertsonian translocation Involves breaks at the extreme ends of the short arms of two nonhomoogous acrocentric chromosomes Small segments are lost but the larger segments fuse at their centromeric region Creates large submetacentric chromosome Accounts for one instance of familial Down syndrome where one parent has a 1421 DG translocation One parent is phenotypically normal through shehe has only 45 chromosomes and is a balanced translocation carrier All offspring that parent has will have Down syndrome due to the double copy of chromosome 21 Fragile Xsyndrome is most common form of inherited mental retardation This syndrome affects 14000 males and 18000 Gene responsible is FMR1 in which a sequence of 3 nucleotides is repeated many times expanding the size of the gene Possible Exam Questions 1 If there is crossing over in a paracentric inversion loop what will happen dicentric bridges and acentric fragments will form 2 A normal child is found to have no let chromosomes but an extra single larger chromosome 2n45 What is likely to have happened Robertsonian translocation involving both 21St chromosomes
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