Dna Rna Structure

8 MCQs9-step worked example
Source: NCERT Structural Organisation in AnimalsPYQ coverage: NEET 2020, 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026

Lesson

NEET regularly tests whether you can distinguish the structural features of DNA from RNA and recall the details of the Watson-Crick model. The high-frequency trap here is confusing the directionality and bonding specifics — getting the strand polarity, base-pairing rules, or groove nomenclature wrong costs marks under negative marking.

The Watson-Crick model (1953) describes DNA as a right-handed double helix with two antiparallel polynucleotide strands. "Antiparallel" means one strand runs 5'→3' while its complement runs 3'→5'. The strands are held together by hydrogen bonds between complementary bases: adenine pairs with thymine via two H-bonds (A=T), and guanine pairs with cytosine via three H-bonds (G≡C). This is Chargaff's rule in structural form — the molar ratio of A/T and G/C each equals 1 (NCERT Class 12 Biology Chapter 5, page 94).

The backbone consists of alternating deoxyribose sugar and phosphate groups linked by 3'–5' phosphodiester bonds. The bases project inward, stacked 0.34 nm apart; one full turn spans 3.4 nm and contains approximately 10 base pairs. The helix has a major groove and a minor groove — proteins that read DNA (transcription factors, regulatory proteins) typically interact via the major groove, where more of the base-pair's chemical identity is exposed.

DNA vs RNA — the four structural differences NEET expects:

FeatureDNARNA
SugarDeoxyribose (–H at 2' carbon)Ribose (–OH at 2' carbon)
BasesA, T, G, CA, U, G, C
StrandsDouble-stranded (usually)Single-stranded (usually)
StabilityMore stable (no 2'–OH)Less stable, more reactive

Thymine in DNA is replaced by uracil in RNA — both pair with adenine, but uracil lacks the methyl group at the 5-position.

Watch-out for NEET: when a question says "which base is absent in RNA," the answer is thymine, not uracil. Reversing this is a common negative-marking loss.

Practice MCQs

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

The distance between two adjacent base pairs in the B-form DNA double helix is:

MCQ 2Easy RecallPractice

The number of hydrogen bonds between adenine and thymine in a DNA double helix is:

MCQ 3Easy RecallPractice

Which of the following bases is present in RNA but absent in DNA?

MCQ 4Direct ApplicationPractice

A DNA molecule has 30% adenine. What is the percentage of guanine in the same molecule?

MCQ 5Direct ApplicationPractice

A DNA molecule contains 600 base pairs. How many complete helical turns does it have in B-form?

MCQ 6Direct ApplicationPractice

The two strands of a DNA double helix are described as antiparallel because:

MCQ 7Concept TrapPractice

DNA is more stable than RNA under physiological conditions. Which structural feature of DNA primarily accounts for this greater stability?

MCQ 8Concept TrapPractice

Regulatory proteins that need to read the DNA sequence without unwinding the helix typically bind at:

Worked Example

  1. 1

    Given

    A double-stranded DNA molecule has 1,000 base pairs. Analysis of one strand shows: A = 200, T = 150, G = 350, C = 300.

  2. 2

    Required

    Find the base composition (number of each base) of the complementary strand and the complete molecule.

  3. 3

    Concept

    In double-stranded DNA, adenine on one strand pairs with thymine on the complementary strand, and guanine pairs with cytosine (Watson-Crick base-pairing). Chargaff's rule ensures that for the whole molecule, A = T and G = C.

  4. 4

    Formula/Rule

    Complementary strand base = partner of the given strand base: - A in given strand → T in complementary strand - T in given strand → A in complementary strand - G in given strand → C in complementary strand - C in given strand → G in complementary strand

  5. 5

    Substitution

    Complementary strand: - T (paired with A = 200) = 200 - A (paired with T = 150) = 150 - C (paired with G = 350) = 350 - G (paired with C = 300) = 300

  6. 6

    Calculation

    Whole molecule totals: - Total A = 200 (strand 1) + 150 (strand 2) = 350 - Total T = 150 (strand 1) + 200 (strand 2) = 350 ✓ (A = T) - Total G = 350 (strand 1) + 300 (strand 2) = 650 - Total C = 300 (strand 1) + 350 (strand 2) = 650 ✓ (G = C) - Grand total = 350 + 350 + 650 + 650 = 2,000 bases = 1,000 base pairs ✓ Note: the counts 200, 150, 350, 300, and 1,000 are exact counting integers — they do not involve significant-figure considerations.

  7. 7

    Final answer

    Complementary strand: A = 150, T = 200, G = 300, C = 350. Whole molecule: A = T = 350; G = C = 650. Total = 2,000 bases.

  8. 8

    Common trap

    Students sometimes apply Chargaff's rule to a single strand (expecting A = T within that strand). Chargaff's rule applies to the double-stranded molecule as a whole. In a single strand, A ≠ T and G ≠ C is perfectly normal.

  9. 9

    Similar NEET-style question

    "If one strand of DNA has the base composition A : T : G : C = 20% : 30% : 15% : 35%, what is the base composition of the complete double-stranded DNA molecule?" (Apply the same complementary-pairing logic, then average across both strands.) ---

Before solving, remember these

Key Fact

DNA structure

Watson-Crick double helix: antiparallel strands; A=T (2 H-bonds), G≡C (3 H-bonds); 10 bp/turn; pitch 3.4 nm; diameter 2 nm. Chargaff's rule: A=T, G=C. Z-DNA, A-DNA, B-DNA forms.

-- NCERT Class 12 Biology, Ch. 5, p. 94

Formulas

Mendel's monohybrid ratio

F2 ratio in monohybrid cross — products of independent assortment of two alleles per locus.

SymbolQuantitySI Unit
ratioF2 progeny ratio-

Valid when

  • Single gene with complete dominance
  • Pure-bred parents

Mendel's dihybrid ratio

F2 ratio in dihybrid cross with two independently-segregating loci, complete dominance, no linkage.

SymbolQuantitySI Unit
ratioF2 phenotype ratio-

Valid when

  • Two unlinked loci
  • Complete dominance

Hardy-Weinberg equation

In an idealised population (no mutation, drift, selection, gene flow, random mating), allele and genotype frequencies remain constant.

SymbolQuantitySI Unit
pfreq of dominant allele A-
qfreq of recessive allele a-

Valid when

  • Idealised population
  • All five conditions met
  • Diploid, autosomal, biallelic locus

Recombination frequency (genetic mapping)

Proportion of recombinant offspring measures genetic distance between linked loci. Capped at 50% (independent assortment).

SymbolQuantitySI Unit
RFrecombination frequency%

Valid when

  • Linked loci on same chromosome

Exam Traps & Common Mistakes

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

5' → 3' synthesis polarity

Category: Sign Convention

DNA polymerase synthesises only 5'→3'. Leading strand: continuous, same direction as fork. Lagging: discontinuous (Okazaki), opposite to fork.

When it triggers

Question on Okazaki, leading vs lagging, primer direction.

How to avoid

Reading template 3'→5'; synthesising 5'→3'. Lagging strand needs short fragments because it can't run continuously against fork direction.

Trap

Hardy-Weinberg disequilibrium causes

Category: Negative Marking

Five forces disturb HW: mutation, gene flow, drift, selection, non-random mating. ANY of these violates equilibrium.

When it triggers

Question asks which factor maintains/disturbs HW.

How to avoid

Random mating + no other forces → equilibrium. ANY of mutation/migration/drift/selection/assortative mating → disequilibrium.

Trap

Genotype vs phenotype ratio confusion

Category: Similar Terms

Monohybrid: genotype 1:2:1 (AA:Aa:aa); phenotype 3:1.

When it triggers

Question asks for one ratio while presenting cross details.

How to avoid

Always note dominance: phenotype merges Aa + AA; genotype keeps them separate.

Trap

X-linked recessive: father→daughter expression

Category: Similar Terms

X-linked recessive (haemophilia, colour-blindness): affects males predominantly; carrier mother → 50% sons affected; affected father → all daughters carriers but not affected.

When it triggers

Pedigree question; carrier vs affected.

How to avoid

Sex chromosomes: XX vs XY. Recessive on X needs both copies (XaXa) in female, only one (XaY) in male.

Trap

Sympatric vs allopatric speciation

Category: Similar Terms

Allopatric: geographic isolation. Sympatric: same area, no physical barrier (e.g. polyploidy in plants, host-shift).

When it triggers

Question gives speciation scenario and asks which type.

How to avoid

If geographic barrier mentioned → allopatric. If population overlaps → sympatric.

Past Year Questions

51 questions from NEET 2020, 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.

NEET 2025

Identify the statement that is NOT correct.

1Constant region of heavy and light chains are located at C-terminus of antibody molecules
2Each antibody has two light and two heavy chains.
3The heavy and light chains are held together by disulfide bonds.
4Antigen binding site is located at C-terminal region of antibody molecules.
NTA Answer: Option 4(final)
NEET 2025

Given below are two statements : Statement I : In the RNA world, RNA is considered the first genetic material evolved to carry out essential life processes. RNA acts as a genetic material and also as a catalyst for some important biochemical reactions in living systems. Being reactive, RNA is unstable. Statement II : DNA evolved from RNA and is a more stable genetic material. Its double helical strands being complementary, resist changes by evolving repairing mechanism. In the light of the above statements, choose the most appropriate answer from the options given below :

1Statement I is incorrect but statement II is correct
2Both statement I and statement II are correct
3Both statement I and statement II are incorrect
4Statement I is correct but statement II is incorrect
NTA Answer: Option 2(final)
NEET 2024

Which one of the following can be explained on the basis of Mendel's Law of Dominance? A. Out of one pair of factors one is dominant and the other is recessive. B. Alleles do not show any expression and both the characters appear as such in F generation. 2 C. Factors occur in pairs in normal diploid plants. D. The discrete unit controlling a particular character is called factor. E. The expression of only one of the parental characters is found in a monohybrid cross. Choose the correct answer from the options given below:

1A, B and C only
2A, C, D and E only
3B, C and D only
4A, B, C, D and E
NTA Answer: Option 2(final)
NEET 2024

Which of the following statement is correct regarding the process of replication in E.coli?

1The DNA dependent DNA polymerase catalyses polymerization in one direction that is 3’ → 5’
2The DNA dependent RNA polymerase catalyses polymerization in one direction, that is 5’ → 3’
3The DNA dependent DNA polymerase catalyses polymerization in 5’ → 3’ as well as 3’ → 5’ direction
4The DNA dependent DNA polymerase catalyses polymerization in 5’ → 3’ direction
NTA Answer: Option 4(final)
NEET 2023

The phenomenon of pleiotropism refers to

1Presence of two alleles, each of the two genes controlling a single trait
2A single gene affecting multiple phenotypic expression
3More than two genes affecting a single character
4Presence of several alleles of a single gene controlling a single crossover
NTA Answer: Option 2(final)
NEET 2023

Expressed Sequence Tags (ESTs) refers to

1All genes that are expressed as proteins.
2All genes whether expressed or unexpressed.
3Certain important expressed genes.
4All genes that are expressed as RNA.
NTA Answer: Option 4(final)
NEET 2023

Which of the following statements are correct about Klinefelter’s Syndrome? A. This disorder was first described by Langdon Down (1866). B. Such an individual has overall masculine development. However, the feminine developement is also expressed. C. The affected individual is short statured. D. Physical, psychomotor and mental development is retarded. E. Such individuals are sterile. Choose the correct answer from the options given below:

1C and D only
2B and E only
3A and E only
4A and B only
NTA Answer: Option 2(final)
NEET 2022

Given below are two statements : Statement I : Mendel studied seven pairs of contrasting traits in pea plants and proposed the Laws of Inheritance. Statement II : Seven characters examined by Mendel in his experiment on pea plants were seed shape and colour, flower colour, pod shape and colour, flower position and stem height. In the light of the above statements, choose the correct answer from the options given below :

1Statement I is incorrect but Statement II is correct
2Both Statement I and Statement II are correct
3Both Statement I and Statement II are incorrect
4Statement I is correct but Statement II is incorrect
NTA Answer: Option 2(final)
NEET 2022

Statements related to human Insulin are given below. Which statement(s) is/are correct about genetically engineered Insulin? (a) Pro-hormone insulin contain extra stretch of C-peptide (b) A-peptide and B-peptide chains of insulin were produced separately in E.coli, extracted and combined by creating disulphide bond between them. (c) Insulin used for treating Diabetes was extracted from Cattles and Pigs. (d) Pro-hormone Insulin needs to be processed for converting into a mature and functional hormone. (e) Some patients develop allergic reactions to the foreign insulin. Choose the most appropria

1(c), (d) and (e) only
2(a), (b) and (d) only
3(b) only
4(c) and (d) only
NTA Answer: Option 3(final)
NEET 2021

Match List-I with List-II. (2) (a)-Replication; (b)-Transcription; (c)-Transduction; (d)-Protein (3) (a)-Translation; (b)-Replication; (c)-Transcription;(d)-Transduction (4) (a)-Replication; (b)-Transcription; (c)-Translation; (d)-Protein Answer (4) Choose the correct answer from the options given 127. Which of the following are not secondary below. metabolites in plants? (a) (b) (c) (d)

1Rubber, gums (1) (ii) (i) (iv) (iii)
2Morphine, codeine (2) (ii) (iv) (i) (iii)
3Amino acids, glucose (3) (iv) (iii) (ii) (i)
4(iii) (i) (iv) (ii) (4) Vinblastin, curcumin
NTA Answer: Option 2(final)
NEET 2020

Which of the following refer to correct example(s) of organisms which have evolved due to changes 97. ÁŸêŸ ◊ ∑§ÊÒŸ, ∞ ‚ ¡ËflÊ ∑ § ‚„Ë ©ŒÊ„⁄UáÊÊ ∑§Ê ‚ ŒÁ÷¸Ã ∑§⁄UÃÊ „Ò in environment brought about by anthropogenic ¡Ê ◊ÊŸfl ∑§Ë Á∑˝§ÿÊ•Ê mÊ⁄UÊ flÊÃÊfl⁄UáÊ ◊ ’Œ‹Êfl ∑ § ∑§Ê⁄UáÊ action ? Áfl∑§Á‚à „È∞ „Ò? (a) Darwin’s Finches of Galapagos islands. (a) ªÒ‹Ê¬ÒªÊ mˬ ◊ «UÊÁfl¸Ÿ ∑§Ë Á» §ø (b) Herbicide resistant weeds. (b) π⁄U¬ÃflÊ⁄UÊ ◊ ‡ÊÊ∑§ŸÊ‡ÊË ∑§Ê ¬˝ÁÃ⁄UÊ œ (c) Drug resistant eukaryotes. (c) ‚‚Ë◊∑ §ãŒ˝∑§Ê ◊ ŒflÊßÿÊ ∑§Ê ¬˝ÁÃ⁄UÊ œ (d) Man-created breeds of domesticated animal

1∑ §fl‹ (d) (1) only (d)
2∑ §fl‹ (a) (2) only (a)
3(a) ∞fl (c) (3) (a) and (c)
4(b), (c) ∞fl (d) (4) (b), (c) and (d) Hindi+English 23 H3
NTA Answer: Option 4(final)
NEET 2020

•ŸÈ‹ πŸ ∑ § ‚◊ÿ «UË.∞Ÿ.∞. ∑§Ë ∑È §«U‹Ë ∑§Ê πÊ ‹Ÿ ◊ ∑§ÊÒŸ‚Ê

1RNA polymerase ∞ ¡Êß◊ ◊ŒŒ ∑§⁄UÃÊ „Ò?
2DNA ligase (1) •Ê⁄U.∞Ÿ.∞. ¬ÊÚÁ‹◊⁄ U$¡
3DNA helicase
4DNA polymerase (2) «UË.∞Ÿ.∞. ‹Êߪ $¡ (3) «UË.∞Ÿ.∞. „Ò‹Ë∑ §$¡ 117. Snow-blindness in Antarctic region is due to : (4) «UË.∞Ÿ.∞. ¬ÊÚ‹Ë◊⁄ U$¡ (1) Damage to retina caused by infra-red rays (2) Freezing of fluids in the eye by low
NTA Answer: Option 1(final)
NEET 2020

¡ËŸ ‘I’ ¡Ê ABO ⁄UÄà flª¸ ∑§Ê ÁŸÿ òÊáÊ ∑§⁄UÃÊ „Ò ©‚∑ § ‚ Œ÷¸ 121. Identify the wrong statement with reference to the gene ‘I’ that controls ABO blood groups. ◊ ª‹Ã ∑§ÕŸ ∑§Ê ¬„øÊÁŸ∞–

1Allele ‘i’ does not produce any sugar. (1) ‘i’ ∞ ‹Ë‹ ∑§Ê ߸ ÷Ë ‡Ê∑¸§⁄UÊ ©à¬ÛÊ Ÿ„Ë ∑§⁄UÃÊ–
2The gene (I) has three alleles. (2) ¡ËŸ (I) ∑ § ÃËŸ ∞ ‹Ë‹ „Ê Ã „Ò –
3A person will have only two of the three (3) ∞∑§ √ÿÁÄà ◊ ÃËŸ ◊ ‚ ∑ §fl‹ ŒÊ ∞ ‹Ë‹ „Ê ª – alleles.
4¡’ IA ∞fl IB ŒÊ ŸÊ ß∑§_ „Ê Ã „Ò , ÿ ∞∑§ ¬˝∑§Ê⁄U ∑§Ë (4) When IA and IB are present together, they ‡Ê∑¸§⁄UÊ •Á÷√ÿÄà ∑§⁄Uà „Ò – express same type of sugar.
NTA Answer: Option 4(final)
NEET 2020

≈˛UÊ ‚‹ ‡ÊŸ (•ŸÈflÊŒŸ/SÕÊŸÊ Ã⁄UáÊ) ∑§Ë ¬˝Õ◊ •flSÕÊ ∑§ÊÒŸ ‚Ë 123. The first phase of translation is : „Ê ÃË „Ò?

1Recognition of an anti-codon (1) ∞∑§ ∞ ≈UË-∑§Ê «UÊÚŸ ∑§Ë ¬„øÊŸ
2Binding of mRNA to ribosome (2) ⁄UÊß’Ê ‚Ê ◊ ‚ mRNA ∑§Ê ’㜟
3Recognition of DNA molecule (3) «UË.∞Ÿ.∞. •áÊÈ ∑§Ë ¬„øÊŸ
4Aminoacylation of tRNA (4) tRNA ∑§Ê ∞ ◊ËŸÊ ∞‚Ë‹ ‡ÊŸ
NTA Answer: Option 4(final)
NEET 2020

◊ «U‹ Ÿ Sflà òÊ M§¬ ‚ ¬˝¡ŸŸ ∑§⁄UŸ flÊ‹Ë ◊≈U⁄U ∑ § ¬ÊÒœ ∑§Ë 130. How many true breeding pea plant varieties did Mendel select as pairs, which were similar except Á∑§ÃŸË Á∑§S◊Ê ∑§Ê ÿÈÇ◊Ê ∑ § M§¬ ◊ øÈŸÊ ¡Ê Áfl¬⁄UËà Áfl‡Ê ∑§Ê in one character with contrasting traits ? flÊ‹ ∞∑§ ‹ˇÊáÊ ∑ § •‹ÊflÊ ∞∑§ ‚◊ÊŸ ÕË?

18 (1) 8
24 (2) 4
32 (3) 2
414 (4) 14 H3 30 Hindi+English
NTA Answer: Option 4(final)

How NEET usually asks this

Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.

Sources

NCERT refs: Class 12 Biology Chapter 5, p.94

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