DNA polymerase directionality explains how DNA is copied, checked, amplified or analysed through specific strand relationships, enzymes and molecular techniques.
Coverage 2013–2013 · Updated 16 Jul 2026
DNA strands have chemically different 5′ and 3′ ends, and DNA polymerase synthesizes only in the 5′→3′ direction.
The 5′ end is associated with the phosphate on the sugar's 5′ carbon, while the 3′ end has a free hydroxyl on the 3′ carbon. Polymerase attaches the 5′ phosphate of an incoming DNA nucleotide to the free 3′ end of the growing strand.
New nucleotide's 5′ end + growing strand's 3′ end → phosphodiester bond. Therefore extension occurs only at 3′ and the new strand lengthens 5′→3′.
A polymerase moving along a template 3′→5′ builds the complementary strand 5′→3′, adding every next nucleotide at the new strand's 3′ end.
5′→3′ describes the direction of new-strand synthesis. Polymerase cannot add to the growing strand's 5′ end.
This objective is assessed through multiple choice, commonly using Explain.
Explain
Build the answer around this relationship: DNA polymerase directionality depends on matching each strand, enzyme or laboratory step to its exact function.
Representative question
How does DNA replicate?
The deoxyribose of a free nucleotide is linked to the phosphate of the last nucleotide in the chain.
The phosphate of a free nucleotide is linked to the deoxyribose of the last nucleotide in the chain.
Nucleotides are linked in a to direction and the new strands are anti-parallel to the template strands.
Nucleotides are linked in a to direction and the new strands are parallel to the template strands.
DNA strands have 5' and 3' ends; DNA polymerase adds nucleotides to the 3' end, so new DNA forms 5' to 3'. Leading strand synthesis is continuous; lagging strand synthesis is discontinuous as Okazaki fragments using repeated RNA primers. In the prokaryotic model, primase starts, DNA polymerase III extends, DNA polymerase I replaces primers, and ligase joins fragments. DNA polymerase III removes mismatched nucleotides from the 3' end; proofreading improves copying accuracy and reduces mutations.
