Chapter 16-Concept 16.1 DNA is the genetic materialEarly 20th century identification of the molecules of inheritance loomed as a major challenge to biologists.In the early 1900’s, T.H. Morgan’s group showed that genes are located in chromosomes.The two components of chromosomes, DNA and protein, became candidates for the genetic material-Genetic MaterialKey factor in determining the genetic material was choosing appropriate experimental organismsRole of DNA in heredity was first discovered by studying bacteria and the virus that infect them-DNA transforming BacteriaDiscovery of the genetic role of DNA began with research by Fredrick Griffith in 1928Griffith worked with two strains of bacterium, one pathogenic and one harmlessWhen he mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain, some cells became pathogenic.In 1924, Oswald Avery, Maclyn McCarty, and Colin Macleod announced that the transforming substance was DNATheir experiments used heat to activate DNA, RNA and Proteins.Their conclusion was based on experimental evidence that only DNA worked in transforming harmless bacteria into pathogenic bacteriaMany biologists remained skeptical, mainly because so little was known about DNA.He called this phenomenon transformation, now defined as a change in genotype and phenotype due to assimilation of foreign DNA-Viral DNA can program CellsMore evidence for DNA as the genetic material came from studies of viruses that infect bacteriaSuch viruses called bacteriophages (phages) are widely used in molecular genetics research.In 1952, Alfred Hersley and Martha Chase preformed experiments showingthat DNA is the genetic material of a phage known as T2To determine this, the designed an experiment showing that only one of the 2 components of T2 (DNA or proteins enters E. Coli cell during infection)Concluded injected DNA of the Phage provides the genetic information.It was known that DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group.1950, Erwin Chargaff reported that DNA composition varies from one species to the next.This evidence of diversity made DNA a more credible candidate for the genetic material.2 findings became known as Chargaff rulesBase composition of DNA varies between species. # of A=T, G=C-Structural Model of DNAMaurice Williams and Rosalind franklin were using a technique called X-Ray crystallography to study molecular structureFranklin produced a picture of the DNA molecule using this techniqueFranklin’s picture of DNA enabled Watson to deduce that DNA was helical, as well the width of the helix and the spacing nitrogenous bases.The pattern in the picture suggested that DNA has two strands forming a double helix.Watson and Crick built models of a double helix to conform to the X-Rays and chemistry of DNAFranklin concluded that there were 2 outer sugar, phosphate backbones, with nitrogenous base paired.Watson built a model in which the backbones were anti parallel.At first thought it was like with like, A with A, instead pairing a purine witha pyrimidine resulted in a uniform width.Watson crick model explains Chargaff’s rules. A=T, G=C.-Concept 16.2Watson and Crick suggested a possible copying mechanism for genetic material.Smooth strain causes pneumonia. Rough strain lacks capsule and is not pathogenic.-Base pairing to a template strand2 Strands of DNA are complementary each strand acts like a template for coping a new strandParent DNA splits, 2 new daughter strands are made base on base pairing rules.3 Models of ReplicationWatson and Crick’s semi conservative model of replication: parent’sjoin with daughter strands.Conservative Model: Parent stays together, two daughter strands stay together.Dispersive Model: Mix and Match of parent’s DNAExperiments by Matthew Meselson and Franklin Stahl support semi-conservative model.Labeled nucleotide of old strands with heavy isotopes, new ones with light isotopes.First replication: Produced band of hybrid DNA (Eliminated conservative model)Second replication: Light Hybrid (Eliminating Dispersive)-Getting StartedOrigins of replication: where the two DNA strands are separated, opening up replication bubble.Eukaryotic has hundreds of originsProkaryotes are circular so they only have one spot to do it.Replication proceeds in both directions from each origin until copied.Replication fork: y-shaped region where new DNA strands are elongated atthe end of each replication bubbleHelicases: Enzymes that untwist the double helix at the replication forks.Single-Strand binding proteins: bind to and stabilize single stranded DNA Topoisomerase: corrects “over-winding” ahead of replication forks b breaking, swiveling and rejoining DNA strands.DNA Polymerase can not initiate synthesis of a polynucleotide, they can only add nucleotides to the 3’ end.Initial nucleotide strand is a short RNA primerThe primer is short (5-10 nucleotides long) and the 3’ end serves asthe starting point for the new DNA strand.An enzyme called primage can start an RNA chain from scratch and adds RNA nucleotides one at a time using the parental DNA as a template.Each somatic cell has DNA consisting of approximately 6 billion base pairs which can be replicated in a few hoursVery few errors occur, only 1 per 10 billion nucleotides-Synthesizing a New DNA strandDNA polymerases: Enzymes that catalyze the elongation of a new DNA at a replication fork.Most DNA polymerases require a primer and a DNA template strandRate of elongation is about 500 nucleotides per second in bacteria and about 50 a second in human cells.Nucleoside triphosphate: each nucleotide that is added to a growing DNA stranddATP supplies adenine to DNA and is similar to the ATP of energy metabolism.dATP uses deoxyribose, ATP uses riboseAs each monomer of dATP joins the DNA strand. It loses two phosphate groups as a molecule of pyrophosphateHydrolysis of the phosphate supplies the DNA polymerase with energy to form the Phophodiester bond-Antiparallel ElongationAnti parallel structure of the double helix affects replicationDNA polymerase adds nucleotides ONLY to the free 3’ end of a growing strand, there for a new DNA strand can elongate in the 5’ to 3’ end.Along one template strand of DNA,
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