Biology · Molecular Basis of Inheritance · NEET
NCERT names DNA-dependent DNA polymerase as the MAIN enzyme of replication, because it does the actual joining of nucleotides using a DNA template. Helicase (which opens the helix) is one of the 'many additional enzymes' the chapter mentions but does not name in detail. For NEET, if a question asks the main/chief enzyme of replication, the answer is DNA-dependent DNA polymerase, not helicase.
DNA polymerase can add a new nucleotide only to a free 3'-OH end, so the new strand can only grow from 5' towards 3'. This is a key NCERT fact and a repeated NEET point. Because of this single-direction rule, one template (3' to 5') is copied continuously and the other template (5' to 3') is copied discontinuously in short pieces.
It is a definite, fixed region in the DNA where replication begins. In E. coli replication does NOT start randomly anywhere; it starts at this specific origin. In recombinant DNA work, a vector must carry an origin of replication so the inserted DNA can be copied inside the host cell.
A replication fork is the small Y-shaped opening in the DNA helix where the two strands separate and copying happens. NCERT says the two strands cannot be separated along their whole length at once because that needs too much energy, so replication happens inside this small opening that moves along the DNA.
No. NCERT clearly states 'DNA polymerases on their own cannot initiate the process of replication.' Replication must begin at the origin of replication, and additional enzymes are needed. This is why DNA polymerase alone is not enough.
The deoxyribonucleoside triphosphates (dNTPs) have a dual role: they are both the substrate (raw material) AND the energy source. The two terminal phosphates are high-energy phosphate bonds, just like in ATP, and releasing them powers the polymerisation reaction. So replication is energetically an expensive process.
DNA polymerase joins nucleotides to build the new strand along the template (5' to 3'). DNA ligase joins the short discontinuous fragments of the lagging strand into one continuous strand. Polymerase builds; ligase seals the gaps. Both are needed for correct replication.
Which of the following statements is correct regarding the process of replication in E. coli?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
DNA-dependent DNA polymerase. It uses a DNA template to join deoxynucleotides and build the new strand. NCERT calls it the main enzyme of replication.
Only 5' to 3'. It cannot work 3' to 5'. This is a very common NEET point.
Because it needs a lot of energy. The deoxyribonucleoside triphosphates supply this energy through their two high-energy terminal phosphates, the same as in ATP.
E. coli has 4.6 x 10^6 base pairs and finishes replication in about 18 minutes, roughly 2000 base pairs per second. So the polymerase must be both fast and highly accurate.
Because replication only starts at an origin. A piece of DNA to be copied in recombinant DNA work must be joined to a vector that provides this origin, or it cannot be replicated inside the host.
DNA ligase. It seals the short fragments made on the discontinuous (lagging) strand into one continuous strand.
