BIOL 1107 1nd Edition Exam 3 Study Guide Lectures 19 24 Each section below includes material from the lecture homework and textbook Lecture 19 20 Chapter 16 The Molecular Basis of Inheritance T H Morgan s group Showed that genes are located on chromosomes Two components of chromosomes DNA and protein became candidates for genetic material Role of DNA in heredity was first discovered by studying bacteria and viruses that infect them Frederick Griffith Discovery of genetic role of DNA began w research by this guy in 1928 He worked with two strains of a bacterium pathogenic one harmless one When he mixed heat killed remains of pathogenic strain w living cells of harmless strain some living cells became pathogenic o Called this phenomenon transformation change in genotype and phenotype due to assimilation of foreign DNA Avery and his group 1944 announced transforming substance was DNA Their conclusion was based on experimental evidence that only DNA worked in transforming harmless bacteria into pathogenic bacteria Many biologists remained skeptical because very little was known about DNA Hershey and Chase Very important experiment 1952 they performed experiments showing that DNA is the genetic material of a phase known as T2 To determine this they designed experiment showing that only one of the two component of T2 DNA or protein enter an E coli cell during infection They concluded that injected DNA od phage provides genetic information Chargaf s findings Base composition of DNA varies between species In any species the of A and T bases are equal and of G and C bases are equal The basis for these rules was not understood until discovery of double helix 1950 he reported that DNA composition varies from one species to the next o This evidence of diversity made DNA a more credible candidate for genetic material o After DNA was accepted as the genetic material challenge was to determine how its structure accounts for its role in heredity Maurice Wilkins Rosalind Franklin They were using a technique called X ray crystallography to study molecular structure Franklin produced picture of DNA molecule using this technique Franklin s X ray crystallographic images of DNA enabled Watson to deduce that DNA was helical The X ray images also enable Watson to deduce the width of the helix and the spacing of the nitrogenous bases He pattern in the photo suggested that the DNA molecule was made up of two strands forming a double helix Watson and Crick Built models of a double helix to conform to the X rays and chemistry of DNA Franklin had concluded that there were two outer sugar phosphate backbones with the nitrogenous bases paired in the molecule s interior Watson built a model in which the backbones were antiparallel their subunits run in opposite directions Pairing a purine with a pyrimidines resulted in a uniform width consistent with the X ray data Relationship b n structure and function is manifest in double helix W C noted that specific base pairing suggested a possible copying mechanism for genetic material They also reasoned that pairing was more specific dictated by the base structures Established that adenine A paired only with thymine T and guanine G paired only with cytosine C This Watson Crick model explains Chargaff s rules in any organism the amount of A T and amount of G C Since two strands of DNA are complementary each strand acts as a template for building a new strand in replication In DNA replication parent molecule unwinds and 2 new daughter strands are built based on base pairing rules Meselson and Stahl Watson and Crick s semi conservative model of replication predicts that when a double helix replicates each daughter molecule will have on old strand derive d or conserved from the parent molecule and one newly made strand Competing models were the conservative model two parent strands rejoin and the dispersive model each strand is a mix of old and new Experiments by Meselson and Stahl supported semiconservative model They labeled the nucleotides of old strands w heavy isotope of nitrogen while any new nucleotides were labeled w lighter isotope DNA Replication Copying of DNA is remarkable in its speed and accuracy More than a dozen enzymes and other proteins participate in DNA replication Replication begins at particular sites called origins of replication where the two 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 entire molecule is copied At the end of each replication bubble is a replication fork a Y shaped region where new DNA strands are elongating Helicases enzymes that untwist double helix replication forks Single stranded binding proteins bind to and stabilize single stranded DNA Topoisomerase corrects overwinding ahead of replication forks by breaking swiveling and rejoining DNA strands Primase an enzyme which starts an RNA chain from scratch and adds RNA nucleotides one at a time using parental DNA as a template Primer short 5 10 nucleotides long and the 3 end serves as the starting point for new DNA strand DNA polymerases enzymes that catalyze the elongation of new DNA a replication fork o Most DNA polymerases require a primer and a DNA template strand o Rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells o Cannot initiate synthesis of a polynucleotide they can only add nucleotides to the 3 end o Initial nucleotide strand is a short RNA primer Each nucleotide that is added to a growing DNA strand is a nucleoside triphosphate dATP supplies adenine to DNA and is similar to the ATP of energy metabolism the difference is in their sugars dATP has deoxyribose while ATP has ribose As each monomer of dATP joins DNA strand it loses two phosphate groups as a molecule of pyrophosphate Antiparallel structure of double helix affects replication DNA polymerases add nucleotides only to free 3 end of a growing strand therefore a new DNA strand can elongate only in the 5 to 3 direction Leading strand DNA polymerase synthesizes this strand continuously moving toward the replication fork Lagging strand DNA polymerase must work in direction away from replication fork in order to synthesize this strand o It is synthesized as a series of segments called Okazaki fragments which are joined together by DNA ligase Proteins that participate in DNA replication form large complex a DNA