The big idea: In prokaryotes such as E. coli, four enzymes besides helicase copy DNA, each with one job: start, build, swap and seal.
| Enzyme | Its job in replication |
|---|---|
| DNA primase | Lays down a short RNA primer where a new strand must start |
| DNA polymerase III | Adds DNA nucleotides to the 3' end of the primer, building the new strand |
| DNA polymerase I | Removes the RNA primers and replaces them with DNA |
| DNA ligase | Seals the gaps between Okazaki fragments, joining the backbone |
Remember it as: Primase starts, pol III builds, pol I swaps, ligase seals.
Real example: E. coli. In E. coli DNA polymerase III adds about 1,000 nucleotides a second, while DNA polymerase I works far more slowly, only over the short stretches where primers were.
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Which enzymes make the backbone bond?: Three of the enzymes form covalent bonds in the sugar-phosphate backbone of DNA. Primase makes RNA, not DNA.
The points to remember
- DNA polymerase III joins each new nucleotide to the 3' end as it builds the strand.
- DNA polymerase I joins DNA nucleotides as it fills the gap left by a removed primer.
- DNA ligase makes the last bond that joins neighbouring Okazaki fragments.
- Primase builds the short RNA primer that DNA polymerase III starts from.
Keep I and III apart: Polymerase III builds the new strands. Polymerase I only swaps the RNA primers for DNA.
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How it is asked: An Explain on the enzymes that form the backbone bond, a 1A item naming an enzyme from its job, or the enzymes inside a longer replication answer.
IB-style questionExplain[2 marks]
Covalent bonds join the nucleotides in the backbone of new DNA. Explain how two enzymes catalyse the formation of these bonds during replication in a bacterium.
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