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Ch. 16

Ch. 16. The search for the genetic material led to DNA. Experiments with bacteria and, later, with phages provided the first strong evidence that the genetic material is DNA. The Hershey-Chase Experiment.

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Ch. 16

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  1. Ch. 16

  2. The search for the genetic material led to DNA • Experiments with bacteria and, later, with phages provided the first strong evidence that the genetic material is DNA.

  3. The Hershey-Chase Experiment • In 1953, Alfred Hershey and Martha Chase confirmed DNA's preeminent role in genetics by demonstrating that DNA is the genetic material of a virus called phage T2. • The phage, which infects E. coli, consists of a head, sheath, tail, and base plate made of different proteins. DNA is packaged within the head of the virus. • When T2 comes in contact with E. coli, the phage attaches to the bacterium by its tail. Next, the phage injects genetic material into the cell. The genetic material directs bacterial enzymes to produce viral offspring.

  4. The Hershey-Chase Experiment • When the life cycle is complete, 100 to 200 progeny phages have been assembled inside each bacterium. The bacterium breaks open, or lyses, and the phages are released. • Since T2 consists of only DNA and protein, Hershey and Chase reasoned that the genetic material must be one of the two components. So they designed an experiment to determine which it is. • For their experiment, the scientists prepared T2 phages that either had radioactive DNA or radioactive proteins.

  5. The Hershey-Chase Experiment • To make the components radioactive, Hershey and Chase infected E. coli with T2, and grew the bacterial cells in two different culture media -- one containing the radioactive isotope phosphorous-32 and the other containing the radioactive isotope sulfur-35. • In the next phase of their experiment, Hershey and Chase infected E. coli with the two types of labeled T2. They then used a blender and a centrifuge to separate the protein parts of the phage -- called the phage ghosts -- from the rest of the infected cell.

  6. The Hershey-Chase Experiment • The scientists discovered that the labeled DNA appeared in the host cell but not the phage ghosts, while the labeled proteins appeared in the phage ghosts but not the host cell. In addition, the progeny of the T2 with labeled DNA were radioactive, while the progeny of the T2 with labeled proteins were not radioactive. • Because genetic material is passed on from parent to offspring, Hershey and Chase concluded that DNA had to be the genetic material of T2. Their results helped convince the scientific community that DNA was the hereditary material.

  7. Watson and Crick discovered the double helix by building models to conform to X-ray data • Watson and Crick discovered that DNA is a double helix. Two antiparallel sugar-phosphate chains wind around the outside of the molecule; the nitrogenous bases project into the interior, where they hydrogen-bond in specific pairs, A with T and G with C.

  8. DNA & RNA Structure

  9. DNA Double Helix

  10. DNA Replication and Repair • During DNA replication, base pairing enables existing DNA strands to serve as templates for new complementary strands. DNA replication is semiconservative: The parent molecule unwinds, and each strand then serves as a template for the synthesis of a new half-molecule according to base-pairing rules.

  11. DNA replication: overview

  12. A large team of enzymes and other proteins carries out DNA replication • Replication begins at origins of replication. Y-shaped replication forks form at opposite ends of a replication bubble, where the two DNA strands separate. DNA polymerases catalyze the synthesis of new DNA strands, working in the 5' 3' direction. DNA synthesis at a replication fork yields a continuous leading strand and short, discontinuous segments of lagging strand. The fragments are then joined together by DNA ligase. DNA synthesis must start on the end of a primer, which is a short segment of RNA.

  13. DNA Replication

  14. DNA Replication Review

  15. Enzymes proofread DNA during its replication and repair damage in existing DNA • DNA polymerase proofreads newly made DNA, replacing any incorrect nucleotides. In mismatch repair of DNA, repair enzymes correct errors in base pairing. In excision repair, enzymes cut out and replace damaged stretches of DNA.

  16. The ends of DNA molecules are replicated by a special mechanism • The ends of the linear DNA molecules of eukaryotic chromosomes, called telomeres, get shorter with each replication. The enzyme telomerase, present in certain cells, can extend the ends.

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