DNA ReplicationDNA Replication

• Converting a single DNA helix into two identical copies.

• Described as semi conservative replication.

New DNA Strands

• Helicase enzyme splits and unwinds the two stranded DNA.

3’3’

5’5’

C5

Naming DNA Direction

• DNA strands are recognised and named according to the carbon atom exposed at the end of the strand.

C1

C2C3

C4

Forming a New Strand

• An enzyme RNA polymerase is used to add a short section of RNA to start the process.

• A topoisomerase enzyme cuts the DNA strand allowing it to twist and relieve

pressure.

3’

5’

3’

5’

Addition of nucleotides

• When nucleotides are added to a new strand they can only do so in a 5’ (5 prime) to 3’ direction.

3’

5’

3’

5’

• An enzyme DNA polymerase III is used to catalyse the addition of DNA nucleotides.

Base Pairing

TT

CC

GG GG

CCAA

TT CCGG

GG

AA

TTAA

TTTT

GG AAAA

CCCC

TT GGAAGG

CCAATTCC

GG

CC

AA

TTCC

• Nucleotides are added. A joins to T, C joins to G.

New DNA Strands

• The leading strand (red) is synthesised continuously.

• The lagging strand (pink) is formed in segments called okazaki fragments.

3’

5’

3’

5’

• An enzyme DNA polymerase I replaces the RNA primers with DNA

Filling the Gaps

• An enzyme DNA Ligase is used to fill in the gaps in the okazaki fragments with nucleotides.

• The DNA strands continue to form in a 5’ to 3’ direction.

3’

5’

3’

5’

Completion

• When DNA replication is complete two molecules are formed.

• Because half of each strand is new and half original it is called semi conservative replication.

Questions

• What is the purpose of DNA replication?• Converting a single DNA helix into two copies.• Name the enzyme that splits and unwinds DNA.• Helicase.• Why is the 5’ end of a DNA strand given this name?• The carbon in the number 5 positions of the sugar unit

is the one exposed.• What is the role of the topoisomerase enzyme?• To cut the DNA strand and allow it to twist and relieve

pressure.

Questions continued

• How is the addition of nucleotides started?• An enzyme RNA polymerase adds a short section of

RNA.• What is the role of DNA polymerase III ?• It catalyses the addition of DNA nucleotides.• In what direction are nucleotides added?• 5’ to 3’• What are the base pairing rules?• A joins with T and C joins with G.• Why does this occur?• A and T form double bonds while C and G form triple

bonds.

Questions continued

• What name is given to the continuously formed strand?• Leading strand.• Why is the lagging strand formed in fragments?• As the helicase enzyme moves it only exposes a small

section of DNA and because DNA forms 5’ to 3’ it does so in sections.

• What name is given to the DNA fragments?• Okazaki fragments.• Which enzyme joins the fragments together?• DNA ligase.