NEET rarely asks you to describe the full Central Dogma in paragraph form. What it does test: can you distinguish the specific biological roles of DNA versus RNA, and can you connect structural features (base pairing, sugar type, strand number) to those functions?
DNA — the information archive. DNA stores the hereditary blueprint of an organism. Its double-stranded helical structure with complementary base pairing (adenine with thymine via 2 hydrogen bonds, guanine with cytosine via 3 hydrogen bonds) provides a built-in mechanism for faithful replication. Each strand serves as a template for copying the other — this is the molecular basis of heredity (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 427).
RNA — the functional messenger. RNA is the working copy. Three major types carry out distinct functions:
The function–structure link NEET exploits. Questions test whether you can connect: (1) double-stranded → replication fidelity (DNA), (2) single-stranded → transient functional roles (RNA), (3) deoxyribose → chemical stability for long-term storage (DNA), (4) ribose → sufficient for short-lived functional molecules (RNA), (5) thymine in DNA versus uracil in RNA.
Watch-out: A common confusion is attributing protein synthesis directly to DNA. DNA does not leave the nucleus in eukaryotes — it is mRNA that carries the message to the cytoplasmic ribosome. Another frequent error: stating that all RNA is single-stranded (some viral RNA is double-stranded, though NEET questions stay within the NCERT scope of single-stranded cellular RNA).
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which of the following is the primary biological function of DNA?
Answer: A. A is correct. DNA's primary biological function is to store the hereditary blueprint of an organism. Its double-stranded, base-paired structure allows faithful replication and transmission of genetic information (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 427).
Why B is wrong: B is wrong — catalysing metabolic reactions is the function of enzymes (proteins), not DNA.
Why C is wrong: C is wrong — transporting amino acids to ribosomes is the function of tRNA, not DNA.
Why D is wrong: D is wrong — forming the structural core of ribosomes is the function of rRNA, not DNA.
Which type of RNA carries the genetic code from DNA to the ribosome for protein synthesis?
Answer: C. C is correct. Messenger RNA (mRNA) is transcribed from DNA and carries the coded sequence to the ribosome, where it is translated into a polypeptide (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 427).
Why A is wrong: A is wrong — tRNA transports amino acids to the ribosome and reads codons via its anticodon; it does not carry the genetic code from DNA.
Why B is wrong: B is wrong — rRNA is a structural and catalytic component of the ribosome, not the carrier of the genetic message.
Why D is wrong: D is wrong — snRNA (small nuclear RNA) is involved in RNA splicing in eukaryotes, not in carrying the genetic code to ribosomes.
The function of tRNA in protein synthesis is to:
Answer: D. D is correct. Transfer RNA (tRNA) carries specific amino acids to the ribosome and matches them to the corresponding mRNA codon using its anticodon loop (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 427).
Why A is wrong: A is wrong — the structural framework of the ribosome is formed by rRNA (along with ribosomal proteins), not tRNA.
Why B is wrong: B is wrong — carrying the genetic code from nucleus to cytoplasm is the function of mRNA, not tRNA.
Why C is wrong: C is wrong — unwinding the DNA double helix during replication is performed by helicase enzymes, not tRNA.
The double-stranded structure of DNA with complementary base pairing directly enables which biological process?
Answer: B. B is correct. The complementary base pairing in the double helix (A-T: 2 H-bonds, G-C: 3 H-bonds) means each strand serves as a template for the other, directly enabling semi-conservative replication — the molecular basis of heredity (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 427).
Why A is wrong: A is wrong — translation occurs at the ribosome using mRNA, tRNA, and rRNA. The double-stranded structure of DNA is not directly involved in translation.
Why C is wrong: C is wrong — polypeptide folding into tertiary structure depends on R-group interactions (H-bonds, ionic bonds, hydrophobic interactions, disulphide bridges), not on DNA's double-stranded structure.
Why D is wrong: D is wrong — pre-mRNA splicing is carried out by the spliceosome (involving snRNA and proteins) and is unrelated to the base-paired double helix structure of DNA.
A student claims that DNA directly synthesises proteins in the cytoplasm. What is the correct explanation?
Answer: A. A is correct. In eukaryotes, DNA remains in the nucleus. The genetic information is first transcribed into mRNA, which exits the nucleus and is translated at cytoplasmic ribosomes into a polypeptide chain (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 427).
Why B is wrong: B is wrong — DNA does not leave the nucleus in eukaryotic cells. The information is transferred via mRNA.
Why C is wrong: C is wrong — DNA is not converted into tRNA. tRNA is transcribed from specific tRNA genes, and its function is amino acid delivery, not carrying the protein-coding sequence.
Why D is wrong: D is wrong — DNA has no catalytic ability to form peptide bonds. Peptide bond formation is catalysed by the peptidyl transferase activity of the ribosome (rRNA).
Which structural feature of DNA makes it more chemically stable than RNA, suiting its role as the long-term hereditary store?
Answer: B. B is correct. Deoxyribose lacks the 2'-OH group present in ribose. This absence makes DNA less susceptible to alkaline hydrolysis of the phosphodiester backbone, giving it the chemical stability required for long-term information storage (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 427).
Why A is wrong: A is wrong — uracil is found in RNA, not DNA. DNA contains thymine. This option describes RNA, not the stability feature of DNA.
Why C is wrong: C is wrong — single-stranded conformation is characteristic of most RNA. DNA's double-stranded structure contributes to its stability, not single-strandedness.
Why D is wrong: D is wrong — the 2'-OH group on ribose actually makes RNA more reactive and susceptible to hydrolysis, not more stable. This is a feature of RNA, not DNA.
If rRNA were absent from a cell, which of the following processes would be directly and immediately affected?
Answer: D. D is correct. rRNA is the structural and catalytic core of the ribosome. Without rRNA, functional ribosomes cannot assemble, and mRNA translation into polypeptides halts immediately (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 427).
Why A is wrong: A is wrong — DNA replication is carried out by DNA polymerase and associated enzymes. rRNA is not involved in replication.
Why B is wrong: B is wrong — transcription is carried out by RNA polymerase and does not require rRNA or ribosomes.
Why C is wrong: C is wrong — pre-mRNA splicing is performed by the spliceosome (snRNPs), not by rRNA or the ribosome.
Consider the following statements about nucleic acid functions:
(i) DNA undergoes semi-conservative replication.
(ii) mRNA acts as the template for translation.
(iii) rRNA delivers amino acids to the ribosome.
(iv) tRNA reads the mRNA codon via its anticodon.
Which statements are correct?
Answer: C. C is correct. Statement (i) is correct — DNA replicates semi-conservatively using each strand as a template. Statement (ii) is correct — mRNA serves as the template read by the ribosome during translation. Statement (iv) is correct — tRNA matches amino acids to mRNA codons via its anticodon. Statement (iii) is incorrect — amino acid delivery is the function of tRNA, not rRNA (NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, page 427).
Why A is wrong: A is wrong — this includes statement (iii), which incorrectly assigns amino acid delivery to rRNA. rRNA forms the structural/catalytic core of the ribosome; tRNA delivers amino acids.
Why B is wrong: B is wrong — this includes statement (iii) (incorrect, as above) and excludes statement (i), which is a correct description of DNA replication.
Why D is wrong: D is wrong — this includes statement (iii), which is incorrect. Amino acid delivery is performed by tRNA, not rRNA.
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Given
• Total base pairs = 200 (exact counting integer)• A–T pairs = 120 (exact counting integer)• G–C pairs = 80 (exact counting integer)
Required
Total number of hydrogen bonds in this DNA segment.
Concept
In Watson–Crick base pairing, each A–T pair is held by 2 hydrogen bonds and each G–C pair is held by 3 hydrogen bonds. The total hydrogen bonds equal the sum across all pairs.
Formula
Total H-bonds = (number of A–T pairs × 2) + (number of G–C pairs × 3)
Substitution
Total H-bonds = (120 × 2) + (80 × 3)
Calculation
Total H-bonds = 240 + 240 = 480
Note on exact values: All numbers in this problem are counting integers (number of base pairs). Counting integers are exact and do not limit significant figures.
Final answer
The DNA segment contains 480 hydrogen bonds.
Common trap
Confusing the hydrogen bond counts: assigning 3 H-bonds to A–T and 2 to G–C (reversed). Remember: G–C has the higher bond count (3) because guanine and cytosine have three complementary H-bonding sites.
Similar NEET-style question
A double-stranded DNA molecule has 30% adenine. If the total number of nucleotides is 2000, how many hydrogen bonds are present?
Approach: If A = 30%, then T = 30% (Chargaff's rule), so A–T pairs = 30% of 1000 base pairs = 300. G + C = 40%, so G–C pairs = 200. Total H-bonds = (300 × 2) + (200 × 3) = 600 + 600 = 1200.
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DNA: deoxyribose sugar, A-T-G-C bases, double helix. RNA: ribose, A-U-G-C bases, mostly single-strand. Phosphate-sugar backbone. Bases: purines (A, G) and pyrimidines (T/U, C). H-bond pairs A=T (DNA), A=U (RNA), G≡C.
-- NCERT Class 12 Chemistry (pre-2023 edition), Chapter 14, p. 428More in Biomolecules: 4 exam traps and mistakes · 3 formulas · 1 question pattern from its other lessons.
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