Chapter 16 Concept 16 1 DNA is the genetic material Early 20th century identification of the molecules of inheritance loomed as a major challenge to biologists chromosomes candidates for the genetic material In the early 1900 s T H Morgan s group showed that genes are located in The two components of chromosomes DNA and protein became Genetic Material experimental organisms virus that infect them Key factor in determining the genetic material was choosing appropriate Role of DNA in heredity was first discovered by studying bacteria and the DNA transforming Bacteria Discovery of the genetic role of DNA began with research by Fredrick Griffith worked with two strains of bacterium one pathogenic and Griffith in 1928 one harmless When 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 DNA Their 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 bacteria Many 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 Cells viruses that infect bacteria genetics research More evidence for DNA as the genetic material came from studies of Such viruses called bacteriophages phages are widely used in molecular In 1952 Alfred Hersley and Martha Chase preformed experiments showing that DNA is the genetic material of a phage known as T2 To 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 This evidence of diversity made DNA a more credible candidate for species to the next the genetic material 2 findings became known as Chargaff rules Base composition of DNA varies between species of A T G C Structural Model of DNA Maurice Williams and Rosalind franklin were using a technique called X Ray crystallography to study molecular structure Franklin produced a picture of the DNA molecule using this technique Franklin 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 and chemistry of DNA with nitrogenous base paired Watson and Crick built models of a double helix to conform to the X Rays Franklin concluded that there were 2 outer sugar phosphate backbones 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 with a pyrimidine resulted in a uniform width Watson crick model explains Chargaff s rules A T G C Concept 16 2 material pathogenic Watson and Crick suggested a possible copying mechanism for genetic Smooth strain causes pneumonia Rough strain lacks capsule and is not Base pairing to a template strand 2 Strands of DNA are complementary each strand acts like a template for Parent DNA splits 2 new daughter strands are made base on base pairing coping a new strand rules 3 Models of Replication join with daughter strands stay together Watson and Crick s semi conservative model of replication parent s Conservative Model Parent stays together two daughter strands Dispersive Model Mix and Match of parent s DNA Experiments by Matthew Meselson and Franklin Stahl support semi conservative model Labeled nucleotide of old strands with heavy isotopes new ones with light isotopes conservative model First replication Produced band of hybrid DNA Eliminated Second replication Light Hybrid Eliminating Dispersive Getting Started up replication bubble Origins of replication where the two DNA strands are separated opening Eukaryotic has hundreds of origins Prokaryotes 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 at the end of each replication bubble Helicases 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 primer The primer is short 5 10 nucleotides long and the 3 end serves as the 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 hours Very few errors occur only 1 per 10 billion nucleotides Synthesizing a New DNA strand replication fork DNA polymerases Enzymes that catalyze the elongation of a new DNA at a Most DNA polymerases require a primer and a DNA template strand Rate 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 strand metabolism dATP supplies adenine to DNA and is similar to the ATP of energy dATP uses deoxyribose ATP uses ribose As each monomer of dATP joins the DNA strand It loses two phosphate groups as a molecule of pyrophosphate form the Phophodiester bond Hydrolysis of the phosphate supplies the DNA polymerase with energy to Antiparallel Elongation Anti parallel structure of the double helix affects replication DNA 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 the DNA polymerase synthesizes a leading strand continuously moving toward the replication fork Also a leading strand found on the complementarity strand of DNA To elongate the other new strand called the lagging strand DNA polymerase must work in the direction away from
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