During the process of transcription, after binding to a promoter, RNA polymerase catalyses and makes the bases

Question

Q. 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

CBSECBSE Class XII Board 2025MCQ· 1mImportance★★★★★

✓ Free question

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.

Note

In prokaryotes, one single RNA polymerase does all the transcription. In eukaryotes (organisms with a nucleus), there are three types: RNA polymerase I (makes ribosomal RNA), RNA polymerase II (makes mRNA — the most important one for protein synthesis), and RNA polymerase III (makes transfer RNA and other small RNAs).

Why This Distinction Matters

If you mix up the two, you will lose marks in exams. Here is the simplest way to keep them straight:

DNA polymerase → makes DNA from DNA (replication). Needs a primer. Has proofreading.
RNA polymerase → makes RNA from DNA (transcription). No primer needed. No proofreading.

Both are essential for life. Without DNA polymerase, cells cannot divide. Without RNA polymerase, genes cannot be expressed and proteins cannot be made. In your NCERT textbook, these definitions appear in the chapters on Molecular Basis of Inheritance — and they are tested directly in multiple-choice and short-answer questions.

Students revising this topic often search for "NCERT biology syllabus types of polymerases", "Types Of Polymerases NEET questions", or "Types Of Polymerases diagram and explanation". This concept is part of the Molecular Basis of Inheritance chapter in the NCERT/CBSE Class 12 Biology syllabus, and revising it thoroughly helps with both board exams and general competitive-exam preparation.

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

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.
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.
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).
Evaluating the Options:
- (A) Deoxyribonucleotide triphosphate: These are the building blocks for DNA synthesis (replication), not RNA synthesis (transcription). They contain deoxyribose sugar.
- (B) Deoxyribonucleoside triphosphate: This option is incorrect for two reasons: it refers to a deoxyribo sugar (for DNA) and a nucleoside (lacks phosphate groups, thus cannot provide energy for polymerization).
- (C) Ribonucleotide triphosphate: This correctly identifies the building blocks. They contain ribose sugar (for RNA) and are triphosphates (providing energy and the necessary phosphate for the backbone).
- (D) Ribonucleoside triphosphate: This option is incorrect because it refers to a nucleoside (lacks phosphate groups, cannot provide energy for polymerization). While it correctly identifies the ribose sugar, the "triphosphate" part is misleadingly attached to "nucleoside" instead of "nucleotide". A ribonucleoside triphosphate would imply a nucleoside with three phosphates, which is essentially a ribonucleotide triphosphate. However, the term "ribonucleoside triphosphate" is not standard for the building blocks; "ribonucleotide triphosphate" is the precise and correct term.

Watch out

Be careful with the terminology: "nucleoside" vs. "nucleotide". A nucleoside lacks phosphate groups, while a nucleotide has them. For synthesis, we need nucleotides, specifically triphosphates, to provide both the monomer unit and the energy.

The bases in the template strand of DNA (A, T, G, C) pair with the incoming ribonucleotide triphosphates (U, A, C, G respectively) according to base-pairing rules, with uracil (U) replacing thymine (T) in RNA.

✓Final answer

During transcription, RNA polymerase makes the bases in the template strand of DNA available for base pairing with the bases of (C) Ribonucleotide triphosphate .

Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2026Set ANNUAL1 markMCQ
$Q. The RNA transcripted by RNA polymerase I is :$
(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).

RNA polymerase III transcribes tRNA, 5S rRNA, and small nuclear RNAs (snRNAs).

So among the given options, the RNA transcribed by RNA polymerase I is 28S rRNA.

✓Final answer
$(a) 28S rRNA.$
CBSE 2026Set ANNUAL1 mark
$Q. Write the name of the enzyme which does DNA replication.$
›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 other enzymes such as DNA helicase (unwinds the double helix), topoisomerase (relieves supercoiling), and DNA ligase (joins Okazaki fragments on the lagging strand).

✓Final answer
$DNA polymerase.$
CBSE 2025Set 57/5/11 markMCQ
$Q. During the process of transcription, after binding to a promoter, RNA polymerase catalyses and makes the bases in the template strand of DNA available for base pairing, with the bases of : (A) Deoxyribonucleotide triphosphate (B) Deoxyribonucleoside triphosphate (C) Ribonucleotide triphosphate (D) Ribonucleoside triphosphate$
›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

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.

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.

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).

Evaluating the Options:

(A) Deoxyribonucleotide triphosphate: These are the building blocks for DNA synthesis (replication), not RNA synthesis (transcription). They contain deoxyribose sugar.

(B) Deoxyribonucleoside triphosphate: This option is incorrect for two reasons: it refers to a deoxyribo sugar (for DNA) and a nucleoside (lacks phosphate groups, thus cannot provide energy for polymerization).

(C) Ribonucleotide triphosphate: This correctly identifies the building blocks. They contain ribose sugar (for RNA) and are triphosphates (providing energy and the necessary phosphate for the backbone).

(D) Ribonucleoside triphosphate: This option is incorrect because it refers to a nucleoside (lacks phosphate groups, cannot provide energy for polymerization). While it correctly identifies the ribose sugar, the "triphosphate" part is misleadingly attached to "nucleoside" instead of "nucleotide". A ribonucleoside triphosphate would imply a nucleoside with three phosphates, which is essentially a ribonucleotide triphosphate. However, the term "ribonucleoside triphosphate" is not standard for the building blocks; "ribonucleotide triphosphate" is the precise and correct term.

Watch out

Be careful with the terminology: "nucleoside" vs. "nucleotide". A nucleoside lacks phosphate groups, while a nucleotide has them. For synthesis, we need nucleotides, specifically triphosphates, to provide both the monomer unit and the energy.

The bases in the template strand of DNA (A, T, G, C) pair with the incoming ribonucleotide triphosphates (U, A, C, G respectively) according to base-pairing rules, with uracil (U) replacing thymine (T) in RNA.

✓Final answer
$During transcription, RNA polymerase makes the bases in the template strand of DNA available for base pairing with the bases of (C) Ribonucleotide triphosphate .$
CBSE 2024Set E1 markMCQ
$Q. Which of the following enzymes is responsible for transcription of ribosomal RNA?$
(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 answer is RNA polymerase I.

(Option 'RNA polymerase' without a number is the single bacterial enzyme and is not the specific eukaryotic answer intended here.)

✓Final answer
$(b) RNA polymerase I.$
CBSE 2024Set ANNUAL1 markMCQ
$Q. Which of the following RNA polymerase transcribes rRNAs (28s, 18s & 5.8s) ?$
(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 messenger RNA (hnRNA) and most small nuclear RNAs; RNA polymerase III transcribes tRNA, 5S rRNA and other small RNAs.

✓Final answer
$(a) I.$
CBSE 2023Set ANNUAL1 markMCQ
$Q. Which of the following RNA polymerase transcribes tRNA?$
(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.

RNA Polymerase III — transcribes tRNA, 5S rRNA, and other small RNAs.

Since the question asks specifically about tRNA, the answer is RNA Polymerase III.

✓Final answer
$RNA polymerase III transcribes tRNA (option c).$
CBSE 2021Set TERM11 markMCQ
$Q. rRNA is transcribed by :$
(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.

RNA Polymerase III transcribes tRNA, 5S rRNA, and some other small RNAs.

Since this question asks specifically about rRNA (the major species), the answer is RNA Polymerase I.

✓Final answer
$(c) RNA Polymerase I.$
CBSE 2020Set ANNUAL1 mark
$Q. Which type of R.N.A. polymerase enzyme transcribes precursor m.R.N.A.?$
›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).

Applying it. The primary transcript for a protein-coding gene contains both introns and exons and is called hnRNA. It is later processed (capping, tailing and splicing) into mature mRNA. Since this precursor is the direct product of the second polymerase, the enzyme responsible is RNA polymerase II. This scheme is standard in the NCERT/CBSE-aligned KSEAB Class-12 syllabus.

✓Final answer
$RNA polymerase II.$
CBSE 2019Set ANNUAL1 mark
$Q. Write any one function of RNA Polymerase.$
›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 $5' \rightarrow 3'$ direction.

It binds at the promoter, opens the DNA duplex, and moves along, synthesising the RNA — in bacteria a single RNA polymerase (with a sigma factor for initiation and rho factor for termination) makes all types of RNA (mRNA, tRNA, rRNA).

✓Final answer
$RNA polymerase catalyses the synthesis of RNA on a DNA template (transcription), adding ribonucleotides in the 5'\to3' direction.$
CBSE 2018Set ANNUAL1 markMCQ
$Q. Which of the following is transcribed by rRNA polymerase-II?$
(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.

RNA polymerase III transcribes tRNA, 5S rRNA, and some snRNAs.

So among the given options, tRNA and 5S rRNA are made by Pol III, while HnRNA is the correct product of RNA polymerase II.

✓Final answer
$(d) HnRNA is transcribed by RNA polymerase II.$
CBSE 2018Set ANNUAL1 markMCQ
$Q. Which of the following is involved in transcription of tRNA, 5Sr RNA and Sn RNA of eukaryotes?$
(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).

RNA Polymerase III — transcribes transfer RNA (tRNA), 5S ribosomal RNA and the small nuclear RNAs (snRNA).

So the enzyme responsible for tRNA, 5S rRNA and snRNA is RNA Polymerase III.

✓Final answer
$(c) RNA Polymerase III.$

Concept and Intuition

Step-by-Step Explanation

🎓Unlock everything free for 14 days

Unlock everything free for 14 days