Lecture 1: Biological Genetics And Evolution

Lecture 1:Biological Geneticsand Evolution

Suggested Reading James F. Crow, Genetic Notes: An Introduction toGenetics, 8th Edition2

Structure of DNA (Deoxyribonucleic Acids)W Discovered by James Watson and FrancisCrick in 1953DNA has double-helical structureThe longitudinal strands made of phosphateand 5-carbon sugar called deoxyriboseThe linkages between two strands arepurine-pyrimidine bridgesHelix makes 360 turn every 10 stepsW&C for Watson and Crick, whodiscovered this structureCPurine-pyrimidine bridge3

The Purine-Pyrimadine Bridge Types of PurinesAdenine (A) (paired with T)Guanine (G) (paired with C) Types of PyrimidineThymine (T) (paired with A)Cytosine (C) (paired with G)4

The Purine-Pyrimidine Bridge Result – 4 Letter AlphabetATTAGCCGNote: AT isn’t TAATGCCGA Sequence carries information1000 steps can carry 41000 differentmessagesT3.4ÅTAHydrogen bonds5

DNA ReplicationAG ATCTTAAG ACTACATTCTGATGTAAG ACTAATTCGATT6

DNA Amounts in HumansHumans have 3.4x106 base pairs (haploid) Total length in diploid cells is 2 m (averagechromosome length is about 4 cm) Arrangement within nucleus is a mess How this sorts itself out not understood 7

DNA in Different Species E. Coli:Yeast:Drosophilae:Silk 0(per unit)8

Cells and Cell Division Cell Structure:NucleusCytosome (Cytoplasm)Cytosome Cell Size:Ostrich egg is single cellE-coli is 2 μ by 0.5 μWhale and Giraffe nerve cellsare several feet longHumans have 1014 cellsNucleus9

Chromosomes in Cell Nucleus Humans are diploidWasps, bees, and ants are haploidPotatoes are tetraploidWheat is hexaploidStrawberries are octaploid10

Mitosis (Nuclear Division) Cytoplasm divides more or less equally between cellsChromosomes undergo precise process that insuresthat an equal number of chromosomes is distributedto each of the new cell11

Mitosis Interphase: Prophase12

Mitosis Metaphase Anaphase Telophase13

Meiosis (Formation of Gametes) Original cell Chromosome doubling Chromosome pairing(note: crossover occurshere)14

Meiosis (Formation of Gametes) Cell Division(Possibility 1) Another Division Each sperm or egg has½ normal number ofchromosomes15

Meiosis (Formation of Gametes) Cell Division(Possibility 2) Another Division16

Mendelian Inheritance Gregor Johann Mendel1822-1884Austrian Roman Catholic Monk and BotanistPerformed experiments with peas in 1860sReported work in 1866Work remained unknown for 35 years17

An Experiment with Tall and Short PeaPlants When tall plant crossed with short plant, he alwaysgot a tall plantThis was true regardless of which parent (male orfemale) was tallThis confirmed earlier observations that both parentscontribute equallyHe then allowed hybrids to self pollinate. He endedup with 787 tall plants and 277 short plants18

Mendel’s First Law :Law of SegregationHeredity characteristics (tallness or shortness) occurin pairs and these pair segregate such that only onemember of the pair is used in a gamete Heredity characteristic unit now known as geneMendel also developed concept of dominance andrecessivenessTested theory using genetic ratios of various matingcombinations19

Explanation of ResultsTTParentalGeneration Results¼ Short¾ Tall Of the tall, 1/3 producedonly tall plants when selffertilizedOf the tall, 2/3 producedtall and short plants whenself-fertilizedFemalegametetttallshortTt1st HybridGeneration(1st Filial)2nd HybridGeneration(2nd rt20

Additional Vocabulary Homozygote: zygote with identical genes(TT or tt) Heterozygote: zygote with different genes (Tt) Alleles: alternate forms of a gene (T or t) Genotype: genetic makeup (TT, Tt, tt) Phenotype: characteristic determined by genotype (tall or short)21

Incomplete Dominance Consider color pattern in cattleOne pair of alleles determines color (completedominance)BB: blackBb: blackbb: red Another pair determines extent of color (incompletedominance)RR: solid colorRr: speckled with whiterr: no color22

Mechanism for DominanceGenes result in production of enzymes For complete dominance, one allele producesenough to achieve a desired effect Often, there will be subtle differences betweenhomozygous and heterozygous phenotypes (afew white hairs on a black mouse) 23

Mendel’s 2nd Law:Law of independenceThe members of one pair of alleles segregateindependently of other pairs (This is only true if they are on separate chromosomes)24

Mendel’s 2nd Law: An Example Round ( R) vs Wrinkled (r) seedsYellow (Y) vs Green (y) seedsRound and Yellow are dominantStep 1: Cross strain producing round yellow seedswith strain producing wrinkled green seedsResult: The F1 seeds are round and yellowStep 2: Self fertilize F1 plantsResult:9/16 of plants are round and yellow3/16 of plants are wrinkled and yellow3/16 of plants are round and green1/16 of plants are wrinkled and green25

Expected Ratios Consider shape(if independent)Likewise for color(if independent)¾ yellow¼ greenRRF1F2rrRrRR Rr Rr rr3/4 round 1/4 wrinkled26

Expected Ratios Thus, if independent we should have9/16 round yellow3/16 round green3/16 wrinkled yellow1/16 wrinkled green This is what is observed27

Gene Interactions:The Punnett SquareConsider the comb shape in poultry GenotypePhenotype R- PR- pprr Prr ppwalnutrosepeasingle28

The Punnett SquareSperm from RrPp (walnut)Egg from PpRrpprPRrPPRrPprrPPrrPprpRrPpRrpprrPPrrpp29

Epistasis: Genes Masking Other Genes Consider mouse coat patternsAllele C necessary for any pigmentGenotype BB and Bb produce black; bb is brownThusC- B- blackC- bb browncc B- whitecc bb white Allele cc masks the color gene30

Mutation Occasionally a gene mutates to another alleleA typical mutation rate for a given gene is one in 105generationsSince there are many genes (say 104) per cell,mutation is pretty commonIn evolutionary termsA high rate weakens populationA low rate keeps population from responding to change31

Mendel’s Insight Used sharply contrasting traitsUsed plants that can be self fertilizedUsed plants that produce large sample sizesHe was lucky (genes are only independent when on differentchromosomes)His luck didn’t hold – he tried (unsuccessfully) moving on tohawkweed which has both sexual and asexual reproductionwhich wasn’t understood for long after his deathThe greatest barrier to acceptance of his theory were traits thatare caused by many traits and influenced by environment(example human height and shape)32

Linkage and Chromosome Mapping Linkage : Genes on the same chromosome tend tostay together in inheritance Consider PoultryLeg length C – creeper (dominant, note CC is lethal)c – normal (recessive)Comb type R – rose comb (dominant)r – single comb (recessive)33

Linkage and Chromosome Mapping ExperimentStep 1: A homozygous rose-combed, normal-legged matedwith a single-combed, short-legged strainStep 2: The resulting creeper hybrids test-crossed withsingle-combed, normal legged strain34

Linkage and Chromosome Mapping RRWhat should happen (comb)step 1RRrstep 2RrrRrRrRrRrRr rrrrRrrrrrThus 50% rose, 50 % single35

Linkage and Chromosome Mapping ccWhat should happen (legs)step 1step 2 CcccCcCcCcccccCccreeper hybridccCcccCcccCcccThus 50 % short legged, 50 % long legged36

Linkage and Chromosome Mapping Thus, by Mendelian principles25% short-legged rose-combed25% normal-legged rose-combed25% short-legged single-combed25% normal-legged single-combed37

Linkage and Chromosome Mapping Actual results1069 normal rose1104 short single6 short rose4 normal single Explanation: The two alleles were on the samechromosome and did not act independently38

Linkage and Chromosome MappingStep 1RcRcResultStep 2Resultmated mated roserosesinglesingle39

Linkage and Chromosome Mapping Question:What about the 6 short rose and 4 normal single ? Answer:Crossover During meiosis the chromosomes can line upside by side and the following can happen:40

Linkages and Chromosome Mapping Importance of crossoverCrossover prevents a beneficial gene from beinginseparably linked to deleterious oneCrossover provides means for two good genes toget togetherExtends benefits of sexual reproduction41

Inheritance of Quantitative TraitsExample: height in humans Genes that control this are essentially identical to other genes, but notphenotypically identifiablecumulative in effectoften influenced by the environment This class of traits is said to be polygenic42

Inheritance of Quantitative TraitsExample: Seed color is some speciesGenotypePhenotypeA’A’B’B’very dark redA’A’B’B,A’AB’B’dark redAA’BB’,A’A’BB,AAB’B’ medium redA’ABB,AAB’Blight redAABBwhite 43

Biological Genetics and Evolution. 2 Suggested Reading James F. Crow, Genetic Notes: An Introduction to Genetics, 8th Edition. 3 Structure of DNA (Deoxyribonucleic Acids) . Microsoft PowerPoint - Lecture01.ppt