DNA Structure Part 1 - PDF

Chapter 7. DNA Structure and Replication - The Genetic Material Must Exhibit Four Characteristics - Evidence Favoring DNA as the Genetic Material - Nucleic Acid Chemistry Essential to DNA Structure - Structure of DNA Key to Its Function - Structure of RNA Is Chemically Similar to DNA Criteria for Ge...

Chapter 7. DNA Structure and Replication - The Genetic Material Must Exhibit Four Characteristics - Evidence Favoring DNA as the Genetic Material - Nucleic Acid Chemistry Essential to DNA Structure - Structure of DNA Key to Its Function - Structure of RNA Is Chemically Similar to DNA Criteria for Genetic Material  To serve as genetic material, molecule must be able to: – Replicate – Store information – Express information – Allow variation by mutation Rediscovery of Mendel  In 1900, Mendel’s hereditary principles were rediscovered  In 1903, Walter Sutton and Theodor Boveri independently described the parallels between chromosome partitioning into gametes and the inheritance of genes 3 DNA as the Candidate Hereditary Material  In 1923, DNA was localized to chromosomes and made a candidate for the hereditary material  However, both proteins and RNA are also found in chromosomes  Lipids and carbohydrates were also considered to be candidates 4 © 2015 Pearson Education, Inc. Appearance of smooth versus rough colonies of Pneumococcus. S (smooth) strain: virulent R (rough) strain: avirulent 5 The Transformation Factor  Frederick Griffith identified two strains of Pneumococcus: S, which caused fatal pneumonia in mice, and R, which did not  A single gene mutation change can convert an S (smooth) strain to an R (rough) strain 6 Griffith’s Transformation Experiment  Frederick Griffith (1927) – Showed avirulent strains of Streptococcus pneumoniae could be transformed to virulence – Speculated transforming principle could be part of polysaccharide capsule or compound required for capsule synthesis – Provided foundation for Avery, MacLeod, and McCarty’s research Griffith’s Transformation Experiment Frederick Griffith (1927), British medical officer Streptococcus pneumonia, Polysaccharide capsule Figure 7-2  In the 1940s, geneticists favored proteins as genetic material – Proteins were diverse and abundant in cells The Avery, MacLeod, and McCarty Experiment 1944 DNA is the hereditary molecule Figure 7-2  Hershey and Chase (1952) – Used Escherichia coli and bacteriophage T2 – Used radioisotopes 32P and 35S – Demonstrated DNA enters bacterial cell during infection an directs viral reproduction – Demonstrated that DNA, not protein, is the genetic material The Hershey-Chase Experiment 1952 Alfred Hershey and Martha Chase experiments in 1952 Radioisotope phosphorous 32P for DNA Radioisotope sulfur 35S for protein Figure 7-4  Genetic material of phage is DNA not protein DNA Nucleotides  Nucleotides – Nucleotides are building blocks of DNA  Nucleotides consist of: – Nitrogenous base (purine or pyrimidine) – Pentose sugar – Phosphate group Pentose sugar  RNA contains ribose sugar  DNA contains deoxyribose – “Deoxy” (without an oxygen) RNA DNA Nitrogenous base (purine or pyrimidine) RNA only (six-member ring) (nine-member ring) DNA only Composition and Structure of DNA (nine-member ring) Hydroxyl group Hydrogen atom (six-member ring) Figure 7-5 Nucleosides and Nucleotides  Nucleoside – Contains nitrogenous base and pentose sugar – Molecule is composed of purine or pyrimidine base and ribose or deoxyribose sugar  Nucleotide – Nucleoside with phosphate group added Mono-, Di-, and Triphosphates  Nucleoside monophosphates (NMP) – A nucleotide  Nucleoside diphosphates (NDP) – Nucleotide with addition of two phosphate groups  Nucleoside triphosphates (NTP) – Nucleotide with addition of three phosphate groups  Phosphodiester bonds – Nucleotides are linked by phosphodiester bonds between phosphate group at C-5 position and OH group on C-3 position Erwin Chargaff’s rules of base composition (1952): 1. The amount of T = A, the amount of C = G 2. The amount of pyrimidine nucleotides (T+C) = purine (A+G) Rosalind Franklin’s critical experimental result Rosalind Elsie Franklin (1920 – 1958), British biophysicist and X-ray crystallographer X-ray diffraction analysis of DNA  DNA is long and skinny  Two similar parts parallel to each other  Spiral-like X-Ray Diffraction  Base composition analysis (Chargaff) and Xray diffraction provided crucial data to Watson and Crick  X-ray diffraction – Studies by Rosalind Franklin (1950–1953) showed DNA had a 3.4-angstrom periodicity, characteristic of helical structure Watson and Crick, 1953 Proposed structure of DNA as a double helix The first model of DNA  Watson and Crick model (1953)  Proposed DNA as: – Double helix – Two anti-parallel strands connected by base pairing – Stacked nitrogenous bases James Watson and Francis Crick -- Nobel Prize 1962. Figure 7-7 Base pairing in DNA Base Pairing – Hydrogen Bonds  Chemical affinity produces hydrogen bonds in pair of bases – A-T and G-C base pairing provides complementarity of two strands and chemical stability to the helix – A-T: Double bond – G-C: Triple bond Figure 10-14 Note direction of DNA strands: 5’-3’ The two backbones are antiparallel Backbone: sugar + phosphate Base pairing: A – T (purine – pyrimidine) 2 hydrogen bonds G – C (purine – pyrimidine) 3 hydrogen bonds DNA Structure Summary - Frederick Griffith’s Transformation Experiment (1927) - The Avery, MacLeod, and McCarty Experiment (1944) - The Hershey-Chase Experiment (1952) - Erwin Chargaff’s rules of base composition (1952) - Rosalind Franklin’s X-ray diffraction data (1950–1953) - Watson and Crick proposed DNA a double helix (1953) James Watson and Francis Crick -- Nobel Prize 1962. End of Part 1