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DNA Structure and Function
Chapter 13
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Miescher Discovered DNA
• 1868• Johann Miescher investigated the chemical
composition of the nucleus• Isolated an organic acid that was high in
phosphorus• He called it nuclein• We call it DNA (deoxyribonucleic acid)
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Mystery of the Hereditary Material
• Originally believed to be an unknown class of proteins
• Thinking was– Heritable traits are diverse– Molecules encoding traits must be diverse– Proteins are made of 20 amino acids and
are structurally diverse
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Structure of the Hereditary Material• Experiments in the 1950s
showed that DNA is the hereditary material
• Scientists raced to determine the structure of DNA
• 1953 - Watson and Crick proposed that DNA is a double helix
Figure 13.2Page 217
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Griffith Discovers Transformation
• 1928• Attempting to develop a vaccine• Isolated two strains of Streptococcus
pneumoniae– Rough strain was harmless– Smooth strain was pathogenic
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Griffith Discovers Transformation
1. Mice injected with live cells of harmless strain R.
2. Mice injected with live cells of killer strain S.
3. Mice injected with heat-killed S cells.
4. Mice injected with live R cells plus heat-killed S cells.
Mice die. Live S cells in their blood.
Mice live. No live R cells in their blood.
Mice die. Live S cells in their blood.
Mice live. No live S cells in their blood.
Figure 13.3Page 218
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Transformation
• What happened in the fourth experiment?
• The harmless R cells had been transformed by material from the dead S cells
• Descendents of the transformed cells were also pathogenic
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Oswald & Avery
• What is the transforming material?• Cell extracts treated with protein-digesting
enzymes could still transform bacteria• Cell extracts treated with DNA-digesting
enzymes lost their transforming ability• Concluded that DNA, not protein,
transforms bacteria
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Bacteriophages
• Viruses that infect bacteria
• Consist of protein and DNA
• Inject their hereditary material into bacteria
cytoplasm
bacterial cell wall plasma
membrane
Figure 13.4bPage 219
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Hershey & Chase’s Experiments
• Created labeled bacteriophages– Radioactive sulfur – Radioactive phosphorus
• Allowed labeled viruses to infect bacteria
• Asked: Where are the radioactive labels after infection?
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virus particle labeled with 35S
virus particle labeled with 32P
bacterial cell (cutaway view)
label outside cell
label inside cell
Hershey and
Chase Results
Figure 13.5Page 219
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Information• Mon., 28 November
– Chapter 14 and 16 highlights• Wed., 30 November
– Final exam review – BRING YOUR QUESTIONS!– Instructor evaluations
• Mon., 12 December, 2:15-4:15pm in C317– Final Exam
• Exam 3 will be returned at the end of this class.
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Structure of Nucleotides in DNA
• Each nucleotide consists of– Deoxyribose (5-carbon sugar)
– Phosphate group
– A nitrogen-containing base
• Four bases– Adenine, Guanine, Thymine, Cytosine
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Nucleotide Bases
phosphate group
deoxyribose
ADENINE (A)
THYMINE (T)
CYTOSINE (C)
GUANINE (G)
Figure 13.6Page 220
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Composition of DNA
• Chargaff showed:– Amount of A relative to G differs among
species
– Always: A=T and G=C
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Rosalind Franklin’s Work
• Expert in X-ray crystallography
• Examined DNA fibers
• Concluded that DNA was some sort of helix
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Watson-Crick Modelof DNA
• 2 nucleotide strands– Run in opposite directions
– Held together by H bonds between bases
• A binds with T and C with G
• Molecule is a double helix
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DNA Structure Helps Explain How It Duplicates
• DNA is 2 nucleotide strands held together by H bonds
• H bonds between 2 strands are easily broken
• Each single strand then serves as template for new strand
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DNA Replication
newnew old old
• Each parent strand remains intact
• Every DNA molecule is half “old” and half “new”
Figure 13.9Page 222
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Base Pairing during
Replication
Each old strand serves as the template for complementary new strand
Figure 13.10Page 223
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Enzymes in Replication
• Enzymes unwind the two strands
• DNA polymerase attaches complementary nucleotides
• DNA ligase fills in gaps
• Enzymes wind two strands together
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A Closer Look at Strand Assembly
Energy for strand assembly is provided by removal of two phosphate groups from free nucleotides
newlyformingDNAstrand
one parent DNA strand
Figure 13.10Page 223
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Continuous and Discontinuous Assembly
Strands can only be assembled in the 5’ to 3’ direction
Figure 13.10Page 223
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DNA Repair
• Mistakes can occur during replication
• DNA polymerase can read correct sequence from complementary strand and, together with DNA ligase, can repair mistakes in incorrect strand
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Information• Mon., 28 November
– Chapter 14 and 16 highlights• Wed., 30 November
– Final exam review – BRING YOUR QUESTIONS!– Instructor evaluation
• Mon., 12 December, 2:15-4:15pm in C317– Final Exam
• Exam 3 will be returned ... Now!
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Exams
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Grades
• A = 100-79 • B = 78-70 • C = 69-61 • D = 60-53
• Average = 63• High = 99