DNA Replication

Learning Objectives Summarize the events of DNA replication. Compare DNA replication in prokaryotes with that of

eukaryotes.

Review of DNA Structure

sugar-phosphatebackbone

nitrogenous bases

double helix

DNA Replication and the Cell Cycle

DNA replication occurs during the phase of the cell cycle.

S

Copying DNA

Direction of replication

Direction of replication

replication fork

DNA polymerase

new nucleotides being added

DNA ReplicationDNA ReplicationEnzyme Enzyme HelicaseHelicase unwinds and separates the 2 DNA unwinds and separates the 2 DNA strands by breaking the strands by breaking the weak hydrogen bondsweak hydrogen bondsSingle-Strand Binding ProteinsSingle-Strand Binding Proteins attach and keep the 2 DNA strands separated and untwisted

DNA ReplicationDNA ReplicationBegins atBegins at Origins of ReplicationOrigins of ReplicationTwo strands open forming Two strands open forming Replication Forks (Y-shaped Replication Forks (Y-shaped region)region)New strands grow at the forksNew strands grow at the forks

ReplicationReplicationForkFork

Parental DNA MoleculeParental DNA Molecule

3’

5’

3’

5’

Semiconservative Replication

The blue strand represents the

DNA strand.

The red strand represents the

DNA strand.

original

new

Telomeres

• Telomeres: the tips of eukaryotic chromosomes• Telomerase helps prevent genes from being damaged or lost during replication.

Telomeres

DNA ReplicationDNA ReplicationAs the 2 DNA strands open at the origin, As the 2 DNA strands open at the origin, Replication Replication BubblesBubbles form formProkaryotes (bacteria) have a single bubbleEukaryotic chromosomes have MANY bubbles

Bubbles Bubbles

Lagging Strand SegmentsLagging Strand Segments• Okazaki FragmentsOkazaki Fragments - - series of short segments on the

lagging strandlagging strand• Must be joined together by an Must be joined together by an enzymeenzyme

Lagging Strand

RNARNAPrimerPrimer

DNADNAPolymerasePolymerase

3’

3’

5’

5’

Okazaki FragmentOkazaki Fragment

Joining of Okazaki FragmentsJoining of Okazaki FragmentsThe enzyme The enzyme LigaseLigase joins the Okazaki fragments joins the Okazaki fragments together to make one strandtogether to make one strand

Lagging Strand

Okazaki Fragment 2Okazaki Fragment 2

DNA ligaseDNA ligase

Okazaki Fragment 1Okazaki Fragment 1

5’

5’

3’

3’

Eukaryotic DNA Replication

Replication forks

Unreplicated DNA

New DNA

Prokaryotic DNA Replication

replication fork

replication fork

new DNA

unreplicated DNA