CHAPTER 11 DNA and Its Role in Heredity

1. CHAPTER 11DNA and Its Role in Heredity

2. The Structure of DNA • In the 1950’s many researchers were trying to determine the structure of DNA. • X-ray crystallography showed that the DNA molecule is a helix. (Franklin & Wilkins) • Chargaff discovered that the amount of adenine equals the amount of thymine and the amount of guanine equals the amount of cytosine. • What does this finding indicate?

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4. The Structure of DNA • Watson and Crick proposed that DNA is a double-stranded helix with the two sides of DNA running in opposite directions (the strands are antiparallel), • The two sides are held together by hydrogen bonds. • What accounts for the uniform diameter of the double helix?

5. Structure of DNA • A purine (A or G) consists of a double ring molecule. A pyrimidine (C or T) consists of a single ring molecule. A purine always bonds with a pyrimidine thus maintaining a constant distance between the two sides of the DNA molecule. • Review Figures 11.6 and 11.7

6. Structure of DNA • What does it mean - the two DNA strands run in opposite directions? • Examine the phosphodiester bonds between nucleotides. • The 3’ carbon of one deoxyribose and the 5’ carbon of another deoxyribose are bonded. • One side of the DNA molecule has an unconnected 5’ phosphate group while the opposite end has an unconnected 3’ hydroxyl group.

7. DNA Structure • Examine the other side of the DNA molecule. • It just the opposite

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11. The Structure of DNA • Three features summarize the molecular architecture of DNA: • The DNA molecule is a double-stranded helix. • The diameter of the DNA molecule is uniform. • The two strands run in different directions (they are antiparallel).

12. Three Models for DNA Replication • Conservative – original plus new strand • Dispersive – fragments of original DNA serve as templates for two DNA molecules. • Semiconservative – parent strand serves as a template for new strand • Review Figure 11.8

13. The Structure of DNA • The sugar–phosphate backbones of each strand coil around the outside of the helix. • The nitrogenous bases point toward the center of the helix. • Hydrogen bonds between complementary bases hold the two strands together. • A always pairs with T (two hydrogen bonds). • G always pairs with C (three hydrogen bonds).

14. Figure 11.7 Base Pairing in DNA Is Complementary

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16. DNA Replication • Meselson and Stahl’s experiment (1957) proved replication of DNA to be semiconservative • A parent strand is a template for synthesis of a new strand • Two replicated DNA helices contain one parent strand and one synthesized strand each.

17. Two Steps of DNA Replication • The DNA is denatured. • New nucleotides are covalently bonded to the each growing strand.

18. The Mechanism of DNA Replication • Nucleotides are always added to the growing 3’ end. Nucleotides are added by complementary base pairing with the template strand • The free hydroxyl group reacts with one of the substrate’s phosphate groups, deoxyribonucleoside triphosphates, a bond breaks releasing two of the phosphate groups, releasing energy for DNA synthesis • Review Figure 11.11

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20. The Mechanism of DNA Replication • No DNA forms without a primer. • A primer is a short segment of DNA or RNA that starts replication. • An enzyme, RNA primase, catalyzes the synthesis of short RNA primers • Review Figure 11.15

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22. The Mechanism of DNA Replication • DNA polymerase action causes the emerging leading strand to grow in the 5’-to-3’ direction. • RNA primer is degraded and DNA replaces it.

23. Many Proteins Assist in DNA Replication • DNA helicases unwind the double helix, • Binding proteins keep the two strands separated. • RNA primases makes the primer strand. • DNA polymerase adds nucleotides, proofreads DNA and repairs it. • DNA ligase seals up breaks in the sugar-phosphate backbone.

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26. The Mechanism of DNA Replication • On the lagging strand, growing away from the replication fork, DNA is made in the 5’-to-3’ direction but synthesis is discontinuous: DNA is added as short fragments to primers, then the polymerase skips past the 5’ end to make the next fragment. • Review Figures 11.16, 11.17 and 11.18

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28. Summary of DNA Replication • The replication begins at origins of replication - specific sequence of nucleotides which recognizes helicase. • Helicase unwinds the parental DNA. • Single-strand binding proteins stabilize the unwound parental DNA. • Replication of DNA then proceeds in both directions.

29. Summary of DNA Replication • Primase joins RNA nucleotides to make a primer (~ 10 nucleotides long) to begin synthesis of the leading strand. • As nucleotides align with complementary bases along a template strand of DNA, they are added by polymerase, to the growing end of the new strand (50/second in human cells). • DNA polymerases add nucleotides only to the free 3’ end of the growing DNA strand.

30. Summary of DNA Replication • The leading strand is synthesized continuously in the 5’ to 3’ direction by DNA polymerase. • The lagging strand is synthesized discontinously. Primase synthesizes short RNA primers to form Okazaki fragments. • The RNA primers are later replaced with DNA. • DNA ligase joins the Okazaki fragment to the growing strand.

31. DNA Proofreading and Repair • There is about about one error in 106 nucleotides bases added in DNA replication. That means about 1000 genes in every cell would be affected each time the cell divided. • Errors are repaired by: proofreading, mismatch repair, and excision repair. • Review Figure 11.19

32. Proofreading Mechanism • DNA polymerase recognizes a typo, an extra base, deletes it and adds the correct base. • Synthesis continues

33. Mismatch Repair Mechanism • The repair mechanism detects the “wrong” base before methylation has occurred. • Methyl groups (-CH3) are added to some cytosines. • Unmethylated strands are targeted for inspections. • A form of colon cancer arises from failure of mismatch repair.

34. Excision Repair Mechanism • Removes abnormal bases due to chemical damages and replaces them with functional bases. (Example, skin cancer) • Enzymes inspect the cell’s DNA and cut the defective strand. • Another enzyme cuts away adjacent bases and the offending bases. • DNA polymerase synthesizes a new correct piece to replace the discarded one. • DNA ligase seals the new base in place.

35. figure 11-19.jpg Figure 11.19 Figure 11.19