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 …
- AP EAPCET 2026Set ap-2026-05-19-AN1 markMCQQ.Thermostable DNA polymerase enzyme which is used in PCR technique was extracted from the following bacteria. (A) Agrobacterium tumefaciens (B) Escherichia coli (C) Thermus aquaticus (D) Salmonella typhimurium
›Reveal solutionSolution
PCR requires a DNA polymerase that survives ~95°C denaturation steps; Taq polymerase, isolated from the thermophilic bacterium Thermus aquaticus, provides this thermostability. Answer: (C).
Concept and Intuition
PCR (Polymerase Chain Reaction) cycles through denaturation (~95°C), annealing (~50-60°C), and extension (~72°C) many times. An ordinary DNA polymerase (like that of E. coli) would denature and lose activity at the high denaturation temperature, requiring fresh enzyme every cycle. This problem was solved by isolating a heat-stable DNA polymerase from Thermus aquaticus, a bacterium that naturally lives in hot springs and geysers, so its enzymes are adapted to withstand high temperatures — this enzyme (Taq polymerase) can survive repeated heating and remains active throughout the PCR cycles.
Step-by-Step Solution
- Agrobacterium tumefaciens (A) is used as a natural vector for plant genetic transformation (T-DNA), not as a source of thermostable polymerase.
- Escherichia coli (B) is a standard cloning host and source of many restriction enzymes/ordinary DNA polymerase, but its polymerase is NOT thermostable. …
- AP EAPCET 2026Set ap-2026-05-20-FN1 markMCQQ.Choose the correct statements among the following A) Transcriptase catalyser the polymerisation in 5′→3′ direction only and the 3′→5′ strand referred as template. B) The chemical method of Khorana was instrumental in synthesis of homopolymers of RNA. C) t RNA amino acid acceptor end has with the bases of complementary to the code. D) 23s r-RNA in bacteria is a ribozyme. (A) A, B, C (B) B, C, D (C) A, C, D (D) A, B, D
›Reveal solutionSolution
Statements A, B and D are correct molecular-biology facts; C wrongly attributes codon-complementarity to the amino-acid acceptor end instead of the anticodon.
Concept and Intuition
Molecular machines that copy nucleic acid information have a fixed directionality: polymerases always add nucleotides at the 3' end of the growing strand, so synthesis proceeds 5'→3' while the enzyme reads the template in the opposite (3'→5') direction. The genetic code was deciphered partly using synthetic RNAs of known, defined sequence (homopolymers/copolymers) — a chemical synthesis capability pioneered by Khorana. A tRNA has two functionally distinct ends: the 3' CCA 'acceptor' end (identical in every tRNA, where the amino acid attaches) and the anticodon loop (which varies between tRNAs and is complementary to the mRNA codon). Finally, ribosomal RNA is not just structural — the large subunit's 23S rRNA in bacteria itself catalyses peptide-bond formation, making it a ribozyme.
Step-by-Step Solution
- A: polymerisation is always 5'→3', template is read 3'→5' — matches known biochemistry — TRUE. …
- AP EAPCET 2025Set ap-2025-05-19-FN1 markMCQQ.Choose the incorrect statement among the following sentences (A) The small opening of DNA helix where replication occur is replication fork. (B) DNA dependent DNA polymerase catalyse polymerisation from 3' → 5'. (C) The template of DNA strand with polarity 3' → 5' is leading strand. (D) DNA dependent DNA polymerase catalyse polymerisation from 5' → 3'.
›Reveal solutionSolution
DNA polymerase only synthesizes new DNA in the 5' to 3' direction, never 3' to 5' — so the statement claiming 3' to 5' polymerization is the incorrect one. Answer: (B).
Concept and Intuition
DNA replication has several well-defined features: the replication fork is the Y-shaped region where the double helix is actively unwound and new strands are being synthesized. All known DNA-dependent DNA polymerases add new nucleotides only to the free 3'-OH end of a growing strand, meaning synthesis always proceeds in the 5' to 3' direction — they read the template strand in the 3' to 5' direction to do this. Because of this fixed directionality, only one template strand (the one running 3' to 5' as the fork opens, called the leading-strand template) allows continuous synthesis in the same direction as fork movement; the other template strand (running 5' to 3') must be copied discontinuously in short Okazaki fragments, forming the lagging strand.
Step-by-Step Solution
- Statement (A): small opening of DNA helix where replication occurs is the replication fork — this is the standard, correct definition. TRUE.
- Statement (B): DNA dependent DNA polymerase catalyse polymerisation from 3' to 5' — this is FALSE; polymerization always proceeds 5' to 3', never 3' to 5'. …
- AP EAPCET 2025Set ap-2025-05-20-FN1 markMCQQ.Which of the following statements are correct ? I. The packaging of chromatin at higher level requires additional set of proteins called non-histone chromosomal proteins. II. In some viruses, the flow of genetic information for protein synthesis is in the reverse direction that is from RNA to DNA. III. RNA is the gentic material that is passed from virus to bacteria was given by Avery, Macleod and Mecarty. IV. RNA polymerase II transcribes the precursor of 5S RNA. (A) I, II (B) II, III (C) III, IV (D) I, IV
›Reveal solutionSolution
Statements I (non-histone proteins needed for higher-order chromatin packaging) and II (reverse RNA→DNA information flow in some viruses) are correct; III and IV each misattribute a classic experiment/enzyme — option (A).
Concept and Intuition
This question tests precise recall of chromatin organisation and the central dogma's exceptions, alongside correct attribution of classic molecular-biology experiments and RNA polymerase specificities.
Step-by-Step Solution
- I: Beyond histones, an additional set of proteins called non-histone chromosomal (NHC) proteins is required to achieve the higher orders of chromatin packaging (looping, scaffolding) seen in condensed chromosomes — true.
- II: In retroviruses (e.g. HIV), the enzyme reverse transcriptase copies the viral RNA genome into DNA — a reversal of the normal DNA→RNA flow, so genetic information flows RNA to DNA in these viruses — true.
- III: The experiment identifying DNA (not RNA) as the transforming principle was done by Avery, MacLeod and McCarty, using bacterial (pneumococcus strain) transformation — not a virus-to-bacterium transfer, and not RNA. This statement is false on multiple counts. …
- AP EAPCET 2024Set ap-2024-05-16-FN1 markMCQQ.Assertion (A): Primers are used in polymerase chain reaction technique Reason (R): Primers is used for the addition of new DNA nucleotides (A) A and R are correct. R is the correct explanation of A (B) A and R are correct. R is not the correct explanation of A (C) A is correct but R is incorrect (D) A is incorrect but R is correct
›Reveal solutionSolution
Primers are essential in PCR, but they only provide the free 3'-OH starting point — it is Taq DNA polymerase, not the primer, that actually adds new nucleotides.
Concept and Intuition
PCR (Polymerase Chain Reaction) amplifies a specific DNA segment through repeated cycles of denaturation, annealing and extension. Short single-stranded DNA primers anneal to the template at the boundaries of the region to be amplified, giving DNA polymerase a free 3'-OH end to begin synthesis from. It is thermostable Taq polymerase that then catalyses the addition of new deoxyribonucleotides, extending the primer along the template.
Step-by-Step Solution
- Assertion: primers are used in PCR — true, they define the amplified region's boundaries and enable annealing. …
- AP EAPCET 2023Set ap-2023-05-23-FN1 markMCQQ.Match the following. Set - I: I. DNA Ligase II. DNA Polymerase III. Splicing IV. Ribozyme Set - II: A. Polymerisation B. Introns C. Joining D. RNA Enzyme Set - III: i. Okazaki fragments ii. Reactive & unstable iii. mRNA iv. One direction (A) I C ii, II A iii, III D iv, IV B i (B) I C i, II D ii, III A iii, IV C iv (C) I A iv, II B i, III C ii, IV D iii (D) I C i, II A iv, III B iii, IV D ii
›Reveal solutionSolution
A molecular-biology match of four terms with their function and a defining characteristic each.
Concept and Intuition
- DNA ligase seals nicks by joining together DNA fragments — most notably the Okazaki fragments on the lagging strand during replication.
- DNA polymerase carries out polymerisation (adding nucleotides) but can only extend a strand in one direction (5'→3').
- Splicing removes introns from a transcript to produce mature mRNA.
- A ribozyme is a catalytic RNA enzyme, and being RNA rather than protein, it is generally more reactive and unstable than protein enzymes.
Step-by-Step Solution
- DNA Ligase → Joining (C) → Okazaki fragments (i).
- DNA Polymerase → Polymerisation (A) → One direction (iv).
- Splicing → Introns (B) → mRNA (iii). …
- AP EAPCET 2022Set ap-2022-07-12-AN1 markMCQQ.During Replication the discontinuously synthesized fragments are known as (A) Okazaki fragments (B) DNA Strands (C) Proteins (D) Amino acids
›Reveal solutionSolution
The lagging strand of DNA is synthesized in short, discontinuous stretches called Okazaki fragments, later joined by DNA ligase.
Concept and Intuition
DNA polymerase can only synthesize new DNA in the 5' to 3' direction. Since the two parental strands are antiparallel, only one new strand (the leading strand) can be synthesized continuously in the same direction as replication fork movement. The other strand (the lagging strand) must be synthesized in the opposite direction relative to fork movement, forcing DNA polymerase to work in short bursts, restarting synthesis repeatedly as the fork opens up more template. Each of these short DNA segments is called an Okazaki fragment, and they are later joined together into a continuous strand by the enzyme DNA ligase, after RNA primers are removed and gaps filled.
Step-by-Step Solution
- Recall that DNA synthesis is always 5'→3', constraining how each template strand can be copied.
- The leading strand is synthesized continuously, matching the direction of fork movement.
- The lagging strand must be synthesized discontinuously, in short pieces, opposite to fork movement. …
- AP EAPCET 2021Set ap-2021-09-03-FN1 markMCQQ.RNA polymerase III transcribes 3 of the following except ____ (A) tRNA (B) 5S rRNA (C) hnRNA (D) snRNA
›Reveal solutionSolution
This tests which RNA polymerase transcribes which RNA class — RNA Pol III makes tRNA, 5S rRNA and snRNA, while hnRNA (pre-mRNA) is made by RNA Pol II.
Concept and Intuition
Eukaryotic cells use three distinct RNA polymerases, each dedicated to a different class of RNA: RNA polymerase I transcribes the large ribosomal RNAs (28S, 18S, 5.8S rRNA); RNA polymerase II transcribes heterogeneous nuclear RNA (hnRNA), the precursor that is processed (capped, spliced, polyadenylated) into mRNA, as well as most snRNAs; RNA polymerase III transcribes small, stable RNA species — tRNA, 5S rRNA, and some snRNAs. Because hnRNA is specifically the domain of RNA polymerase II, it is the odd one out among the four listed options.
Step-by-Step Solution
- Recall RNA Pol III's transcripts: tRNA, 5S rRNA, and snRNA — three of the four options listed. …
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