dna structure and function chapter 13. miescher discovered dna 1868 johann miescher investigated the...
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DNA Structure and Function
Chapter 13
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)
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
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
Griffith Discovers Transformation
• 1928
• Attempting to develop a vaccine
• Isolated two strains of Streptococcus pneumoniae– Rough strain was harmless
– Smooth strain was pathogenic
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
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
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
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
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?
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
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.
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
Nucleotide Bases
phosphate group
deoxyribose
ADENINE (A)
THYMINE (T)
CYTOSINE (C)
GUANINE (G)
Figure 13.6Page 220
Composition of DNA
• Chargaff showed:
– Amount of A relative to G differs among
species
– Always: A=T and G=C
Rosalind Franklin’s Work
• Expert in X-ray crystallography
• Examined DNA fibers
• Concluded that DNA was some sort of helix
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
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
DNA Replication
newnew old old
• Each parent strand
remains intact
• Every DNA
molecule is half
“old” and half “new”
Figure 13.9Page 222
Base Pairing during
Replication
Each old strand serves as the template for complementary new strand
Figure 13.10Page 223
Enzymes in Replication
• Enzymes unwind the two strands
• DNA polymerase attaches
complementary nucleotides
• DNA ligase fills in gaps
• Enzymes wind two strands together
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
Continuous and Discontinuous Assembly
Strands can only be assembled in the 5’ to 3’ direction
Figure 13.10Page 223
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
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!
Exams
Grades
• A = 100-79
• B = 78-70
• C = 69-61
• D = 60-53
• Average = 63
• High = 99