In some viruses, DNA is synthesised by using RNA as template. Such a DNA is called: (a) A-DNA (b) B-DNA (c) cD

NCERT Exemplar · Q21

Q. In some viruses, DNA is synthesised by using RNA as template. Such a DNA is called:

(a) A-DNA

(b) B-DNA

(d) rDNA

Uttarakhand UbseMCQ· 1mImportance★★★★★

36% · 35/96 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

Concept understanding — Types Of Polymerases

You have probably never heard the word "polymerase" before, but you already understand what it does. Think of a polymer as a long chain — like a necklace made of many identical or similar beads. In biology, the most famous polymers are DNA and RNA, which are chains of smaller units called nucleotides. A polymerase is simply the enzyme (a protein machine) that links those beads together to build the chain.

In the context of your syllabus, the term "polymerases" almost always refers to DNA polymerases and RNA polymerases. These are the two main types you need to know. They are not interchangeable; each has a specific job and a specific set of rules.

1. DNA Polymerase — The Copyist

DNA polymerase is the enzyme responsible for DNA replication — making an exact copy of the entire DNA molecule before a cell divides. Imagine you have a master blueprint (the original DNA). DNA polymerase reads that blueprint and builds a matching second copy.

Key features to remember:

It can only add new nucleotides to an existing strand (it needs a "primer" to start).
It works in one direction only: from the 5' end to the 3' end of the new strand.
It has a proofreading ability. If it accidentally puts the wrong bead on the chain, it can cut it out and replace it. This is why DNA replication is so accurate.

Important

In NCERT, you will encounter three types of DNA polymerases in prokaryotes (like E. coli): DNA polymerase I, II, and III. Of these, DNA polymerase III is the main enzyme that does the bulk of replication. DNA polymerase I helps remove the RNA primer and fill in gaps. DNA polymerase II is mainly involved in repair.

2. RNA Polymerase — The Transcriber

RNA polymerase does a different job. It is responsible for transcription — copying a specific gene from DNA into a messenger RNA (mRNA) molecule. Think of it as a photocopier that makes a working copy of just one page of the blueprint, not the whole book.

Key differences from DNA polymerase:

It does not need a primer to start. It can begin building the RNA chain from scratch.
It uses DNA as a template but builds an RNA strand (which uses uracil instead of thymine).
It is less accurate than DNA polymerase — it has no proofreading ability. This is acceptable because RNA molecules are temporary and get replaced. …

Start your 14-day free trial to unlock the full solution →

When a virus uses RNA as a template to make DNA, that DNA is called complementary DNA (cDNA).

The question touches on a fascinating exception to the central dogma of molecular biology — the flow of genetic information. Normally, DNA makes RNA, and RNA makes protein. But some viruses, most famously retroviruses like HIV, reverse this flow. They carry their genetic information as RNA, and once inside a host cell, they use that RNA as a template to synthesise DNA.

This process is catalysed by an enzyme called reverse transcriptase. The DNA produced in this way is not identical to any pre-existing DNA in the cell. Instead, it is a complementary copy of the viral RNA. That is why it is called complementary DNA, or cDNA.

Note

The term "complementary" here means that the DNA sequence is exactly complementary to the RNA template — each RNA base pairs with its DNA counterpart (A with T, U with A, G with C, and C with G).

Now, let us look at the options given:

A-DNA and B-DNA are structural forms of DNA — different helical conformations that DNA can adopt under certain conditions. They have nothing to do with the template used for synthesis. A-DNA is a right-handed helix that is shorter and wider than the more common B-DNA, but both are simply physical shapes of the DNA molecule.
rDNA stands for recombinant DNA — DNA that has been artificially created by combining genetic material from different sources. This is a tool used in biotechnology, not a natural product of viral replication. …

Start your 14-day free trial to unlock the full solution →

←Previous questionQ20 — While analysing the DNA of an organism a total number of 5386 nucleoti…→Next questionQ22 — If Meselson and Stahl's experiment is continued for four generations i…

Q.In some viruses, DNA is synthesised by using RNA as template. Such a DNA is called:

🎓Unlock everything free for 14 days

Unlock everything free for 14 days