3 Steps to DNA Replication: Step 13 Steps to DNA Replication: Step 11.1. The double helix must “unwind”The double helix must “unwind”

The hydrogen bonds between the The hydrogen bonds between the nitrogenous bases must be brokennitrogenous bases must be broken

DNADNA HelicaseHelicase- enzyme that separates or - enzyme that separates or “unzips” DNA“unzips” DNA

The places where the DNA splits are called The places where the DNA splits are called replication forksreplication forks

DNA Replication

3 Steps to DNA Replication: Step 23 Steps to DNA Replication: Step 22.2. Nucleotides must be added in the Nucleotides must be added in the

correct order to create a complementary correct order to create a complementary strandstrand

PrimasePrimase- enzyme that adds RNA primers so - enzyme that adds RNA primers so nucleotides can be attachednucleotides can be attached

DNADNA PolymerasePolymerase- enzyme that adds - enzyme that adds nucleotides (and proof-reads, and replaces nucleotides (and proof-reads, and replaces RNA primers with DNA nucleotides)RNA primers with DNA nucleotides)

NOTE: DNA Polymerase can only read DNA NOTE: DNA Polymerase can only read DNA in the 3’ in the 3’ 5’ direction, it synthesizes a new 5’ direction, it synthesizes a new complementary strand 5’ complementary strand 5’ 3’ 3’

We will see how the other strand is used as a We will see how the other strand is used as a template in a momenttemplate in a moment DNA Replication

3 Steps to DNA Replication: Step 33 Steps to DNA Replication: Step 33.3. The New Strand must be sealed up The New Strand must be sealed up

– DNADNA LigaseLigase- the enzyme that seals the - the enzyme that seals the phosphate-sugar backbone of DNA togetherphosphate-sugar backbone of DNA together

DNA Replication

Okazaki FragmentsOkazaki FragmentsProblem: DNA Polymerase can only read DNA in Problem: DNA Polymerase can only read DNA in the 3’ the 3’ 5’ direction, it synthesizes a new 5’ direction, it synthesizes a new complementary strand 5’ complementary strand 5’ 3’ 3’

The leading strand proceeds without difficulty The leading strand proceeds without difficulty because DNA polymerase can read itbecause DNA polymerase can read it

The lagging strand must proceed in chunks, The lagging strand must proceed in chunks, called Okazaki fragmentscalled Okazaki fragments

3’ AAATGCCGTTAG 5’5’ TTTACGGCAATC 3’

3’ AAATGCCGTTAG 5’

5’ TTTACGGCAATC 3’Original Strand

Template Strand

Leading Strand5’ TTTACGGCAATC 3’Template Strand3’ AAA CGT 5’

Lagging Strand

DNA Replication and Okazaki Fragments

Prokaryotes vs EukaryotesProkaryotes vs EukaryotesReplication AnimationReplication Animation

In prokaryotes DNA Replication starts in one In prokaryotes DNA Replication starts in one place and continues in two different directions place and continues in two different directions until the chromosome is copieduntil the chromosome is copied

In eukaryotes DNA Replication occurs in many In eukaryotes DNA Replication occurs in many places at the same timeplaces at the same time