DNA: The Carrier of Genetic Information Chapter 12
Learning Objective 1 • What evidence was accumulated during the 1940s and early 1950s demonstrating that DNA is the genetic material?
The Mystery of Genes • Many early geneticists thought genes were proteins • Proteins are complex and variable • Nucleic acids are simple molecules
Evidence for DNA • DNA (deoxyribonucleic acid) • Transformation experiments • DNA of one strain of bacteria can transfer genetic characteristics to related bacteria
Bacteriophage Experiments • Bacteriophage (virus) infects bacterium • only DNA from virus enters the cell • virus reproduces and forms new viral particles from DNA alone
KEY CONCEPTS • Beginning in the 1920s, evidence began to accumulate that DNA is the hereditary material
Learning Objective 2 • What questions did these classic experiments address? • Griffith’s transformation experiment • Avery’s contribution to Griffith’s work • Hershey–Chase experiments
Griffith’s Transformation Experiment • Can a genetic trait be transmitted from one bacterial strain to another? • Answer: Yes
Experiment 1 Experiment 2 Experiment 3 Experiment 4 R cells injected S cells injected Heat-killed S cells injected R cells and heat-killed S cells injected Mouse dies Mouse lives Mouse lives Mouse dies Fig. 12-1, p. 261
Animation: Griffith’s Experiment CLICKTO PLAY
Avery’s Experiments • What molecule is responsible for bacterial transformation? • Answer: DNA
Hershey–Chase Experiments • Is DNA or protein the genetic material in bacterial viruses (phages)? • Answer: DNA
1 35S 32 P Bacterial viruses grown in 35S to label protein coat or 32P to label DNA 2 Viruses infect bacteria Fig. 12-2, p. 262
3 Agitate cells in blender Agitate cells in blender 4 Separate by centrifugation Separate by centrifugation 32 P 35S 5 Bacteria in pellet contain 32P-labeled DNA 35S-labeled protein in supernatant Fig. 12-2, p. 262
6 Viral reproduction inside bacterial cells from pellet 7 Cell lysis 32P 5 6 7 Fig. 12-2, p. 262
Learning Objective 3 • How do nucleotide subunits link to form a single DNA strand?
Watson and Crick • DNA Model • Demonstrated • how information is stored in molecule’s structure • how DNA molecules are templates for their own replication
Nucleotides • DNA is a polymer of nucleotides • Each nucleotide subunit contains • a nitrogenous base • purines (adenine or guanine) • pyrimidines (thymine or cytosine) • a pentose sugar (deoxyribose) • a phosphate group
Forming DNA Chains • Backbone • alternating sugar and phosphate groups • joined by covalent phosphodiester linkages • Phosphate group attaches to • 5′ carbon of one deoxyribose • 3′ carbon of the next deoxyribose
Thymine Nucleotide Adenine Cytosine Phosphate group Guanine Phosphodiester linkage Deoxyribose (sugar) Fig. 12-3, p. 264
Animation: Subunits of DNA CLICKTO PLAY
KEY CONCEPTS • The DNA building blocks consist of four nucleotide subunits: T, C, A, and G
Learning Objective 4 • How are the two strands of DNA oriented with respect to each other?
DNA Molecule • 2 polynucleotide chains • associated as double helix
Sugar–phosphate backbone Minor groove Major groove 3.4 nm 0.34 nm 2.0 nm = hydrogen = carbon = oxygen = atoms in base pairs = phosphorus Fig. 12-5, p. 266
Double Helix • Antiparallel • chains run in opposite directions • 5′ end • phosphate attached to 5′ deoxyribose carbon • 3′ end • hydroxyl attached to 3′ deoxyribose carbon
KEY CONCEPTS • The DNA molecule consists of two strands that wrap around each other to form a double helix • The order of its building blocks stores genetic information
Animation: DNA Close Up CLICKTO PLAY
Learning Objective 5 • What are the base-pairing rules for DNA? • How do complementary bases bind to each other?
Base Pairs • Hydrogen bonding • between specific base pairs • binds two chains of helix • Adenine (A) with thymine (T) • forms two hydrogen bonds • Guanine (G) with cytosine (C) • forms three hydrogen bonds
Adenine Thymine Deoxyribose Deoxyribose Guanine Cytosine Deoxyribose Deoxyribose Fig. 12-6b, p. 267
Chargaff’s Rules • Complementary base pairing • between A and T; G and C • therefore A = T; G = C • If base sequence of 1 strand is known • base sequence of other strand can be predicted
KEY CONCEPTS • Nucleotide subunits pair, based on precise pairing rules: T pairs with A, and C pairs with G • Hydrogen bonding between base pairs holds two strands of DNA together
Learning Objective 6 • What evidence from Meselson and Stahl’s experiment enabled scientists to differentiate between semiconservative replication of DNA and alternative models?
(a) Hypothesis 1: Semiconservative replication Parental DNA First generation Second generation Fig. 12-7a, p. 268
(b) Hypothesis 2: Conservative replication Parental DNA First generation Second generation Fig. 12-7b, p. 268
(c) Hypothesis 3: Dispersive replication Parental DNA First generation Second generation Fig. 12-7c, p. 268
Meselson-StahlExperiment • E. coli • grown in medium containing heavy nitrogen (15N) • incorporated 15N into DNA • Transferred from 15N to 14N medium • after one or two generations, DNA density supported semiconservative replication
Some cells continue to grow in 14N medium. Some cells are transferred to 14N (light) medium. Bacteria are grown in 15N (heavy) medium. All DNA is heavy. First generation Second generation High density Low density Cesium chloride (CsCl) DNA The greater concentration of CsCl at the bottom of the tube is due to sedimentation under centrifigal force. DNA is mixed with CsCl solution, placed in an ultracentrifuge, and centrifuged at very high speed for about 48 hours. 15N (heavy) DNA 14N (light) DNA 14N – 15N hybrid DNA DNA molecules move to positions where their density equals that of the CsCl solution. Fig. 12-8a, p. 269
14N (light) DNA 14N – 15N hybrid DNA 14N – 15N hybrid DNA 15N (heavy) DNA Before transfer to 14N One cell generation after transfer to 14N Two cell generations after transfer to 14N The location of DNA molecules within the centrifuge tube can be determined by UV optics. DNA solutions absorb strongly at 260 nm. Fig. 12-8b, p. 269
Semiconservative Replication • Each daughter double helix consists of • 1 original strand from parent molecule • 1 new complementary strand
Learning Objective 7 • How does DNA replicate? • What are some unique features of the process?