DNA replication

steps

occurs before mitosis during interphase


1. initiation
2. elongation
3. termination

Initiation

1. proteins attach to the **origin of replication**
2. **Gyrase** weakens H+ bonds (undoes supercoiling)
3. **Helicase** breaks H+ bonds
4. **Single-stranded binding proteins** (SSB) keep strands apart (don’t allow them to bond)

replication bubble

replication bubble is where these steps are taking place, origin of replication is in the middle and the enzymes move in opposite directions from the middle (outwards). Replication fork is the end of the bubble

Elongation

1. **primase** lays down primer acting as a starting point for **DNA polymerase III**
2. DNA poly III binds to the primer


1. moves along in the 5 to 3 direction of the NEW strand, creates the matching nucleotide
2. the strand being created in the direction of the gyrase and helicase is the leading strand
3. lagging strand goes the other direction
3. **DNA polymerase I** removes primers and replaces them with the proper nucleotide
4. **DNA Ligase** joins Okazaki fragments together (

lagging strand

on the original DNA chain that goes 5 to 3, therefore the new strand is being created in the opposite direction. requires multiple primers for DNA poly 3 to attach to and creates **Okazaki fragments** (bottom strand in the gif)

* this is called **discontinuous replication**

Repair

DNA polymerase 1 and 3 can ‘spell check’, and ensure all nucleotides are correctly matched. If not, it is fixed by **exonuclease.**

Termination

Once new strands are formed, DNA is automatically rewinded into the double helix structure