Q.In some viruses, DNA is synthesised by using RNA as template. Such a DNA is called:
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🔒 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.
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. …
The key here is the template. When a virus uses RNA as a template to make DNA, the process is reverse transcription, and the DNA produced is complementary to the original RNA.
- cDNA stands for complementary DNA. It is synthesised from an RNA template using the enzyme reverse transcriptase. This is exactly what happens in retroviruses like HIV.
- A-DNA and B-DNA are just different structural forms (conformations) of the DNA double helix — they have nothing to do with the template used for synthesis. …
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.
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:
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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.
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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. …
Decode what each answer's own name implies before considering the biology: 'A-DNA' and 'B-DNA' name helical conformations — shape variants of ordinary double-stranded DNA — not a synthesis route. 'rDNA' names DNA assembled in vitro from multiple sources (recombinant DNA), a laboratory construction technique, not a natural viral product. Only 'cDNA' names …
- CBSE 2026Set ANNUAL1 markMCQQ.The RNA transcripted by RNA polymerase I is :(a) 28S rRNA(b) tRNA(c) Sn rRNA(d) mRNA
›Reveal solutionSolution
In eukaryotes, RNA polymerase I transcribes the major ribosomal RNAs (28S, 18S and 5.8S rRNA).
Eukaryotic cells have three distinct RNA polymerases, each transcribing a defined set of genes:
- RNA polymerase I transcribes the ribosomal RNAs — 28S, 18S, and 5.8S rRNA.
- RNA polymerase II transcribes the precursor of mRNA, i.e. heterogeneous nuclear RNA (hnRNA). …
- CBSE 2026Set ANNUAL1 markQ.Write the name of the enzyme which does DNA replication.
›Reveal solutionSolution
DNA replication is catalysed by the enzyme DNA polymerase (DNA-dependent DNA polymerase).
During DNA replication, the enzyme DNA polymerase (specifically DNA-dependent DNA polymerase) catalyses the polymerisation of deoxyribonucleotides in the 5'→3' direction, using the parental strand as a template, to synthesise the new complementary daughter strand. It requires a primer (a short RNA segment made by the enzyme primase) to begin synthesis, and works alongside o …
- CBSE 2025Set 57/5/11 markMCQQ.During the process of transcription, after binding to a promoter, RNA polymerase catalyses and makes the bases in the template strand of DNA available for base pairing, with the bases of : (A) Deoxyribonucleotide triphosphate (B) Deoxyribonucleoside triphosphate (C) Ribonucleotide triphosphate (D) Ribonucleoside triphosphate
›Reveal solutionSolution
During transcription, RNA polymerase synthesizes RNA by using ribonucleotide triphosphates as the building blocks, which pair with the exposed bases of the DNA template strand. The correct option is (C).
Concept and Intuition
Transcription is the fundamental biological process where genetic information from a DNA segment is copied into an RNA molecule. Think of it like making a temporary working copy of a blueprint. The enzyme responsible for this crucial task is RNA polymerase.
For RNA polymerase to build a new RNA strand, it needs raw materials, much like a construction worker needs bricks to build a wall. These raw materials are individual molecular units that will be linked together to form the long RNA polymer.
Here's the intuition:
- What is being built? An RNA molecule. RNA is made of ribonucleotides. This immediately tells us the sugar must be ribose, not deoxyribose (which is for DNA).
- How are these units linked? The formation of phosphodiester bonds between nucleotides requires energy. In biological synthesis, this energy is typically provided by the hydrolysis of high-energy phosphate bonds. Therefore, the building blocks are not just single phosphates, but triphosphates. The breaking off of two phosphates (as pyrophosphate) releases the energy needed for the reaction.
- What's the difference between a nucleoside and a nucleotide? A nucleoside is a base + sugar. A nucleotide is a base + sugar + phosphate(s). For polymerization, we need nucleotides, specifically triphosphates.
Combining these points, the building blocks must be ribonucleotide triphosphates.
Important- A nucleoside consists of a nitrogenous base (A, G, C, T/U) linked to a pentose sugar (deoxyribose in DNA, ribose in RNA).
- A nucleotide is a nucleoside with one or more phosphate groups attached to the sugar.
- Triphosphates (like ATP, GTP, CTP, UTP) are used as building blocks in nucleic acid synthesis because the hydrolysis of their terminal phosphate bonds provides the necessary energy for forming phosphodiester bonds.
Step-by-Step Explanation
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Understanding Transcription: Transcription is the first step of gene expression, where a specific segment of DNA is copied into an RNA molecule. This process is catalyzed by RNA polymerase. The DNA double helix unwinds locally, exposing the bases of the template strand.
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Role of RNA Polymerase: RNA polymerase moves along the DNA template strand, reading its sequence. It then recruits complementary building blocks from the surrounding cellular environment and links them together to form a new RNA strand. For example, if the DNA template has an 'A', RNA polymerase will incorporate a 'U' into the growing RNA strand. If the DNA has a 'G', it will incorporate a 'C', and so on.
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Identifying the Correct Building Blocks:
- Nature of the product: The product is RNA. RNA is a polymer of ribonucleotides. This means the sugar component of the building blocks must be ribose, not deoxyribose.
- Energy requirement: The formation of phosphodiester bonds between incoming nucleotides requires energy. This energy is supplied by the hydrolysis of the high-energy phosphate bonds present in nucleotide triphosphates. When a ribonucleotide triphosphate is added to the growing RNA chain, two phosphate groups are cleaved off as pyrophosphate (PPi), releasing energy that drives the polymerization reaction.
- Combining these facts: The building blocks must be ribonucleotide triphosphates. These are adenosine triphosphate (ATP), guanosine triphosphate (GTP), cytidine triphosphate (CTP), and uridine triphosphate (UTP).
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Evaluating the Options: …
- CBSE 2024Set E1 markMCQQ.Which of the following enzymes is responsible for transcription of ribosomal RNA?(a) RNA polymerase(b) RNA polymerase I(c) RNA polymerase II(d) RNA polymerase III
›Reveal solutionSolution
Ribosomal RNA (except 5S) is transcribed by RNA polymerase I in eukaryotes.
Eukaryotes have three nuclear RNA polymerases with a division of labour: RNA polymerase I transcribes the ribosomal RNAs (28S, 18S and 5.8S rRNA); RNA polymerase II transcribes the precursor of mRNA (hnRNA); and RNA polymerase III transcribes tRNA, 5S rRNA and small nuclear/snRNAs. Since the question asks specifically about ribosomal RNA transcription, the answe …
- CBSE 2024Set ANNUAL1 markMCQQ.Which of the following RNA polymerase transcribes rRNAs (28s, 18s & 5.8s) ?(a) I(b) II(c) III(d) None of these
›Reveal solutionSolution
Eukaryotes use three RNA polymerases; Pol I makes the major rRNAs.
Eukaryotic cells have three distinct RNA polymerases, each transcribing a different set of RNAs: RNA polymerase I transcribes ribosomal RNA (rRNA), specifically the 28S, 18S and 5.8S rRNAs; RNA polymerase II transcribes the precursor of me …
- CBSE 2023Set ANNUAL1 markMCQQ.Which of the following RNA polymerase transcribes tRNA?(a) I(b) II(c) III(d) None of these
›Reveal solutionSolution
In eukaryotes, three RNA polymerases have separate jobs; RNA Pol III makes tRNA, 5S rRNA, and other small RNAs.
Eukaryotic cells have three distinct nuclear RNA polymerases, each transcribing a different class of RNA:
- RNA Polymerase I — transcribes ribosomal RNA (rRNA), specifically 28S, 18S, and 5.8S rRNA.
- RNA Polymerase II — transcribes precursor of mRNA (heterogeneous nuclear RNA, hnRNA) as well as most snRNAs. …
- CBSE 2021Set TERM11 markMCQQ.rRNA is transcribed by :(a) RNA Polymerase III(b) RNA Polymerase II(c) RNA Polymerase I(d) RNA Polymerase IV
›Reveal solutionSolution
In eukaryotic cells, the three main rRNA species (except 5S rRNA) are transcribed by RNA Polymerase I inside the nucleolus.
Eukaryotes have three main nuclear RNA polymerases, each dedicated to different classes of RNA:
- RNA Polymerase I transcribes the pre-rRNA that is processed into the 28S, 18S and 5.8S rRNAs (in the nucleolus).
- RNA Polymerase II transcribes heterogeneous nuclear RNA (hnRNA), the precursor of mRNA, as well as most snRNAs. …
- CBSE 2020Set ANNUAL1 markQ.Which type of R.N.A. polymerase enzyme transcribes precursor m.R.N.A.?
›Reveal solutionSolution
RNA polymerase II transcribes the precursor of mRNA (heterogeneous nuclear RNA, hnRNA) in eukaryotes.
Concept. Unlike bacteria, which use a single RNA polymerase for all RNAs, eukaryotic cells possess three nuclear RNA polymerases, each dedicated to a specific class of RNA:
- RNA polymerase I → transcribes rRNAs (28S, 18S, 5.8S).
- RNA polymerase II → transcribes the precursor of mRNA, i.e. heterogeneous nuclear RNA (hnRNA).
- RNA polymerase III → transcribes tRNA, 5S rRNA and small nuclear RNAs (snRNAs). …
- CBSE 2019Set ANNUAL1 markQ.Write any one function of RNA Polymerase.
›Reveal solutionSolution
RNA polymerase catalyses transcription — it synthesises an RNA strand from a DNA template, joining ribonucleotides in the 5′→3′ direction.
Concept
Transcription is the copying of genetic information from DNA into RNA. The enzyme responsible is RNA polymerase, which reads the template strand of DNA and builds a complementary RNA strand.
Function
- RNA polymerase catalyses the polymerisation of ribonucleotides (using the DNA template) to make RNA, adding nucleotides only in the 5′→3′ direction. …
- CBSE 2018Set ANNUAL1 markMCQQ.Which of the following is transcribed by rRNA polymerase-II?(a) tRNA(b) 5s rRNA(c) Sn RNA(d) Hn RNA
›Reveal solutionSolution
Of the three eukaryotic RNA polymerases, RNA polymerase II is specifically responsible for transcribing hnRNA (the mRNA precursor).
Eukaryotic cells have three distinct RNA polymerases, each transcribing a different class of RNA:
- RNA polymerase I transcribes ribosomal RNA (rRNA), except 5S rRNA.
- RNA polymerase II transcribes heterogeneous nuclear RNA (HnRNA), the precursor that is processed (capping, tailing, splicing) into mature mRNA; it also transcribes some snRNAs. …
- CBSE 2018Set ANNUAL1 markMCQQ.Which of the following is involved in transcription of tRNA, 5Sr RNA and Sn RNA of eukaryotes?(a) RNA Polymerase I(b) RNA Polymerase II(c) RNA Polymerase III(d) All of these
›Reveal solutionSolution
The three eukaryotic RNA polymerases divide the work: Pol III makes tRNA, 5S rRNA and snRNA.
Eukaryotes have three nuclear RNA polymerases with distinct roles:
- RNA Polymerase I — transcribes the large ribosomal RNAs (28S, 18S, 5.8S).
- RNA Polymerase II — transcribes precursor mRNA (hnRNA). …
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