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DNA REPLICATION

DNA REPLICATION. STRUCTURE.

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DNA REPLICATION

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  1. DNA REPLICATION

  2. STRUCTURE • DNA is a double stranded helix, where each strand is composed of a sequence of deoxyribonucleotides connected to each other via phosphodiester linkages. The two strands are connected via hydrogen bonding between complimentary base pairs. Purines pair with their complimentary pyrimidine.

  3. Chargraff's Rule • Due to complimentary base pairing we know that the amount of A in DNA is always equal to the amount of T. Also, the amount of G is always equal to the amount of C • A +G = T + C • So if a molecule of DNA is 26% adenine, what is the % of T? G? C?

  4. Possible Mechanisms

  5. WHERE? • In the nucleus

  6. WHEN? • During the S phase of the cell cycle

  7. Why? • For cell division

  8. How? • Through semiconservative replication

  9. THE BIG PICTURE

  10. INITIATION • DNA helicase unwinds the double helix by breaking the hydrogen bonds. • Single-stranded binding proteins keep apart the strands -> polymerization • DNA gyrase relieves tension by cutting the strands of DNA Eukaryotic cells have multiple origins of replication to get the job done quickly. See page 220 Figure 4

  11. ELONGATION • DNA polymerase I, II and III function in replication and repair, while in eukaryotes, there are 5 types of DNA polymerases • DNA polymerase III uses each strand of DNA as a template to bring in complimentary base pairs. • It can only add nucleotides to the 3’-OH end of the growing chain (5’3’ growth direction) • This means that only one of the template strands is in the correct direction as the two strands are antiparallel. This strand is synthesized continuously (LEADING STRAND), while the other is synthesized discontinuously (LAGGING STRAND).

  12. LEADING VS. LAGGING STRANDS

  13. LEADING STRAND • Elongation begins by addition of a complimentary deoxyribonucleotide triphosphate to the 3’ end of the primer. This condensation reaction uses up energy in the form of the phosphate groups. • Question: When is your body doing a lot of DNA replication? • During times when cells are reproducing either for growth or repair • So how does eating well and resting help this?

  14. LAGGING STRAND • RNA primers are continuously added as the replication fork forms, and DNA polymerase builds short segments in between the gaps known as Okazaki fragments • DNA polymerase I removes the primers on both leading and lagging strands and replaces them with the appropriate deoxyribonucleotide. • There is a small gap in the sequence that is filled by DNA ligase with new phosphodiester bonds created.

  15. TERMINATION • Each of the two new strands will begin to twist and coil into their double helix form. • DNA polymerase I and III proofread the newly synthesized strands and function as exonucleases when an error occurs • Incorrect bases are excised and the correct nucleotides are inserted. • Why are these repairs needed immediately? • See summary pg. 224 Fig. 5.19 • Do # 19-24 pg. 227 & pg. 229 # 111

  16. Let’s watch  • http://youtu.be/teV62zrm2P0

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