I. DNA as Genetic Material Figure 1: Griffith s Experiment. Frederick Griffith:

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1 I. DNA as Genetic Material Figure 1: Griffith s Experiment Frederick Griffith: a) Frederick Griffith suspected that some component of the S strain was being passed along to the R strain, causing it to become pathogenic (disease-causing). He called this phenomenon Transformation. b) He was never able to identify the transforming agent (DNA) responsible for the conversion of the R cells into lethal S cells. Today, we understand transformation to be a process by which a cell can absorbed free DNA from its surroundings & thus express a new trait. Oswald Avery attempted to identify the mysterious transforming agent that Griffith could not. He attempt to exposed living R cells to components of dead S cells. Whichever S cell component resulted in the transformation of the living R cells must be the transforming agent in Griffith s original experiment Figure 2: Avery s Experiment (Pt. 1)

2 Figure 2: Avery s Experiment (Pt. 2) Oswald Avery: a) Avery s findings were met with skepticism because, although DNA may be the genetic material of bacteria, it was believed that protein was a more likely candidate for the genetic material of higher organisms. Figure 3: Hershey-Chase Experiment Hershey & Chase attempted to identify whether DNA or protein was the genetic material. They radioactively labeled two cultures of the T2 bacteriophage: a) Culture 1: Proteins of the phage outer coat were labeled w/radioactive sulfur. b) Culture 2: DNA of the phage was labeled with radioactive phosphorous. The phages were then allowed to infect bacteria & each culture was spun in a centrifuge to separate the viral particles from the bacteria, which collected into a dense pellet. Whichever radioactive component was detected in the pellet, must be the genetic material Conclusion: Hershey-Chase:

3 II. DNA Chemistry Figure 4: Nucleotides & Nucleotide Chains Nucleotide: a) A DNA nucleotide can bear 1 of 4 Nitrogenous Bases which are classified into 2 groups: Purines: adenine & guanine (one 5-sided + one 6-sided ring). Pyrimidines: cytosine & thymine (one 6-sided ring). Nucleotide Chain: Figure 5: X-Ray Diffraction This technique was used to take photographs of molecules like DNA. When exposed to X-rays, the atoms of the sample scatter the rays onto photographic film behind the sample. This causes the film to develop wherever the X-rays strike. Analysis of the pattern produced by DNA was studied to determine the precise structure of the molecule Method of X-Ray Diffraction X-Ray Photo of DNA

4 Figure 6: DNA Double Helix Structure Antiparallel Arrangement of Nucleotide Chains Arrangement of Nucleotide Bases Watson & Crick: built scale models of the DNA components & then fit them together according to X-ray diffraction data. After several trials, they worked out a model that fit the experimental data. a) Each strand of the molecule is arranged in an Antiparallel manner. This means that is one strand begins with a 5 phosphate at one end & ends with a 3 hydroxyl at the other, the neighboring strand runs in the reverse manner. b) The DNA molecule was found to have a constant diameter (.2nm). In order to maintain this diameter, the nitrogenous bases would have to be organized such that each purine from one strand of the molecule is paired with a pyrimidine from the other. c) The nitrogenous bases of each strand must pair in Complementary manner. This means that the base sequence on one strand determines that of the other: 5 -ATACGGATCGATCGATCTA-3 3 -TATGCCTAGCTAGCTAGAT-5 Double Helix:

5 III. DNA Replication Figure 7: Semiconservative Model of DNA Replication The semiconservative model suggests that when DNA replicates during the S phase of the cell cycle, both strands of the molecule separate. Each strand can then serve as a template (guide) for the construction of a new complementary strand. The result is the formation of 2 genetically identical daughter DNA molecules. Semiconservative Replication: Stages of DNA Replication Figure 8: Unzipping & Unwinding: Origins of Replication The origins of replication are areas along DNA where the molecule can begin to replicate. Multiple origins of replication make the overall process proceed faster.

6 Figure 8.1: Unzipping & Unwinding: Replication Fork Replication Fork Helicase: Topoisomerase: Single Stranded Binding Proteins: Figure 8.2: Priming of DNA Templates

7 Figure 8.3: Creating Complementary DNA Chains: Action of DNA Polymerase DNA Polymerase: a) This enzyme is also capable of proofreading the base sequence on the new complementary strand to check for errors that would lead to mutations. Figure 8.4: Creating Complementary DNA Chains Polymerase III forms a new chain in the 3 direction by adding nucleotides complementary to those of template. Ultimately, a problem arises concerning the extension of the new strand growing in the opposite direction to which the replication fork is expanding

8 Helicase & Topoisomerase continue to unzip & unwind the helix to expand the replication fork. Consequently, a gap opens up behind the 5 end of each RNA primer. Primase deposits an additional RNA primer BEHIND the pre-existing ones near at each replication fork... DNA Polymerase III attaches to each new primer & creates new DNA in the gaps created by the expanding replication forks. Eventually, new gaps will open up behind the newly-formed strand as the replication forks continue to expand

9 Once again, primase deposits new primers at each replication fork & DNA polymerase III creates new DNA to fill the gaps. At this point, two distinct DNA strands begin to appear along each template Leading Strand: Lagging Strand: a) Once the construction of the leading & lagging strands is complete, DNA polymerase I will remove the primers one the lagging strand & replace the resulting gaps with DNA

10 The gaps created by the removal of RNA primers along the lagging strands are filled-in by DNA polymerase I The incomplete sugar-phosphate backbone in between each Okazaki fragment along the lagging strands are joined by DNA Ligase to form a continuous sugar-phosphate backbone. DNA Ligase:

11 Figure 8.5: Summary of DNA Replication Figure 9: Telomeres & Telomerase With each round of replication, the DNA molecule loses some nucleotides as primers on the leading strand are removed & not replaced with DNA Telomere: Telomerase:

12 IV. Genetic Mutations Figure 10: Base Substitutions Base Substitution: Effects of Base Substitutions Figure 11: Nonsense Mutations Nonsense Mutation: base substitution creating a stop codon w/in mrna where none previously existed. Results in a shorter than normal polypeptide leading to an almost always nonfunctional protein.

13 Figure 11.1: Missense Mutations Missense Mutation: base substitution resulting in the alteration of mrna codons. For example, if an AGU is changed to an AGA, the protein will have an arginine where a serine was meant to go. This might alter the final 3-D shape & function the resulting polypeptide/protein. Figure 11.2: Silent Mutations Silent Mutation: base substitution that changes the base sequence without changing the amino acid sequence of the resulting a protein.

14 Figure 12: Deletions, Insertions, & Frameshifts Deletion: Insertion: Frameshift:

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