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DNA Replication

DNA Replication. Packet #43 Chapter #16. Historical Facts About DNA. Historical DNA Discoveries. 1928 Federick Griffith finds a substance in heat-killed bacteria that “transforms” living bacteria 1944

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DNA Replication

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  1. DNA Replication Packet #43 Chapter #16

  2. Historical Facts About DNA

  3. 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

  4. 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

  5. Griffith Experiment • The Griffith experiment, conducted in 1928, was one of the first experiments suggesting that bacteria are capable of transferring genetic information through a process known as transformation.

  6. Hershey Chase Experiment • Hershey and Chase conduced an experiment using viral DNA to show that the DNA was the genetic material being inserted into the bacteria and used to replicate more viruses.

  7. Structure of DNA

  8. Introduction I • DNA is an organic macromolecule known as a nucleic acid. • Nucleic Acids are composed of building blocks known as nucleotides. • Nucleotides have three parts: - • Phosphate • Sugar • Nitrogenous bases

  9. DNA Nucleotides • Multiple DNA nucleotide subunits link together to form a single DNA strand. • DNA nucleotides are composed of: - • Phosphate • Sugar • Deoxyribose • Nitrogenous Bases • Purines (Two Rings) • Adenine • Guanine • Pyrimidines (One Ring) • Thymine • Cytosine

  10. DNA Nucleotides II • Nucleotides are linked together by covalent phosphodiester bonds • Each phosphate attaches to the 5’ end (carbon #5) of one deoxyribose and to the 3’ end (carbon #3) of the neighboring deoxyribose • Makes up the sugar-phosphate backbone

  11. DNA Strands • Each DNA strand, that is composed of multiple nucleotides, has a head and a tail. • Head = 5’ end • Phosphate group • Tail = 3’ end • Hydroxyl group

  12. DNA Molecule • Each DNA molecule consists of two DNA strands (polynucleotide chains) that associate as a double helix • The two strands/chains run antiparallel

  13. Base-Pairing Rules for DNAChargaff Rules • The two DNA strands are joined together at the nitrogenous bases. • Holding the bases together, and allowing the formation of the double helix, are hydrogen bonds.

  14. Base-Pairing Rules for DNAChargaff Rules II • Adenine forms two hydrogen bonds with thymine • Guanine forms three hydrogen bonds with cytosine • These pairings are known as Chargaff’s rules • A always pairs with T • G always pairs with C • Complementary base pairing

  15. Chargaff Rules III

  16. Models of DNA Replication

  17. Models of DNA Replication • There were three models proposed about how DNA replicates. • However, the one that stood the test was semi-conservative replication.

  18. DNA Replication Introduction • In semi-conservative replication, each “old” strand of DNA is used to create a new complementary strand.

  19. Introduction to DNA Replication The Players

  20. 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

  21. Introduction to the Strands II • 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

  22. Origin of Replication & Bi-directionality. • 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

  23. Introduction to the Making of the Complementary Strand. • DNA replication/synthesis, of the complementary strands, proceed in a 5’ to 3’ direction. • Nucleotides can ONLY be added to the 3’ end.

  24. Introduction to the Making of the Complementary Strand. • Since DNA nucleotides can only be added to the 3’ end, it 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

  25. Enzymes of DNA Replication & The Steps of DNA Replication

  26. 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

  27. Enzymes of DNA Replication II • 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

  28. Enzymes of DNA Replication III • 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

  29. DNA Replication—The Big Picture

  30. DNA Replication—Lagging Strand

  31. Post DNA Replication

  32. DNA Excision RepairDNA Polymerase II • On some occasions, errors in nucleotides may occur while making the new DNA strand. • Errors such as mismatches & dimers may occur. • To correct these errors, the enzymes nuclease, DNA polymerase III and DNA ligase are used during the process known as excision repair.

  33. Telomeres, Telomerase & DNA Shortening • At the end of eukaryotic chromosomes are known as telomeres • Short, repetitive DNA sequences that do not contain genes. • Typically 100 to 1000 nucleotides • TTAGGG (Humans) • Telomeres help protect the organism's genes from being eroded through successive rounds of DNA replication.

  34. Telomeres, Telomerase & DNA Shortening • Telomeres shorten each cell cycle (DNA replication sequence) but can be extended using the enzyme telomerase • Absence of telomerase in certain cells may be the cause of “cell aging” • Cells having a limited number of cell divisions • Most cancer cells have telomerase to maintain the telomeres and possibly resist apoptosis.

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