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Explore the historical discoveries of DNA, including Griffith's transformation experiments and Hershey-Chase experiments. Learn about nucleotides, their structure, and base pairing rules. Understand the enzymes involved in DNA replication.
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Historical DNA Discoveries • 1928 • Federick Griffith finds a substance in heat-killed bacteria that “transforms” living bacteria • 1944 • Oswald Avery, Cloin MacLeod and Maclyn McCarty chemically identify Griffith’s transforming principle as DNA • 1949 • Erwin Chargaff reports relationships among DNA bases that provide a clue to the structure of DNA • 1953 • Alfred Hersey and Martha Chase demonstrate that DNA , not protein, is involved in viral reproduction. • 1953 • Rosalind Franklin produces an x-ray diffraction image of DNA
Historical DNA Discoveries II • 1953 • James Watson and Francis Crick propose a model of the structure of DNA. • 1958 • Matthew Meselson and Franklin Stahl demonstrate that DNA replication is semi conservative replication • 1962 • James Watson, Francis Crick and Maurice Wilkins are awarded the Nobel Prize in Medicine for discoveries about the molecular structure of nucleic acids. • 1969 • Alfred Hershey is awarded the Nobel Prize in Medicine for discovering the replication mechanism and genetic structure of viruses
Homework Assignment • Know Griffith’s Transformation Experiments • Know The Hershey-Chase Experiments
Nucleotides • Nucleotide subunits link together to form a single DNA strand • DNA Nucleotides • Nucleotides are the building blocks of nucleic acids • Phosphate • Sugar • Deoxyribose • Base • Purines (Two Rings) • Adenine • Guanine • Pyrimidines (One Ring) • Thymine • Cytosine
Nucleotides II • Nucleotides are linked together by covalent phosphodiester bonds • Each phosphate attaches to the 5’ end of one deozyribose and to the 3’ carbon of the neighboring deoxyribose • Makes up the sugar-phosphate backbone
Orientation of DNA • Each DNA molecule consists of two polynucleotide chains that associate as a double helix • The two chains run antiparallel
Base-Pairing Rules for DNAChargaff Rules • Hydrogen bonds hold the chains of the double helix together • Adenine forms two hydrogen bonds with thymine • Guanine forms three hydrogen bonds with cytosine • Chargaff’s rules • A always pairs with T • G always pairs with C • Complementary base pairing
Semi-conservative Replication • When Ecoli cells are grown for many generations in a medium containing heavy nitrogen, 15N they incorporate the 15N into their DNA. • When researchers transfer cells from a 15N medium to a 14N medium and isolate them after either one or two generations, the density of the DNA in each group is what would be expected if DNA replication were semi conservative. • In semi conservative replication, each daughter double helix consists of an original strand from the parent molecule and a newly synthesized complementary strand.
Introduction to the Strands • Template Strands {The Parental Strands} • Are the strands being copied • The original DNA strands • During DNA replication, both strands are copied • This means that there are TWO template strands • Complementary Strands {The Daughter Strands} • The NEW DNA strands produced from the Template Strands • During DNA replication, there are TWO complementary strands • Always remember that the process started with TWO template strands
Introduction to DNA Replication • DNA replication is bidirectional and starts at the origin of replication • The process proceeds in both directions from that point. • A eukaryotic chromosome may have multiple origins of replication • Allows the process to occur faster and more efficient • DNA replication/synthesis, of the complementary strands, proceed in a 5’ to 3’ direction. • Nucleotides can ONLY be added to the 3’ end. • This causes one of the complementary strands to be produced continuously and the other discontinuous • The continuous strand is called the leading strand • The discontinuous strand is called the lagging strand • Is first synthesized as short Okazaki fragments before becoming one strand
Enzymes of DNA Replication • Helicase • Unzips DNA double-helix • Topoisomerases • Prevents tangling and knotting of DNA as the while the strands are unzipped. • RNA primase • Initiates the formation of “daughter” strands • Forms a segment known as the RNA primer • The RNA primer contains the nitrogenous base Uracil • DNA Polymerase III • Enzyme that catalyzes the polymerization (making) of nucleotides • Adds Deoxyribonucleotides (nucleotides only found in DNA, as opposed to RNA) to the 3’ end of a growing nucleotide chain • Acts at the replication fork • DNA Polymerase I • A type of DNA polymerase will change the RNA primers into DNA • Changing the base Uracil into Thymine
Enzymes of DNA Replication • DNA Ligase • Enzyme responsible for joining Okazaki fragments forming the Lagging Strand • Gyrase • Returns the DNA strands into a Double Helix • Zips the DNA back together
Telomeres & Telomerase • End of eukaryotic chromosomes are known as telomeres • Short, non-coding, repetitive DNA sequences • Shorten each cell cycle but can be extended using the enzyme telomerase • Absence of telomerase in certain cells may be the cause of cell aging • Cells using their ability to divide after a limited number of cell divisions • Most cancer cells have telomerase to maintain the telomeres and possibly resist apoptosis.
