DNA polymerase can't start a strand on its own
- The idea
- Predict
- See it
- Why
- Check
The idea
“DNA polymerase can start a new strand anywhere on the template, as long as nucleotides are around.”
Polymerase is the enzyme that builds DNA, so it seems like it should start new strands too. It can only lengthen one that already exists.
The short version
The mistake: DNA polymerase can start a new strand anywhere on the template, as long as nucleotides are around.
What's actually true: DNA polymerase can only add nucleotides to the 3' end of a strand already paired to the template, so it needs a primer.
- DNA polymerase joins each nucleotide to the 3' hydroxyl at the end of a growing strand. With no strand, there is nothing to join to.
- Primase can start a new chain on bare template. Its short RNA primer, paired to the template, supplies the first 3' end.
- The leading strand is primed once at the origin. The lagging strand needs a primer for every Okazaki fragment. Primers are later replaced with DNA.
On the exam: Polymerase only adds to a paired 3' end. No primer, no new DNA.