DNA Replication
Biology: IB Diploma Higher Level, topic 1.2.4
DNA Replication
A replication fork forms first. Then DNA splits into a leading strand and a lagging strand. Each strand is copied in a different way.
Leading strand synthesis
- DNA polymerase adds nucleotides in the 5' to 3' direction only.
- The parental strands are antiparallel.
- The strands run in opposite directions.
- The leading strand is made continuously towards the replication fork.
Lagging strand synthesis
- The lagging strand is made in short sections called Okazaki fragments.
- Each fragment needs its own RNA primer to start synthesis.
- The leading strand only needs one primer, because it is made continuously.
Topoisomerase
- DNA tends to overwind ahead of the replication fork.
- Topoisomerase relieves the strain by cutting and resealing the DNA backbone.
Primer removal and ligation
- DNA polymerase I removes the RNA primers and replaces them with DNA nucleotides.
- DNA ligase seals the remaining nicks between the DNA fragments.
- Ligase joins adjacent fragments with a phosphodiester bond.
Test yourself on DNA Replication
Which strand is iscomplementary to the 3' to 5' parental DNA strand?