of Inheritance BIOL 222

Transcription

1 h. 16 he Molecular Basis of Inheritance of Inheritance BIOL 222 Overview: Life s Opera:ng Instruc:ons James Watson and Francis rick 1953 produced double helical model for the structure of DN. H. Morgan he Search for the ene:c Material Showed that genes are located on chromosomes herefore wo components of chromosomes DN and protein became candidates for the geneoc material he role of DN in heredity First studied in bacteria/bacteriophages

2 Evidence hat DN an ransform Bacteria Frederick riffith 1928 two strains of a bacterium one pathogenic and one harmless heat killed pathogenic remains mixed with living harmless strain alled transforma:on some living cells became pathogenic hange in genotype and phenotype due to assimilaoon of foreign DN Fig EXPERIMEN Living S cells (control) Living R cells (control) Heat-killed S cells (control) Mixture of heat-killed S cells and living R cells RESULS Mouse dies Mouse healthy Mouse healthy Mouse dies Living S cells Evidence hat DN an ransform Bacteria Oswald very, Maclyn Mcarty, and olin MacLeod 1944 transforming substance was DN only DN transformed harmless bacteria into pathogens SequenOally deacovated proteins, RN, or DN

3 Evidence hat Viral DN an Program ells More evidence for DN as the geneoc material came from studies of viruses that infect bacteria bacteriophages widely used in molecular geneocs research Phage head Bacterial cell ail sheath ail fiber DN 100 nm Evidence hat Viral DN an Program ells lfred Hershey and Martha hase 1952 showed DN is geneoc material phage known as 2 Looked to see if viral DN or protein enters the bacterium concluded injected DN provides geneoc informaoon Fig EXPERIMEN Batch 1: sulfur ( 35 S) Phage Bacterial cell protein DN DN Batch 2: phosphorus ( 32 P)

4 Fig. Fig EXPERIMEN Phage Bacterial cell protein Empty protein shell Batch 1: sulfur ( 35 S) DN Phage DN DN Batch 2: phosphorus ( 32 P) Fig EXPERIMEN Phage Bacterial cell protein Empty protein shell Radioactivity (phage protein) in liquid Batch 1: sulfur ( 35 S) DN Phage DN entrifuge DN Pellet (bacterial cells and contents) Batch 2: phosphorus ( 32 P) entrifuge Pellet Radioactivity (phage DN) in pellet Evidence hat DN Is the ene:c Material DN is a polymer of nucleoodes base, a sugar, and a phosphate group Erwin hargaff 1950 DN composioon varies from one species to the next evidence of diversity points to DN hargaff s rules equal number of and bases and equal and bases

5 Structural Model of DN Rosalind Franklin and Maurice Wilkins X ray crystallography Franklin produced a picture of the DN (a) Rosalind Franklin (b) Franklin s X-ray diffraction photograph of DN Watson and rick bases paired like with like resulted in a non uniform width purine with a pyrimidine Structural Model of DN uniform width consistent with X ray Purine + purine: too wide Pyrimidine + pyrimidine: too narrow Purine + pyrimidine: width consistent with X-ray data herefore Watson and rick pairing more specific Structural Model of DN dictated by the base structures Determined adenine () with thymine () guanine () with cytosine () explains hargaff s rule: in any organism the amount of =, and the amount of =

6 Fig. Fig denine () hymine () uanine () ytosine () Fig end Hydrogen bond 3 end 1 nm 3.4 nm 3 end 0.34 nm 5 end (a) Key features of DN structure (b) Partial chemical structure (c) Space-filling model DN Replica:on Watson and rick specific base pairing suggested a possible copying mechanism two strands of DN are complementary each strand acts as a template

7 Fig. Fig (a) Parent molecule Fig (a) Parent molecule (b) Separation of strands Fig (a) Parent molecule (b) Separation of strands (c) Daughter DN molecules, each consisting of one parental strand and one new strand

8 Semiconserva:ve model Each daughter molecule one old, one new strand ompeong models conservaove model the two parent strands rejoin dispersive model DN Replica:on each strand is a mix of old and new Fig Parent cell First replication Second replication (a) onservative model (b) Semiconservative model (c) Dispersive model DN Replica:on DN replicaoon remarkable speed and accuracy More than a dozen enzymes and other proteins parocipate DN Replication

9 Origins of replica:on Sites where two DN strands separate opens replicaoon bubble eukaryooc chromosome DN Replica:on hundreds thousands of origins of replicaoon ReplicaOon proceeds in both direcoons from each origin unol the enore molecule is copied Fig Origin of replication Parental (template) strand Daughter (new) strand Doublestranded DN molecule wo daughter DN molecules Replication bubble Replication fork 0.5 µm (a) Origins of replication in E. coli Origin of replication Double-stranded DN molecule Parental (template) strand Daughter (new) strand Bubble Replication fork 0.25 µm wo daughter DN molecules (b) Origins of replication in eukaryotes Replica:on fork Y shaped region Helicases end of each replicaoon bubble where new DN strands are elongaong untwist DN at replicaoon forks Single strand binding protein Binds/stabilizes single stranded DN opoisomerase DN Replica:on unol it can be used as a template corrects overwinding ahead of replicaoon forks by breaking, swiveling, and rejoining DN strands

10 Fig. Fig Single-strand binding proteins Primase opoisomerase 3 5 RN 3 primer 5 3 Helicase 5 Primer DN Replica:on Short RN strand required for inioaoon Primase Enzyme that starts an RN chain from scratch adds RN nucleoodes Uses DN as a template 5 10 nucleoodes long 3 end starong point for new DN DN polymerases DN Replica:on catalyze elongaoon of new DN at a replicaoon fork 500 nucleoodes per second in bacteria 50 per second in human cells