Molecular Basis of Inheritance
DNA Polymerase
DNA polymerase is the enzyme that builds a new DNA strand, adding one nucleotide at a time onto the 3' end of an existing strand.
When DNA copies itself, something has to add the new building blocks in the right order. That is DNA polymerase. It reads the old strand as a template and matches each base, A with T and G with C, joining the new nucleotides into a growing chain.
It can only add bases in one direction, from 5' to 3'. That single rule is why one new strand is built smoothly (the leading strand) and the other is built in short pieces (the lagging strand).
Where students slip up
In gene cloning, students often think you add DNA polymerase from outside to copy the inserted gene. You do not. Once the recombinant plasmid is inside the bacterium, the bacterium's own DNA polymerase copies it, along with the rest of its DNA. That is the whole point of using a living host.
Hold onto this
It adds bases 5' to 3', always onto a 3' end.
Why it helps in the exam: two favourites: the 5' to 3' direction and why it forces leading versus lagging strands, and the host cell's own polymerase copying the plasmid during cloning.
Where this connects
The one-direction rule looks like a limitation, but it is the reason replication needs primers, short Okazaki fragments, and a whole team of enzymes. A single constraint shapes the entire machinery.
Quick check
DNA polymerase adds new nucleotides:
- ATo the 5' end, in the 3' to 5' direction
- BTo the 3' end, in the 5' to 3' direction
- CTo either end freely
- DOnly at the origin of replication