BSC197 Lecture notes 10.19 - 10.23
BSC197 Lecture notes 10.19 - 10.23 BSC197
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Date Created: 10/24/15
BSC197 Lecture Notes 10192015 10232015 10192015 Lecture Extending Mendelian Genetics for a Single Gene 0 Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations 0 When alleles are not completely dominant or recessive 0 When a gene has more than two alleles 0 When more than one gene encode a single trait 0 When the environment affects the phenotype Degrees of Dominance 0 occurs when phenotypes of the heterozygote and the dominant homozygote are identical the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties o In two dominant alleles affect the phenotype in separate distinguishable ways The Relation between Dominance and Phenotype o A dominant allele does not subdue a recessive allele alleles don t interact o Alleles are simply variations in a gene s nucleotide sequence 0 For any character dominancerecessiveness relationships of alleles depend on the level at which we examine the phenotype o TaySachs disease is fatal a dysfunctional enzyme causes an accumulation of lipids in the brain 0 At the organismal level the allele is recessive 0 At the biochemical level the phenotype ie the enzyme activity level is incompletely dominant 0 At the molecular level the alleles are codominant Multiple Alleles 0 Most genes exist in populations in more than two allelic forms 0 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 1 quot IB and i o The enzyme encoded by the IA allele adds the A carbohydrate whereas the enzyme encoded by the 13 allele adds the B carbohydrate the enzyme encoded by the i allele adds neither Polvgenic Inheritance 0 Quantitative characters are those that vary in the population along a continuum 0 Quantitative variation usually indicates polygenic inheritance an additive effect of two or more genes on a single phenotype 0 Skin color in humans is an example of polygenic inheritance Nature and Nurture The Environmental Impact on Phenotype 0 Another departure from Mendelian genetics arises when the phenotype for a character depends on the environment as well as genotype Overview Locating Genes along Chromosomes o Mendel s hereditary factors were genes though this wasn t known at the time 0 Today we can show that genes can show that genes are located on chromosomes 0 The location of a particular gene can be seen by tagging isolated chromosomes with uorescent dye that highlights the gene Morgan s Experimental Evidence 0 The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan an embryolo gist o Morgan s experiments with fruit ies provided convincing evidence that chromosomes are the location of Mendel s heritable factors 0 Morgan noted wild type or normal phenotypes that were common in the y populations 0 Traits alternative to the are called the mutant phenotypes 10212015 Lecture Sexlinked Genes Exhibit Unique Patterns of Inheritance o In humans and some other animals there is a chromosomal basis of sex determination o In humans and other mammals there are two varieties of sex chromosomes a larger X chromosomes and a smaller Y chromosome 0 Only the ends of the Y chromosome have regions that are homologous with the X chromosome 0 The SRY gene on the Y chromosome codes for the development of testes 0 Females are XX and males are XY 0 Each ovum contains an X chromosome while a sperm may contain either an X or a Y chromosome 0 Other animals have different methods of sex determination Inheritance of Sexlinked Genes o The sex chromosomes have genes for many characters unrelated to sex A gene located on either sex chromosome is called a In humans sexlinked usually refers to a gene on the larger X chromosome Sexlinked genes follow specific patterns of inheritance For a recessive selinked trait to be expressed o A female needs two copies of the allele 0 A male needs only one copy of the allele 0 Some disorders caused by recessive alleles on the X chromosome in humans 0 Color blindness o Duchenne muscular dystrophy o Hemophilia X Inactivation in Female Mammals In mammalian females one of the two X chromosomes in each cell is randomly inactivated during embryonic development The inactive X condenses into a If a female is heterozygous for a particular gene located on the X chromosome she will be a mosaic for that character Linked Genes Tend to be inherited together because they are Located near Each Other on the Same Chromosome Each chromosome has hundreds or thousands of genes Genes located on the same chromosome that tend to be inherited together are called How Linkage Affects Inheritance Morgan did other experiments with fruit ies to see how linkage affects inheritance of two characters Morgan crossed ies that differed in traits of body color and wing size Morgan found that body color and wing size are actually inherited together in specific combinations parental phenotypes He noted that these genes do not assort independently and reasoned that they were on the same chromosome However nonparental phenotypes were also produced Understanding this result involves exploring offspring with combinations of traits differing from either parent the production of Genetic Recombination and Linkage The genetic findings of Mendel and Morgan relate to the chromosomal basis of recombination Recombination of Unlinked Genes Mendel observed that combinations of traits in some offspring differ from either parent Offspring with a phenotype matching one of the parental phenotypes are called Offspring with nonparental phenotypes new combinations of traits are called or A 50 frequency of recombination is observed for any two genes on different chromosomes Morgan discovered that genes can be linked but the linkage was incomplete as evident from the recombinant phenotypes Morgan proposed that some process must sometimes break the physical connection between genes on the same chromosome That mechanism was the of homologous chromosomes Mapping the Distance between Genes Alfred Sturtevant one of Morgan s students constructed a genetic map an ordered list of the genetic loci along a particular chromosome Sturtevant 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 A linkage map is a genetic map of a chromosome based on recombination frequencies Distances between genes can be expressed as map units one map unit or centimorgan represents a 1 recombination frequency Map units indicate relative distance and order not precise locations of genes Genes that are far apart on the same chromosome can have a recombination frequency near 50 Such genes are physically linked but genetically unlinked and behave as if found on different chromosomes Alterations of Chromosome Number or Structure Cause some Genetic Disorders Largescale chromosomal alterations often lead to spontaneous abortions miscarriages or cause a variety of developmental disorders Abnormal Chromosome Number In nondisjunction pairs of homologous chromosomes do not separate normally during meiosis As a result one gamete receives two of the same type of chromosome and another gamete receives no copy Aneuploidy results from the fertilization of gametes in which nondisjunction occurred Offspring with this condition have an abnormal number of a particular chromosome A monosomic zygote has only one copy of a particular chromosome A trisomic zygote has three copies of a particular chromosome Polyploidy is a condition in which an organism has more than two complete sets of chromosomes 0 Triploidy 3n is three sets of chromosomes 0 Tetraploidy 4n is four sets of chromosomes Polyploidy is common in plants but not in animals Polyploids are more normal in appearance than aneuploids Alterations of Chromosome Structure Breakage of a chromosome can lead to four types of changes in chromosome structure 0 Deletion removes a chromosomal segment 0 Duplication repeats a segment 0 Inversion reverses a segment within a chromosome 0 Translocation moves a segment from one chromosome to another Human Disorders Due to Chromosomal Alterations Alterations of chromosome number and structure are associated with some serious disorders Some types of aneuploidy appear to upset the genetic balance less than others resulting in individuals surviving birth and beyond These surviving individuals have a set of symptoms or syndrome characteristic of the type of aneuploidy Down syndrome Trisomy 21 Down syndrome is an aneyploid condition that results from three copies of chromosome 21 It 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 Aneuploidv of Sex Chromosomes Nondisjunction of sex chromosomes produces a variety of aneuploid conditions Klinefelter syndrome is the result of an extra chromosome in a male producing XXY individuals Monosomy X called Turner syndrome produces X0 females who are sterile it is the only known viable monosomy in humans Many Proteins Work Together in DNA Replication and Repair The relationship between structure and function is manifest in the double helix Watson and Crick noted that the specific base pairing suggested a possible copying mechanism for genetic material Watson and Crick s semiconservative model of replication predicts that when a double helix replicates each daughter molecule with have one old strand derived or conserved from the parent molecule and one newly made strand Competing models were the conservative model the two parent strands rejoin and the disperse model each strand is a mix of old and new Getting Started Replication begins as special sites called origins of replication where the two DNA strands are separated opening up a replication bubble A eukaryotic chromosome may have hundreds or even thousands of origins of replication Replication proceeds in both directions from each origin until the entire molecule is copied At the end of each replication bubble is a new replication fork a Yshaped region where new DNA strands are elongated Helicases are enzymes that untwist the double helix at the replication forks Singlestrand binding protein binds to and stabilizes singlestranded DNA until it can be used as a template Topoisomerase corrects overwinding ahead of replication forks by breaking swiveling and rejoining DNA strands DNA polymerases cannot initiate synthesis of a polynucleotide they can only add nucleotides to the 3 end The initial nucleotide strand is a short RNA primer 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 The primer is short 510 nucleotides long and the 3 end serves as the starting point for the new DNA strand Synthesizing a New DNA Strand Enzymes called DNA polymerases catalyze the elongation of new DNA at a replication fork Most DNA polymerases require a primer and a DNA template strand The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells Antiparallel Elongation The antiparallel structure of the double helix two strands oriented in opposite directions affects replication 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 To elongate the other new strand called the lagging strand DNA polymerase must work in the direction away from the replication fork The lagging strand is synthesized as a series of segments called Okazaki fragments which are joined together by DNA ligase Proofreading and Repairing DNA DNA polymerases proofread newly made DNA replacing any incorrect nucleotides In mismatch repair of DNA repair enzymes correct errors in base pairing DNA can be damaged by chemicals radioactive emissions Xrays UV light and certain molecules in cigarette smoke for example In nucleotide excision repair a nuclease cuts out and replaces damaged stretches of DNA
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