Biology · Biotechnology: Principles and Processes · NEET
DNA is double-stranded, so after denaturation you get two separate template strands running in opposite directions. Each strand needs its own primer to mark where copying should start. One primer (forward) binds one strand and the other (reverse) binds the second strand. NCERT clearly says 'two sets of primers' — this is a favourite NEET point.
Primers bind in the ANNEALING step, not denaturation. Denaturation only separates the two DNA strands using heat (around 94-95 degrees C). Once single strands are ready, the temperature is lowered so primers can base-pair (anneal) to their complementary regions. NEET 2026 asked exactly this.
PCR primers are short DNA pieces (oligonucleotides), chemically synthesised in the lab. This is different from natural DNA replication inside cells, where the primer is a short RNA made by primase. For NEET, remember: in PCR the primers are DNA and are added by the scientist, not made by an enzyme.
The template is the original long DNA whose gene you want to copy. The primer is a very short DNA piece complementary to a small region at the end of the target. The template is 'read', the primer is the 'start point'. Polymerase extends the primer using the template as a guide.
No. DNA polymerase cannot start a new strand from scratch — it can only ADD nucleotides to an existing free 3' end. The primer provides that free 3' end. So without a primer, Taq polymerase has nothing to extend and no amplification happens.
Yes. Because primers are complementary only to chosen regions flanking the target gene, they bind only there. This means the primers define the exact segment that will be amplified. Choosing different primers lets you amplify different genes from the same genomic DNA.
During PCR, primers bind to the DNA strands in the ______ step.
Match the enzymes with their functions: (a) Restriction endonuclease (b) Restriction exonuclease (c) DNA ligase (d) Taq polymerase — with (i) Joins the DNA fragments (ii) Extends primers on genomic DNA template (iii) Cuts DNA at specific position (iv) Removes nucleotides from the ends of DNA.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Primers give DNA polymerase a fixed starting point on each template strand. They are complementary to the ends of the target region, so they bind there and let polymerase extend a new complementary strand only from that point.
Because double-stranded DNA opens into two single strands running in opposite directions. Each strand needs its own primer, so a forward primer and a reverse primer are both required — NCERT calls this 'two sets of primers'.
They are small chemically synthesised oligonucleotides (short single-stranded DNA) that are complementary to specific regions of the target DNA.
In the annealing step of each PCR cycle, after denaturation separates the strands. Polymerase then extends the primers in the extension step.
Yes, primers are added in excess because each cycle uses fresh primers to bind the newly formed single strands, allowing the DNA to be amplified up to about a billion copies.
