SCI-02 Evolution: A Primer Session 1, 12 Sept. 2017

Transcription

1 SCI-02 Evolution: A Primer Session 1, 12 Sept Location: Old U Park Room weeks: 9/12/ /3/2017 Meets 8:45 AM-10:00 AM ADT on Tuesdays

2 Instructor Ed Murphy

3 Ed s hearing Wears hearing aids, but: 1. Too many loud noises (tools, guns, concerts, 2. Too much swimming in cold waters PLEASE SPEAK UP!

4 Today s Topics Some definitions Chromosomes, Genes, DNA Examples of ALLELES Genetic Variation

5 FACT An observation that has been repeatedly confirmed For all practical purposes is accepted as true ; e.g., pure water freezes at 32 F or 0 C Truth in science: rarely final or absolute What is accepted as true today may be modified or even discarded tomorrow

6 LAW A descriptive generalization about how some aspect of the natural world behaves under stated circumstances Example: Newton s law of universal gravitation (an excellent approximation of forces of gravity in most situations) (Vs. the Law of Averages not true)

7 HYPOTHESIS A tentative statement about the natural world that can be TESTED and either verified (rarely possible) or rejected (suggesting that one or more of any number of alternative hypotheses must be true) KEY: TESTABLE

8 Hypothesis: An example Watching the moon, we can see that it rises in the east and sets in the west, night after night A reasonable, common-sense hypothesis: The moon travels from east to west as it rotates around the earth (28-day cycle) Can you think of a way to test this hypothesis?

9 Examples of possible tests Note compass position of moon at the same time of night, night-afternight Note the compass position of successive phases of the moon 3 rd option: recent opportunity

10 Moon in Real Time Today: last quarter Set: 4:21 pm Rise 10:43 pm Tomorrow: Set: 5:37 pm Rise 11:28 pm

11 What s the answer? Moon travels from WEST TO EAST as it rotates around the earth Opposite of what common sense tells us

12 Solar Eclipse 21 August 2017 Moon passed from east to west across path of sun Arrow: direction of moon s path past sun

13 THEORY In science, a well-substantiated explanation that is built on facts, laws, inferences, and tested hypotheses NOT a guess, or an unsubstantiated hypothesis Theory is NOT synonymous with hypothesis; don t say Oh, that s just a theory

14 Evolution: definitions Evolution is a robust theory Originally: descent with modification change in the characteristics of populations over time Currently: changes in ALLELE FREQUENCIES over time

15 Chromosomes, genes, and loci Chromosomes: in nucleus, contain GENES GENES: instructions for most functions in organism Locus (loci): physical location of a gene on a chromosome Shown: Human karyotype (2n = 46 chromosomes) (Dogs: 2n = 78)

16 Human karyotype (chromosome diagram) Human gametes: --22 autosomes --1 sex chromosome Females: X Males: X or Y Fertilized egg: --2n = 46 chromosomes Note that each chromosome appears double (occurs only when cell is about to divide) chromosomes easiest to see at this stage

17 Genetic Information Two sources DNA in nucleus (in chromosomes) DNA outside of nucleus (in organelles) Key feature: nucleotide bases (rungs of ladder) Purines (Adenine and Guanine) Pyrimidines (Thymine and Cytosine) Bonds: ONLY A-T and G-C

18 DNA outside the nucleus Typically Inherited only from mother Mitochondria in plants and animals Fuel burning: sugars to CO 2 and H 2 0: huge energy capture Chloroplasts in plants Genes that convert sunlight to sugars (photosynthesis: conversion of CO 2 to simple sugars) Why? Many more mitochondria in eggs than sperm (e.g., humans: 200,000 vs. 5, on average) High failure rates of those in sperm

19 DNA structure: rungs of ladder are the key: A-T and C-G bonds

20 DNA to protein: via a messenger, mrna

21 Very predictable bonding within DNA and in message via RNA Within DNA: G-C, C-G and A-T, T-A ONLY messenger RNA: has U instead of T; ONLY A U, T A, G C, C G

22 Genetic Code 3 DNA (RNA) base sequence ( codon ) = 1 amino acid (building block of protein) 64 (4 3 ) possible combinations, but only 23 needed: 20 amino acids 1 start (AUG) 3 stops 3rd position may be silent Molecular clock

23 DNA to Protein ALL ORGANISMS (bacteria to humans) Example: CGU Arginine GGU Glycine UAU Tyrosine??? Threonine

24 Genetic code From mrna at step 2, Threonine could be coded by: ACU ACC ACA ACG (And, from DNA, at step 1, by TGA, TGG, TGT, or TGC in DNA)

25 Lysozyme, an antibacterial enzyme Enzymes: proteins that speed up reactions Amino acid sequence:

26 Enzymes (e.g., Lysozyme) Speed reaction times Complex 3- dimensional structure Active site Mutation options Silent (same amino acid) Minor effects Major effects

27 Alleles: Definition Recall modern definition of evolution: changes in allele frequencies Alleles: Particular forms or variants of a gene Versions of the same gene that differ in their nucleotide (A,T,C,G) sequence One example:

28 Dominant and recessive alleles Dominant: Expressed if either one or two copies of that allele are present Recessive: Only expressed if both copies of that allele are present

29 WINTER coat color in Arctic Foxes 1 gene: the coat color gene 2 alleles: B (blue) and W (white) 2 phenotypes: blue phase (BB, BW) and white phase (WW) (Blue is a DOMINANT allele and white is a RECESSIVE allele)

30 Arctic Fox Winter Coat Color 1 Gene: MC1R 2 Alleles Blue: Position 5: glycine (UG_ cysteine (GG_) Position 280: phenylalanine (UU_) cysteine (UG_)

31 Can a Blue Phase male and a Blue phase female produce a white phase pup? Not if one or both is homozygous for the Blue allele (BB) Yes, if both are heterozygous for the Blue and White alleles If so: 25% chance [pup possibilities: BB (blue), BW (blue), WB (blue), WW (white)]

32 Eye color in humans: greatly simplified 2 of >10 genes, each with 2 alleles: Chromosome 15, bey2 locus: 2 alleles Brown blue Chromosome 19, gey locus: 2 alleles Green blue (letter that is underlined is used in table below)

33 Dominance and interactions between eye color genes A table to help sort things out: Just part of the picture: hazel, eye color changes, Outcome bey2 gey BB any Brown Bb any Brown bb GG, Gb Green bb bb Blue

34 Question Is it possible for 2 brown-eyed parents to produce a blue-eyed child? Why or why not?

35 Answer Yes, if both are heterozygous for brown and blue alleles at bey2 locus and carry blue allele at gey locus If both parents are heterozygous for both Brown and blue and Green and blue: 1 in 16 chance

36 Example, continued Cards: Brown or clear for bey2 Green or blue for gey

37 Dominant-recessive patterns No limit on number of alleles of a particular gene within a population or a species Only some ALLELES show such patterns One allele can be dominant over a second allele, recessive to a third allele, and co-dominant to a fourth, etc.

38 Incomplete Dominance Example: Snapdragon Flower color Rr genotype expresses flowers with less red pigment than RR genotype resulting in pink flowers (Colors are not blended together, the dominant trait is just expressed less strongly)

39 Co-dominance Two or more alleles: each expressed in phenotype Example: white bull (WW) mates with a red cow (RR): offspring exhibit codominance expressing both white and red coat colors

40 Co-dominance: Human Example Human ABO blood types: 3 alleles (A,B, and O) AA and AO: Type A blood BB and BO: Type B blood OO: Type O blood AB: Type AB blood (Both A and B modifications expressed; codominance )

41 ABO Blood Group types: One Gene One genetic locus, the ABO locus 3 primary alleles: A, B, and O A child receives one of the three alleles from each parent Six possible genotypes (AA, AO,BB,BO,AB,OO) Four possible blood types (phenotypes): A,B,AB,O

42 ABO Locus Chromosome 9 at 9q34.1- q34.2 Contains 7 exons (protein coding regions) that span more than 18 kb of genomic DNA Exon 7 is the largest and contains most of the coding sequence Exon 6 contains the deletion that is found in most O alleles and results in a loss of enzymatic activity

43 ABO molecules Polysaccarides (complex sugars) O is missing the leftside arm of the molecule A and B differ slightly Example of ANTIGENS: Antibody antagonists

44 ABO Blood Cell Types AA, AO: A antigens on surface No A antibodies in blood BB, BO: B antigens on surface No B antibodies in blood AB OO Both A and B antigens on surface No A or B antibodies in blood No antigens on surface Both A and B antibodies in blood

45 Example: Type-A blood Body produces Type B-antibodies: to attack bacteria that have B-like epitopes (areas on are open to attack by matching antibodies) Body does not produce Type A antibodies (selftolerant or compatible)

46 Blood Transfusions Only safe if donor blood cells A. Have same antigens as recipient s or B. Lack antigens present in recipient s Example: O has no A or B antigens but has both A and B antibodies, so can t be a recipient of any blood but O

47 ABO Blood Types: Geographic Variation Right: Allele-by-allele Note high frequency of B allele in most, but not all, Asian populations

48 ABO Blood Types Why so much geographic variation? One possibility

49 ABO: >>3 alleles Each allele (A, B, and O) is actually a group of 2 or more alleles (see chart) All A-type alleles produce the same A antigen and all B-type alleles produce the same B antigen

50 Genetic Variation: General Although any one individual of a diploid organism has at most two different alleles at any one locus, most genes exist in a large number of allelic versions in the population as a whole Bottom line: Lots of genetic variation exists in natural populations

51 Modern genetic techniques 1970 s: electrophoresis Protein samples placed on a gel slab and subjected to an electric current Crude way to detect different alleles if different in size, surface charge Example: sickle-cell vs normal hemoglobin

52 DNA variability in instructions for specific enzymes Electrophoresis: different alleles detected if enzymes differ in size or electrical charge (crude tool) 28% of genes coding for enzymes in humans have 2 or more alleles, as detected by electrophoresis House mice also about 28%; fruit flies slightly higher

53 Human genetic variation Human genome: about (billion) base pairs (rungs of the ladder) of DNA On average, we differ from each in about 1 in every 1000 base pairs, or 3 million overall Consequently, no two humans, save identical twins, likely ever have been, or will be, genetically identical WOW!

54 Human Variation Estimates based on electrophoresis Genetic distance Demonstrates moderate, aut varying, isolation among populations

55 Human DNA variability 85% of all variability is within populations 15% among populations Take-home message: Except for close kin, we have about the same genetic similarity to everyone else on earth

56 Human genetic variability The few genes coding for skin color and other features that seem to define races and cultures are a biased sample Most alleles of most genes coding for enzymes are present in similar proportions in all populations and cultures Except for close relatives, you are almost as similar genetically to any human on earth as you are to others in your culture or race

57 1000 Genomes Project Genomes of 2,504 humans (2015 publication) 26 populations Broad spectrum of genetic variation in total, over 88 million variants, mostly (84.7 million) single amino acid differences (SNP s)

58 Findings A typical genome differs from the reference human genome at 4.1 million to 5.0 million sites More variant: African populations, melting pot populations

59 Pie Colors Dark gray: variant alleles found on all continents Light gray: variant alleles found on 2 or more continents Light color: only on that continent Dark color: only in that population

60 Microevolution Definition: change in allele frequencies that occurs over time within a population 4 possible causes: Mutation Selection (natural and artificial) Gene flow Genetic drift Also important: mixing the pot: Independent Assortment Crossing-over