Describe the function of DNA polymerase
WHAT is DNA polymerase?
Key facts to lock in:
- It works only in the 5′ → 3′ direction (it adds new nucleotides to the 3′-OH end).
- It cannot start from nothing — it needs a pre-existing primer with a free 3′-OH to extend.
- It has a proofreading ability (3′→5′ exonuclease) that removes wrongly-paired bases.
WHY does it work this way? (first principles)
HOW it builds the strand (the mechanism, derived)
Imagine the template strand running . The new strand must be antiparallel, so it grows .
Step by step (each step has a Why?):
- Read the template base. Why? The next base must be complementary, so the enzyme must "see" what it's pairing against.
- Select the matching dNTP. A free triphosphate nucleotide floats in and tests its fit (A↔T, G↔C). Why? Only the correct pair forms stable hydrogen bonds and fits the enzyme's shape — this is how accuracy is enforced.
- Form the bond. The 3′-OH of the growing chain attacks the innermost phosphate of the incoming dNTP. A phosphodiester bond forms and pyrophosphate (PPᵢ) is released. Why? Breaking the high-energy triphosphate releases the energy needed to make the bond — no separate ATP required.
- Proofread. If a wrong base was added, the polymerase's 3′→5′ exonuclease snips it out and tries again. Why? This lowers the error rate dramatically (≈1 in 10⁹ bases), keeping genetic information accurate.
- Repeat, moving along the template.
Leading vs lagging strand (a consequence of 5′→3′ only)
Worked Examples
Common Mistakes (Steel-manned)
Recall Feynman: explain to a 12-year-old
DNA polymerase is like a super-careful typist copying one half of a zipper. It looks at each tooth on the old half and snaps on the matching tooth to build a new half. It can only add teeth to one end (never the start), so a little helper sticks down the first tooth for it. And it's so careful it checks every tooth and rips out any that don't fit. That's how your cells make near-perfect copies of all your DNA before they divide.
Active Recall Flashcards
What is the function of DNA polymerase?
In which direction does DNA polymerase synthesise the new strand?
Why can DNA polymerase only add to the 3′ end?
What must be present before DNA polymerase can begin?
Where does the energy for joining nucleotides come from?
What is proofreading and which activity does it?
Which enzyme unwinds DNA (not polymerase)?
What are Okazaki fragments?
Template is 3′–GCTA–5′; what new strand forms (5′→3′)?
What joins Okazaki fragments together?
Connections
- DNA Structure — antiparallel strands explain the 5′→3′ rule.
- Complementary Base Pairing — the rule polymerase enforces.
- Helicase — unwinds DNA ahead of polymerase.
- Primase and RNA Primers — provide the starting 3′-OH.
- DNA Ligase — seals nicks and joins Okazaki fragments.
- Semiconservative Replication — overall process polymerase serves.
- Leading and Lagging Strands — direct consequence of polymerase's one-way action.
Concept Map
Hinglish (regional understanding)
Intuition Hinglish mein samjho
Dekho, DNA polymerase basically ek "copy karne wala" enzyme hai. Replication ke time DNA ke do strands alag ho jaate hain (woh helicase karta hai, polymerase nahi), aur phir DNA polymerase har purane strand ko template maan ke uske saamne naya strand banata hai — base pairing rule follow karke (A ke saamne T, G ke saamne C). Yaad rakho: yeh enzyme sirf 5′ se 3′ direction mein naya strand banata hai, matlab woh template ko 3′→5′ padhta hai.
Ek important baat: DNA polymerase zero se shuru nahi kar sakta. Usko ek free 3′-OH chahiye hota hai jisko woh aage badha sake. Isliye pehle primase ek chhota RNA primer banata hai, aur uske baad polymerase usko extend karta hai. Energy ke liye alag se ATP nahi chahiye — har naya nucleotide khud triphosphate (dNTP) form mein aata hai, aur jab woh judta hai to pyrophosphate (PPi) nikalta hai, wahi energy de deta hai.
Aur sabse mast feature hai proofreading. Agar galat base lag jaaye, to polymerase ka 3′→5′ exonuclease usko nikaal ke sahi base laga deta hai — isi wajah se replication itna accurate hota hai (galti almost 1 in a billion). Exam mein common galti yeh hoti hai ki students bolte hain "polymerase DNA ko unwind karta hai" ya "naya strand khud se start karta hai" — dono galat. Unwind helicase, start primer/primase, aur leading strand continuous banta hai jabki lagging strand chhote Okazaki fragments mein, jinhe ligase jodta hai. Yeh saaf clear rakhoge to question pakka sahi jaayega.