CHAPTER 8 CHAPTER 8 THE REPLICATION OF DNA 王青 生物科学类

OUTLINE DNA polymerization DNA replication Action of Telomerase

DNA polymerization 1.The Chemistry of DNA SynthesisThe Chemistry of DNA Synthesis 2. The Mechanism of DNA PolymeraseThe Mechanism of DNA Polymerase 3. The Replication ForkThe Replication Fork

The Chemistry of DNA Synthesis deoxynucleoside triphosphates primer:template junction primer Hydrolysis of pyrophosphate (PPi) Metal ion

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The Mechanism of DNA Polymerase Thumb Palm Fingers

palm Palm domain hydrogen bonds to base pairs in minor groove Hydrogen bonds only form if nucleotides are correctly base paired

Fingers Finger domain associates with template region 90 。 turn of template helps avoid confusion in the active site

Thumb Holds the primer:template junction in the active site Reduces the rate of dissociation

Exonucleases proofread newly synthesized DNA back

Replication fork

DNA replication 1. The Specialization of DNA Polymerases 2. DNA Synthesis at the Replication Fork 3. Initiation of DNA Replication

The replication fork Leading strand Lagging strand Okazaki fragment Replication fork Need the help of DNA pol,DNA helicase, SSB, primase, DNA topoisomeraseDNA helicase SSBDNA topoisomerase

DNA helicse Back

SSB Back

DNA topoisomerase Back

DNA Pol III holoenzyme, helicase and primase interact with each other to form replisome, a finely tuned factory for DNA synthesis with the activity of each protein is highly coordinated.

replisome

the trombone model for coordinating replication by two DNA polymeraes at the repolication fork back

Three different functions of initiator protein: (1) binds to replicator, (2) distorts/unwinds a region of DNA, (3) interacts with and recruits additional replication factors

Action of Telomerase 1. Binding and Unwinding: origin selection and activation by the intiator protein 2. Finishing Replication

Telomere replication The end of linear chromosomes can’t be fully replication by semi-discontinuous replication as there is no DNA ti elongate to replace the RNA removed from the 5’-end of the lagging strand. Thus,genetic information sould be lost from the DNA. To over come this, the ends of eukaryotic chromosomes( telomeres) consist of hundreds of copies of a simple, noninformational repeat sequence with the 3’-end overhanging the 5’-end. The enzyme telomerase contains a short DNA molecule part of whose sequence is complementary to this repeat. This RNA acts as a template for the addition of these repeats to the 3’-overhang by repeated cycles of elongation and translocation

Repulication of telomeres by telomerase(FIGURE 8-36) Telomerase uses its RNA component to the 3 ’ -end of the ssDNA region of the telomere. Telomerase then uses its reverse transcription activity to synthesize DNA to the end of the RNA template. Telomerase then displaces the RNA from the DNA product and rebinds at the end of the telomere and repeats the process

Role of Telomeres and Telomerase in Cancer and Aging

G1 G2 M S G0

G1 :the longest phase during which the cells are preparing for DNA replication S: the only period in the cell cycle during which DNA is replication G2: a short gap before mitosis M: mitosis and cell division

G0: cell can also exit the cell cycle after mitosis and enter a nonprotiferative state

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