DNA Replication

Synthesis Phase (S phase) S phase in interphase of the cell cycle. Nucleus of eukaryotes Mitosis -prophase -metaphase -anaphase -telophase G1G1 G2G2 S phase interphase DNA replication takes place in the S phase.

DNA Nucleotide O O=P-O OPhosphate Group Group N Nitrogenous base (A, G, C, or T) (A, G, C, or T) CH2 O C1C1 C4C4 C3C3 C2C2 5 Sugar Sugar(deoxyribose)

Figure 11.7 Base Pairing in DNA Is Complementary

DNA Replication DNA must replicate during each cell division THREE alternative models for DNA replication were hypothesized: Semiconservative replication Conservative replication Dispersive replication DNA Replication

Figure 11.8 Meselson and Stahl’s experiment proved replication of DNA to be semiconservative - See Practical - See pages   

Watson & Crick predicted that each DNA strand could serve as a template for the replication of a new strand Q: What is the mode of replication?

The Mechanism of DNA Replication DNA replication is catalyzed by DNA polymerase DNA polymerase needs an RNA primer DNA polymerase adds nucleotides to the 3’ end of the growing strand Nucleotides are added by complementary base pairing with the template strand

Figure 11.11

The Mechanism of DNA Replication Many proteins assist in DNA replication DNA helicases unwind the double helix, the template strands are stabilized by other proteins

Figure 11.16

The Mechanism of DNA Replication Many proteins assist in DNA replication DNA helicases RNA primase catalyzes the synthesis of short RNA primers, to which nucleotides are added. DNA polymerase III extends the strand in the 5’-to-3’ direction

Figure RNA primase

The Mechanism of DNA Replication DNA synthesis on the leading strand is continuous. It reads the template in the 3’-to - 5’ direction (the same direction as helicase) The lagging strand grows in the opposite direction to the Replication Fork. DNA is always made in the 5’-to-3’ direction. Therefore, DNA synthesis on the lagging strand is discontinuous DNA is created as short fragments (Okazaki fragments) that are subsequently ligated together

Figure 11.17

The Mechanism of DNA Replication Many proteins assist in DNA replication DNA helicases RNA primase DNA polymerase III DNA polymerase I degrades the RNA primer and replaces it with DNA DNA ligase joins the DNA fragments into a continuous daughter strand

Figure 11.18

Figure 11.8 a

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