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THE MOLECULAR BASIS OF INHERITANCE

THE MOLECULAR BASIS OF INHERITANCE. Chapter 16. What you must know. The structure of DNA. The major steps to replication. The difference between replication, transcription, and translation. The general differences between the bacterial chromosome and eukaryotic chromosomes.

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THE MOLECULAR BASIS OF INHERITANCE

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  1. THE MOLECULAR BASIS OF INHERITANCE Chapter 16

  2. What you must know • The structure of DNA. • The major steps to replication. • The difference between replication, transcription, and translation. • The general differences between the bacterial chromosome and eukaryotic chromosomes. • How DNA is packaged into a chromosome.

  3. Problem: Is the genetic material of organisms made of DNA or proteins?

  4. Frederick Griffith (1928)

  5. Frederick Griffith (1928) Conclusion: living R bacteria transformed into deadly S bacteria by unknown, heritable substance Oswald Avery, et al. (1944) • Discovered that the transforming agent was DNA

  6. Hershey and Chase (1952) • Bacteriophages: virus that infects bacteria; composed of DNA and protein Protein = radiolabel S DNA = radiolabel P

  7. Hershey and Chase (1952) Conclusion: DNA entered infected bacteria  DNA must be the genetic material!

  8. Edwin Chargaff (1947) Chargaff’s Rules: • DNA composition varies between species • Ratios: • %A = %T and %G = %C

  9. Rosalind Franklin (1950’s) • Worked with Maurice Wilkins • X-ray crystallography = images of DNA • Provided measurements on chemistry of DNA

  10. James Watson & Francis Crick (1953) • Discovered the double helix by building models to conform to Franklin’s X-ray data and Chargaff’s Rules.

  11. Structure of DNA DNA = double helix • “Backbone” = sugar + phosphate • “Rungs” = nitrogenous bases

  12. Structure of DNA Nitrogenous Bases • Adenine (A) • Guanine (G) • Thymine (T) • Cytosine (C) • Pairing: • purine + pyrimidine • A = T • G Ξ C purine pyrimidine

  13. Structure of DNA Hydrogen bondsbetween base pairs of the two strands hold the molecule together like a zipper.

  14. Structure of DNA Antiparallel: one strand (5’ 3’), other strand runs in opposite, upside-down direction (3’  5’)

  15. DNA Comparison Prokaryotic DNA Eukaryotic DNA • Double-stranded • Circular • One chromosome • In cytoplasm • No histones • Supercoiled DNA • Double-stranded • Linear • Usually 1+ chromosomes • In nucleus • DNA wrapped around histones (proteins) • Forms chromatin

  16. Problem: How does DNA replicate?

  17. Replication: Making DNA from existing DNA 3 alternative models of DNA replication

  18. Meselson & Stahl

  19. Meselson & Stahl

  20. Replication is semiconservative

  21. DNA Replication Video http://www.youtube.com/watch?v=4jtmOZaIvS0&feature=related

  22. Major Steps of Replication: • Helicase:unwinds DNA at origins of replication • Initiation proteins separate 2 strands  forms replication bubble • Primase: puts down RNA primer to start replication • DNA polymerase III: adds complimentary bases to leading strand (new DNA is made 5’  3’) • Lagging strand grows in 3’5’ direction by the addition of Okazaki fragments • DNA polymerase I: replaces RNA primers with DNA • DNA ligase: seals fragments together

  23. 1. Helicase unwinds DNA at origins of replication and creates replication forks

  24. 3. Primase adds RNA primer

  25. 4. DNA polymerase III adds nucleotides in 5’3’ direction on leading strand

  26. Replication on leading strand

  27. Leading strand vs. Lagging strand

  28. Okazaki Fragments: Short segments of DNA that grow 5’3’ that are added onto the Lagging Strand DNA Ligase: seals together fragments

  29. Proofreading and Repair • DNA polymerases proofread as bases added • Mismatch repair: special enzymes fix incorrect pairings • Nucleotide excision repair: • Nucleases cut damaged DNA • DNA poly and ligase fill in gaps

  30. Nucleotide Excision Repair Errors: • Pairing errors: 1 in 100,000 nucleotides • Complete DNA: 1 in 10 billion nucleotides

  31. Problem at the 5’ End • DNA poly only adds nucleotides to 3’ end • No way to complete 5’ ends of daughter strands • Over many replications, DNA strands will grow shorter and shorter

  32. Telomeres: repeated units of short nucleotide sequences (TTAGGG) at ends of DNA • Telomeres “cap” ends of DNA to postpone erosion of genes at ends (TTAGGG) • Telomerase: enzyme that adds to telomeres • Eukaryotic germ cells, cancer cells Telomeres stained orange at the ends of mouse chromosomes

  33. Telomeres & Telomerase

  34. BioFlix: DNA Replication http://media.pearsoncmg.com/bc/bc_0media_bio/bioflix/bioflix.htm?8apdnarep

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