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AP Biology Chapter 9

AP Biology Chapter 9. Griffith 1928. Proved transformation of bacteria into a mouse Had two strains of bacteria An avirulent or nonlethal strain (R) A virulent of lethal strain (S). Griffith’s Experiment. When he put the virulent strain in the mouse, it died

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AP Biology Chapter 9

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  1. AP Biology Chapter 9

  2. Griffith 1928 • Proved transformation of bacteria into a mouse • Had two strains of bacteria • An avirulent or nonlethal strain (R) • A virulent of lethal strain (S)

  3. Griffith’s Experiment • When he put the virulent strain in the mouse, it died • When he put the avirulent strain in the mouse, the mouse lived! • Then he heated the virulent strain and then put it into the mouse and the mouse lived! • When he put the heated virulent strain + the nonvirulent strain into the mouse, the mouse died  • Why?

  4. Explanation • Transformation had occurred • The nonvirulent bacteria took up something in the dead heated virulent strain: a “transforming principle” and changed the nonvirulent strain into a virulent strain!!

  5. Other Scientists of interest • Before 1940, biologists thought that proteins were the information molecules because they were so complex and had a lot of variety • Avery, Macleod and McCarty in 1944 proved that the transformation principle in Griffith’s experiment was DNA!

  6. Other Scientists of interest • Hershey and Chase in 1952 proved that bacteriophages (viruses that attack bacteria) inject DNA into bacterial cells • Franklin and Wilkins used x ray diffraction on DNA to determine the distances between molecules • Watson and Crick in 1953 came up with the model of DNA

  7. Chargaff’s rules • He found a simple relationship in DNA called the base pairing rules • Adenine = Thymine • Guanine = Cytosine

  8. Watson and Crick • Crick: English phage geneticist at the Cavendish labs at Cambridge University, London England • Watson: American postdoc in Crick’s lab • Both visited Wilkins & Franklin routinely 1951-53 • Derived the overall concept of the chemical relationship • Considered how Chargaff’s rules represented the structure of DNA • Franklin’s X ray data • Built little tin models of the nucleotides and put the DNA model together like a TinkerToy set • Correctly deduced the structure of DNA (double helix)

  9. The Double Helix • This is the Watson and Crick model worked out in 1953 and published in a single-page article in Nature of that year. • Was convincing structurally: gave evidence for how DNA replicated • Most famous biology paper ever written!

  10. DNA Structure • Calleddeoxyribonucleic acid • Made up ofnucleotides which have 3 parts • sugar – deoxyribose • Phosphate • Nitrogen base

  11. Deoxyribose • Pentose sugar = 5 carbons • Carbons on the sugar are numbered 1 through 5 and the first carbon (1’) is linked to one of the four nitrogen bases (ACTG) nitrogen base

  12. phosphate • Is attached to the 5’ and 3’ carbon, making a phosphodiester linkage • Forms the sugar phospate backbone of DNA (or the ladder)

  13. Nitrogen base • Remember these are connected to the 1’ carbon of the sugar • 2 groups • Purines • Have two ring structures • Adenine and guanine • Pyrimidines • Have one ring structure • Thymine and cytosine • The number of purines = number of pyrimidines

  14. DNA molecule • Consists of 2 polynucleotide chains arranged in a coiled double helix • Helix is like a ladder • Two strands run in opposite directions and are said to be antiparallel to each other • The nitrogen bases are bonded by hydrogen bonds • A = T and C=G according to Chargaff’s rules

  15. Hydrogen bonds

  16. What is the complement of 3’AGCTAC5’? 5' TCGATG 3'

  17. How Does DNA Replicate? • Several research groups worked on this. We’ll discuss one • 1957: Matthew Meselsohn and Fred Stahl • They had 3 hypotheses

  18. DNA replication is “semiconservative” • One old strand kept with each of the new molecules; one old paired with one new strand

  19. 2. DNA replication is “conservative” • Double strand maintained intact; new strands are together in the new molecule

  20. 3. DNA replication is “dispersive” • Strands cut up and the old and new DNA interspersed in both new strands Which hypothesis was right?

  21. Semiconservativeis correct! • Each strand acts as a template for the other, and so the mutation will propagate through successive generations.

  22. How Does Replication Start? • The replication complex binds at the origin of replication, which is identified by a particular base sequence. This is initiated by RNA primer • Helicase unwinds the DNA, which is held open with helix-destabilizing proteins. Replication starts in the Y-shaped replication fork. DNA polymerase origin of replication RNA primer proteins DNA polymerase helicase RNA primer

  23. Replication Proceeds on Two Strands • Nucleotides are always added to the 3’ end by DNA polymerase, thus moving in the 3’ to 5’ direction • but the new strands elongate in opposite directions • The leading strand elongates into the fork • The lagging strand elongates away from the fork • Elongation proceeds smoothly on the leading strand leading strand DNA helix RNA primer DNA polymerase replication fork lagging strand

  24. Leading and Lagging Strands • As the fork grows, both new strands elongate further • Subunit addition to the lagging strand is by 100-2000 base Okazaki fragments. • The lagging strand grows in a discontinuous manner because of the size of the Okazaki fragments • That’s why it lags leading strand RNA primer Okazaki fragments

  25. The Lagging Strand • Notice that the lagging strand is always growing away from the replication fork • The gaps between the Okazaki fragments are joined together by DNA ligase leading strand DNA ligase

  26. In Action!

  27. Review of Enzymes helicase • Unwinds the DNA • Puts down RNA primer • Adds bases to strands • Seals Okazaki fragments up • Winds the DNA molecule back together primase DNA polymerase ligase gyrase

  28. DNA Repair • DNA polymerase proofreads what bases had been laid down • If there is a mistake, it will go back and remove the wrong base and fix it

  29. How does DNA fit in the cell? • By histones • Positively charged proteins (due to the high number of amino acids) • Are able to associate with DNA which is negatively charged (due to the phosphate groups) • Histones and DNA form structure called nucleosome

  30. Nucleosome • Are 8 histones with DNA wrapped around it • Are part of the chromatin • Prevent DNA strands from becoming tangled

  31. Cellular Ageing and DNA • The replication process never entirely completes at the ends of the chromosomes • However, DNA is protected at its ends with long strands that do not carry any genetic information, called telomeres • as we age, they become shorter • They are repaired and lengthened with an enzyme called telomerase • Loss of telomerase activity may be an important cause of cellular aging

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