Prentice Hall c2002 Chapter 19 1

DNA Replication

• Initiation• Elongation• Leading and

Lagging strand synthesis

• DNA Ligase• Eukaryotic synthesis• Proofreading

Prentice Hall c2002 Chapter 19 2

DNA Can Be Supercoiled

• “Relaxed” circular DNA with the B conformation (10.4 base pairs/turn) would lie flat on a surface

• If strands are broken, and two ends of linear DNA twisted in opposite directions and rejoined, DNA supercoils to restore 10.4 bp/turn

• Each supercoil compensates for one turn of the double helix

Prentice Hall c2002 Chapter 19 3

Supercoiling

• Most bacterial chromosomes are supercoiled, and regions of eukaryotic DNA are supercoiled

• Topoisomerases - enzymes that can alter the topology of DNA helixes by:(1) Cleaving one or both DNA strands(2) Unwinding or overwinding the double helix

by rotating the strands(3) Rejoining ends to create (or remove) supercoils

Prentice Hall c2002 Chapter 19 4

Human topoisomerase I bound to DNA

• Topoisomerases can add or remove supercoils in DNA

• Cleave one or both DNA strands, unwind or overwind by rotating cleaved ends, then rejoin ends

Prentice Hall c2002 Chapter 19 5

Initiation

• Origin of replication: 245 bp• Single strand binding proteins • ATP hydrolysis

Prentice Hall c2002 Chapter 19 6

Helicase

• Separates DNA strands

• ATP hydrolysis

Prentice Hall c2002 Chapter 19 7

RNA Primer

• RNA polymerase • Primase• 5-10 nucleotides

long• DNA Polymerase

then binds to the DNA

Prentice Hall c2002 Chapter 19 8

Polymerization Reaction

• DNA Pol I and III• 5’ to 3’

polymerization reaction

• 3’ to 5’ exonucleaseactivity

Prentice Hall c2002 Chapter 19 9

Leading and lagging strands

Prentice Hall c2002 Chapter 19 10

Replication in 5’ to 3’ direction

Prentice Hall c2002 Chapter 19 11

Primer Removal by DNA Pol I

• DNA Pol I = 5’ to 3’ exonuclease activity• DNA ligase

Prentice Hall c2002 Chapter 19 12

Eukaryotic modifications

• Multiple origins• Polymerases• Linear

chromosomes

Prentice Hall c2002 Chapter 19 13

Telomeres

Prentice Hall c2002 Chapter 19 14

DNA Proofreading

• 3’ to 5’ exonuclease• Base pairing