3 Steps to DNA Replication: Step 1 1. The double helix must “unwind”  The hydrogen bonds between the nitrogenous bases must be broken  DNA Helicase- enzyme that separates or “unzips” DNA  The places where the DNA splits are called replication forks DNA Replication

3 Steps to DNA Replication: Step 2 2. Nucleotides must be added in the correct order to create a complementary strand  Primase- enzyme that adds RNA primers so nucleotides can be attached  DNA Polymerase- enzyme that adds nucleotides (and proof-reads, and replaces RNA primers with DNA nucleotides)  NOTE: DNA Polymerase can only read DNA in the 3’  5’ direction, it synthesizes a new complementary strand 5’  3’ We will see how the other strand is used as a template in a moment DNA Replication

3 Steps to DNA Replication: Step 3 3. The New Strand must be sealed up –DNA Ligase- the enzyme that seals the phosphate-sugar backbone of DNA together DNA Replication

Okazaki Fragments Problem: DNA Polymerase can only read DNA in the 3’  5’ direction, it synthesizes a new complementary strand 5’  3’ The leading strand proceeds without difficulty because DNA polymerase can read it The lagging strand must proceed in chunks, called Okazaki fragments 3’ AAATGCCGTTAG 5’ 5’ TTTACGGCAATC 3’ 3’ AAATGCCGTTAG 5’ 5’ TTTACGGCAATC 3’ Original Strand Template Strand Leading Strand 5’ TTTACGGCAATC 3’Template Strand 3’ AAA CGT 5’ Lagging Strand DNA Replication and Okazaki Fragments

Prokaryotes vs Eukaryotes Replication Animation Replication Animation In prokaryotes DNA Replication starts in one place and continues in two different directions until the chromosome is copied In eukaryotes DNA Replication occurs in many places at the same time