Unit 4.3: DNA Replication HL

Directionality of DNA Polymerases

• DNA Polymerase always functions in a 5’ to 3’ terminal direction

  • Requires a 3’ where a free nucleotide can be added → Primers create this 

Terminal nucleotides

• EIther a sugar or phosphate group that 

  • The phosphate group is 5’ terminal

  • Sugar group is a 3’ terminal 

Fork Movement

THe direction in which the DNA is unzipped

Lagging Strand

DNA Polymerase adds nucleotides in the opposite direction of the fork movement → Since it needs to be opposite direction of the template strand

• okazaki fragments

• Slower process since the DNA polymerase needs to keeps starting up 

Okazaki Fragments

• SHort lengths that are added in series

Leading Strand

 DNA Polymerase adds nucleotides in the direction of the fork movement 

• Continuous replication - quicker 

Enzymes within DNA Replication

1. Primase

2. Polymerase III

3. Polymerase I

4. Ligase

DNA Primase

Type of RNA polymerase 

• MAde of about 10 RNA nucleotides that bind to the template strand to provide a site for DNA polymerase III to bind and add more nucleotides to the newly created 3’ end 

• Added every 100-200 nucleotides for lagging strands 

DNA Polymerase III

Principle Polymerase in DNA replication 

• Binds to the 3’ of the RNA primer on the template strand

• Continuous to assemble a chain of DNA nucleotides that compliment those on the strand 

  • Keeps going until the end or another primer 

• does proofreadings

DNA Proofreading

• When errors are made they are immediately corrected to prevent mutations 

  • Immediately after the wrong base is added it changes it 

DNA Polymerase I

Both an Exonuclease and a polymerase

• Can break bonds between nucleotides and link them 

• It removes RNA fragments and replaces them with DNA nucleotides 

  • Mainly on the lagging strand where it keeps detaching 

• Leaves a gap in the chain of nucleotides - sugar to phosphate bond is missing 

DNA Ligase

Connects the gap

• It will go back and fill any gaps in the chain DNA polymerase I left by forming sugar-phosphate bonds