DNA Replication

Synthesis Phase (S Synthesis Phase (S phase)phase)

S phase in interphase of the cell cycle.

Nucleus of eukaryotes

Mitosis-prophase-metaphase-anaphase-telophase

G1 G2

Sphase

interphase

DNA replication takesDNA replication takesplace in the S phase.place in the S phase.

DNA NucleotideDNA Nucleotide

OO=P-O O

PhosphatePhosphate GroupGroup

NNitrogenous baseNitrogenous base (A, G, C, or T)(A, G, C, or T)

CH2

O

C1C4

C3 C2

5

SugarSugar(deoxyribose)(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 206-207

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 11.15

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

DNA ReplicationAnimations

http://www.bioteach.ubc.ca/TeachingResources/MolecularBiology/DNAReplication.swf

http://bcs.whfreeman.com/thelifewire/content/chp11/1102002.html

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