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DNA: The Genetic Material

Explore the fascinating world of DNA through key experiments and discoveries that unveiled its structure and replication mechanism. Learn about nucleotides, Chargaff's Rules, X-ray diffraction studies, and the double-helix model proposed by Watson and Crick.

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DNA: The Genetic Material

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  1. DNA: The Genetic Material Chapter 14

  2. Fig. 14.6

  3. Fig. 14.7

  4. E. coli grown in 15N medium E. coli shifted to 14N medium Samples taken at 0, 1 generation, and 2 generations (replication rounds) samples centrifuged at high speeds The positions of radioactive 15N in the centrifuge tubes are visualized by placing tubes against photographic film

  5. Fig. 14.18

  6. Fig. 14.19

  7. Fig. 14.20

  8. Fig. 14.17

  9. Fig. 14.22

  10. Fig. 14.23

  11. Fig. 14.24

  12. Fig. 14.26

  13. Fig. 14.26

  14. The Genetic Material Frederick Griffith, 1928 studied Streptococcus pneumoniae, a pathogenic bacterium causing pneumonia there are 2 strains of Streptococcus: - S strain is virulent - R strain is nonvirulent Griffith infected mice with these strains hoping to understand the difference between the strains

  15. The Genetic Material Griffith’s results: - live S strain cells killed the mice - live R strain cells did not kill the mice - heat-killed S strain cells did not kill the mice - heat-killed S strain + live R strain cells killed the mice

  16. The Genetic Material Griffith’s conclusion: - information specifying virulence passed from the dead S strain cells into the live R strain cells - Griffith called the transfer of this information transformation

  17. The Genetic Material Avery, MacLeod, & McCarty, 1944 repeated Griffith’s experiment using purified cell extracts and discovered: - removal of all protein from the transforming material did not destroy its ability to transform R strain cells - DNA-digesting enzymes destroyed all transforming ability - the transforming material is DNA

  18. The Genetic Material Hershey & Chase, 1952 - investigated bacteriophages: viruses that infect bacteria - the bacteriophage was composed of only DNA and protein - they wanted to determine which of these molecules is the genetic material that is injected into the bacteria

  19. The Genetic Material - Bacteriophage DNA was labeled with radioactive phosphorus (32P) - Bacteriophage protein was labeled with radioactive sulfur (35S) - radioactive molecules were tracked - only the bacteriophage DNA (as indicated by the 32P) entered the bacteria and was used to produce more bacteriophage - conclusion: DNA is the genetic material

  20. DNA Structure DNA is a nucleic acid. The building blocks of DNA are nucleotides, each composed of: • a 5-carbon sugar called deoxyribose • a phosphate group (PO4) • a nitrogenous base • adenine, thymine, cytosine, guanine

  21. DNA Structure The nucleotide structure consists of • the nitrogenous base attached to the 1’ carbon of deoxyribose • the phosphate group attached to the 5’ carbon of deoxyribose • a free hydroxyl group (-OH) at the 3’ carbon of deoxyribose

  22. DNA Structure Nucleotides are connected to each other to form a long chain phosphodiester bond: bond between adjacent nucleotides • formed between the phosphate group of one nucleotide and the 3’ –OH of the next nucleotide The chain of nucleotides has a 5’ to 3’ orientation.

  23. DNA Structure Determining the 3-dimmensional structure of DNA involved the work of a few scientists: • Erwin Chargaff determined that • amount of adenine = amount of thymine • amount of cytosine = amount of guanine This is known as Chargaff’s Rules

  24. DNA Structure Rosalind Franklin and Maurice Wilkins • Franklin performed X-ray diffraction studies to identify the 3-D structure • discovered that DNA is helical • discovered that the molecule has a diameter of 2nm and makes a complete turn of the helix every 3.4 nm

  25. DNA Structure James Watson and Francis Crick, 1953 • deduced the structure of DNA using evidence from Chargaff, Franklin, and others • proposed a double helixstructure

  26. DNA Structure The double helix consists of: • 2 sugar-phosphate backbones • nitrogenous bases toward the interior of the molecule • bases form hydrogen bonds with complementary bases on the opposite sugar-phosphate backbone

  27. DNA Structure The two strands of nucleotides are antiparallel to each other • one is oriented 5’ to 3’, the other 3’ to 5’ The two strands wrap around each other to create the helical shape of the molecule.

  28. DNA Replication Matthew Meselson & Franklin Stahl, 1958 investigated the process of DNA replication considered 3 possible mechanisms: • conservative model • semiconservative model • dispersive model

  29. DNA Replication Bacterial cells were grown in a heavy isotope of nitrogen, 15N all the DNA incorporated 15N cells were switched to media containing lighter 14N DNA was extracted from the cells at various time intervals

  30. DNA Replication The DNA from different time points was analyzed for ratio of 15N to 14N it contained After 1 round of DNA replication, the DNA consisted of a 14N-15N hybrid molecule After 2 rounds of replication, the DNA contained 2 types of molecules: • half the DNA was 14N-15N hybrid • half the DNA was composed of 14N

  31. DNA Replication Meselson and Stahl concluded that the mechanism of DNA replication is the semiconservative model. Each strand of DNA acts as a template for the synthesis of a new strand.

  32. DNA Replication DNA replication includes: • initiation – replication begins at an origin of replication • elongation – new strands of DNA are synthesized by DNA polymerase • termination – replication is terminated differently in prokaryotes and eukaryotes

  33. Prokaryotic DNA Replication The chromosome of a prokaryote is a circular molecule of DNA. Replication begins at one origin of replication and proceeds in both directions around the chromosome.

  34. Prokaryotic DNA Replication The double helix is unwound by the enzyme helicase DNA polymerase III (pol III) is the main polymerase responsible for the majority of DNA synthesis DNA polymerase III adds nucleotides to the 3’ end of the daughter strand of DNA

  35. Prokaryotic DNA Replication DNA replication is semidiscontinuous. • pol III can only add nucleotides to the 3’ end of the newly synthesized strand • DNA strands are antiparallel to each other leading strand is synthesized continuously (in the same direction as the replication fork) lagging strand is synthesized discontinuously creating Okazaki fragments

  36. Prokaryotic DNA Replication The enzymes for DNA replication are contained within the replisome. The replisome consists of • the primosome - composed of primase and helicase • 2 DNA polymerase III molecules The replication fork moves in 1 direction, synthesizing both strands simultaneously.

  37. Eukaryotic DNA Replication The larger size and complex packaging of eukaryotic chromosomes means they must be replicated from multiple origins of replication. The enzymes of eukaryotic DNA replication are more complex than those of prokaryotic cells.

  38. Eukaryotic DNA Replication Synthesizing the ends of the chromosomes is difficult because of the lack of a primer. With each round of DNA replication, the linear eukaryotic chromosome becomes shorter.

  39. Eukaryotic DNA Replication telomeres – repeated DNA sequence on the ends of eukaryotic chromosomes • produced by telomerase telomerase contains an RNA region that is used as a template so a DNA primer can be produced

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