Download - DNA Structure and Function
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DNA Structure and Function
Chapter 12
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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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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.
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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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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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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
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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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Hershey and Chase Results
virus particlelabeled with 35S
DNA (blue)being injected into bacterium
35S remainsoutside cells
virus particlelabeled with 32P
DNA (blue)being injected into bacterium
35P remainsinside cells
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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
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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
sugar (deoxyribose)
adenine A
base with a double-ring
structure
guanine (G)
base with a double-ring
structure
cytosine (C)
base with a single-ring structure
thymine (T)
base with a single-ring structure
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Composition of DNA
• Chargaff showed:– Amount of adenine relative to guanine
differs among species
– Amount of adenine always equals amount of thymine and amount of guanine always equals amount of cytosine
A=T and G=C
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Watson-Crick Model
• DNA consists of two nucleotide strands
• Strands run in opposite directions
• Strands are held together by hydrogen bonds between bases
• A binds with T and C with G
• Molecule is a double helix
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Watson-Crick Model
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Patterns of Base Pairing
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Rosalind Franklin’s Work
• Was an expert in x-ray crystallography
• Used this technique to examine DNA fibers
• Concluded that DNA was some sort of helix
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DNA Structure Helps Explain How It Duplicates
• DNA is two nucleotide strands held together by hydrogen bonds
• Hydrogen bonds between two 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”
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Base Pairing during
Replication
Each old strand serves as the template for complementary new strand
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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
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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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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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Cloning
• Making a genetically identical copy of an individual
• Researchers have been creating clones for decades
• These clones were created by embryo splitting
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• Showed that differentiated cells could be used to create clones
• Sheep udder cell was combined with enucleated egg cell
• Dolly is genetically identical to the sheep that donated the udder cell
Dolly: Cloned from an Adult Cell
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More Clones
• Mice• Cows• Pigs• Goats• & endangered species