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DNA History and Structure Pre-AP Biology, Mrs. Krouse Early ideas about the genetic material of the cell Why did scientists originally think protein was the genetic material (aka genetic molecule of the cell?) Proteins have a very complex structure -20 different amino acids can be combined in various orders to create long polypeptide chains -Polypeptides can be hundreds of amino acids in length (Rubisco, an enzyme involved in photosynthesis, is composed of a polypeptide that is approximately 500 amino acids long) -Many proteins consist of MULTIPLE polypeptides folded around each other The 20 amino acids Polypeptides and Proteins A single polypeptide A protein (contains multiple polypeptides) Protein folding at the secondary, tertiary, and quaternary levels determines the protein’s function because in biology, structure (shape) function What do we mean when we say “genetic material?” • A coded molecule in the cell that provides instructions for creating cell substances and directing cell activities • This molecule is passed down through generations of offspring Review from our Biochemistry Unit (Unit 2) • What do we call the building blocks of the four macromolecules (carbohydrates, lipids, proteins, and nucleic acids)? (Hint: Amino acids are examples of these building blocks) • What do we call chains of building blocks of the four macromolecules? (Hint: Polypeptides are an example of these chains) Frederick Griffith’s Experiment • R vs. S strain pneumonia bacteria Frederick Griffith’s Experiment (ctd) • Why did the mouse in the fourth trial die? Heat destroyed the S strain bacteria by breaking apart the bacterial cells, but it did not destroy the S strain DNA. Some R strain bacteria were “transformed” by taking in S strain DNA, which gave them the ability to build the smooth outer capsule and become S strain bacteria. The mouse’s immune system was unable to destroy the S strain bacteria (due to the protective capsule), so it died. What is transformation? • Transformation occurs when a cell takes in DNA from its surroundings (like the R strain bacteria taking in DNA from the heat-killed S strain bacteria in Trial 4 of Griffith’s experiment) Did Frederick Griffith know that the genetic molecule of the bacteria was DNA? • He did not know that the genetic molecule of the bacteria was DNA. He knew some molecule had been passed from the heat-killed S strain to the R strain bacteria to cause them to build a smooth capsule. • He called this unknown molecule the “transforming principle.” Avery, McCarty, and Macleod’s Experiment • Tried to determine which molecule was the “genetic molecule” of the pneumonia bacteria used in Griffith’s experiment Avery, McCarty, and Macleod’s Experiment (ctd) • Results = transformation of the R strain bacteria to make them S strain bacteria… -did occur when enzymes for breaking down RNA, proteins, lipids, and carbohydrates were used -did NOT occur when enzymes for breaking down DNA were used • Conclusions = DNA must be the genetic molecule of the bacteria cells because when it is destroyed, the R strain bacteria are not transformed Hershey and Chase Experiment Hershey and Chase Experiment (ctd) • Trial #1: Radioactively labeling sulfur (35S) found in the protein coat of the virus • Results: The radioactive sulfur was only found outside the bacterial host cell. • Conclusion: The protein coat of the virus (aka capsid) does not get injected into the host cell during infection Hershey and Chase Experiment (ctd) • Trial #2: Radioactively labeling phosphorus (32P) found in the DNA of the virus • Results: The radioactive sulfur was only found inside the bacterial host cell • Conclusion: The DNA of the virus gets injected into the host cell during infection Hershey and Chase Experiment (ctd) • Overall Conclusion = DNA is the genetic molecule of viruses because it is injected into host cells during infection and directs the host cell to create baby viruses Franklin + Wilkins and Watson + Crick • X-ray crystallography (aka X-ray diffraction) DNA Structure • DNA is an example of which type of macromolecule (carbohydrate, lipid, protein, nucleic acid)? DNA Structure (ctd) • Full Name of DNA = deoxyribonucleic acid • Function of DNA = to store genetic information • Monomer of DNA = Nucleotide • Overall Structure of DNA = two coiled chains of nucleotides called the double helix -Double because there are two chains -Helix because the chains are twisted around each other in a spiral fashion DNA Structure (ctd) Nucleotide Structure • Three parts of a nucleotide -Phosphate group -Deoxyribose (aka 5-carbon sugar or pentose sugar) -Nitrogenous base (aka nitrogen base) DNA Double Helix • Untwisted double helix looks like a ladder -Sides of the ladder (backbone of each chain of nucleotides) = phosphate groups bonded to deoxyribose sugars -Rungs of the ladder = nitrogen bases bonded together by weak hydrogen bonds DNA is Antiparallel • 5 prime (5’) end of a DNA strand (one chain of nucleotides) = ends with a phosphate group • 3 prime (3’) end of a DNA strand = ends with a sugar • The strands of the double helix are antiparallel, which means they are side-by-side (parallel) but their 5’ and 3’ ends are opposite (anti) one another Nitrogen Bases • Two types of bases = -Purines have a double-ring structure -Pyrimidines have a single- ring structure Memory trick = the name lengths are opposite the size of the molecule. “Purine” is a shorter word but has a larger (double ring) structure. “Pyrimidine” is a longer word but has a smaller (single ring) structure • Purines = adenine and guanine • Pyrimidines = thymine and cytosine Hydrogen Bonds between Nitrogen Bases • Purines only pair with pyrimidines across the DNA double helix. They are connected by hydrogen bonds. • Two hydrogen bonds connect adenine and thymine. • Three hydrogen bonds connect guanine and cytosine. Why do purines only pair with pyrimidines? • Pyrimidines cannot pair with pyrimidines because they are so small that they don’t get close enough to each other (given the width of the double helix) to form hydrogen bonds with each other • Purines cannot pair with purines because they are so large that they would not fit within the width of the double helix Chargaff’s Experiment • Determined the percentages of each nitrogen base in the DNA double helix. These frequencies (in humans) were… - A = 30% - T = 30% - C = 20% - G = 20% Chargaff’s Experiment (ctd) • He looked at the frequencies of nitrogen bases for the DNA in other species as well! Chargaff’s Experiment (ctd) • Conclusion: Adenine must bond (aka pair) with thymine across the double helix because they have equal frequencies. This means where you find an adenine, you will always find a thymine and vice versa. For the same reason, he concluded that guanine must pair with cytosine. • As such, A / T and G / C are said to be complementary to each other. Their bonding (aka pairing) across the double helix is therefore called complementary base pairing. Practice Questions • If there is 30% adenine in a particular DNA molecule, how much cytosine is present? Practice Questions (ctd) • Write out the nitrogen base sequence of a DNA strand complementary to the following strand. T T A G C A T G G