DNA/RNA Structure — Homi Bhabha Class 9 Biology MCQs with Solutions
Free Homi Bhabha Class 9 Biology DNA/RNA Structure MCQs with step-by-step solutions (17 questions). Part of Heredity & Genetics. Practise online on Prepizo — no login needed.
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Questions with solutions
Q1 — DNA/RNA Structure · medium · theory
The molecule that carries the hereditary (genetic) information in most living organisms is:
A. RNA only
B. protein
C. DNA ✓ Correct
D. glucose
Solution: DNA (deoxyribonucleic acid) stores and transmits genetic information.
Q2 — DNA/RNA Structure · hard · theory
The double-helix structure of DNA was proposed by:
A. Darwin
B. Watson and Crick ✓ Correct
C. Mendel
D. Robert Hooke
Solution: James Watson and Francis Crick described the DNA double helix in 1953.
Q3 — DNA/RNA Structure · medium · theory
The four nitrogenous bases found in DNA are adenine, thymine, guanine and:
A. ribose
B. cytosine ✓ Correct
C. glucose
D. uracil
Solution: DNA contains A, T, G and C; uracil occurs in RNA, not DNA.
Q4 — DNA/RNA Structure · hard · theory
In RNA, the base that replaces thymine (found in DNA) is:
A. cytosine
B. guanine
C. uracil ✓ Correct
D. adenine
Solution: RNA uses uracil (U) in place of the thymine found in DNA.
Q5 — DNA/RNA Structure · medium · theory
A segment of DNA that controls a particular trait is called a:
A. ribosome
B. gene ✓ Correct
C. tissue
D. cell
Solution: A gene is a unit of heredity — a stretch of DNA coding for a characteristic.
Q6 — DNA/RNA Structure · hard · theory
In the DNA double helix, adenine (A) always pairs with:
A. uracil (U)
B. cytosine (C)
C. guanine (G)
D. thymine (T) ✓ Correct
Solution: Complementary base pairing: A pairs with T, and G pairs with C.
Q7 — DNA/RNA Structure · medium · theory
The thread-like structures in the nucleus that carry the genes are called:
A. lysosomes
B. chromosomes ✓ Correct
C. vacuoles
D. ribosomes
Solution: Chromosomes, made of DNA and protein, carry the genes.
Q8 — DNA/RNA Structure · hard · theory
Structurally, DNA is usually ______, while RNA is usually ______:
A. both double-stranded
B. both single-stranded
C. single-stranded; double-stranded
D. double-stranded; single-stranded ✓ Correct
Solution: DNA is typically a double-stranded helix; RNA is generally single-stranded.
Q9 — DNA/RNA Structure · hard · theory
A single unit (building block) of DNA, consisting of a sugar, a phosphate and a base, is called a:
A. monosaccharide
B. nucleotide ✓ Correct
C. amino acid
D. fatty acid
Solution: A nucleotide (sugar + phosphate + nitrogenous base) is the monomer of nucleic acids.
Q10 — DNA/RNA Structure · medium · theory
In eukaryotic cells, most of the DNA is located in the:
A. cytoplasm
B. cell wall
C. ribosome
D. nucleus ✓ Correct
Solution: The bulk of a eukaryotic cell's DNA is found in the nucleus (some in mitochondria/chloroplasts).
Q11 — DNA/RNA Structure · easy · theory
What does the abbreviation DNA stand for?
A. Dinucleic ribose acid
B. Diribonucleic acid
C. Deoxyribonucleic acid ✓ Correct
D. Deoxyribose amino acid
Solution: DNA stands for Deoxyribonucleic acid, the molecule that carries genetic information. The "deoxy" refers to its sugar (deoxyribose), which lacks one oxygen compared with ribose.
Q12 — DNA/RNA Structure · easy · theory
Which sugar is present in a molecule of RNA?
A. Sucrose
B. Deoxyribose
C. Ribose ✓ Correct
D. Glucose
Solution: RNA contains the five-carbon sugar ribose. DNA contains deoxyribose, which has one fewer oxygen atom. This difference in sugar is one of the main ways DNA and RNA differ.
Q13 — DNA/RNA Structure · easy · theory
In a DNA molecule, the base adenine (A) always pairs with which base?
A. Cytosine (C)
B. Thymine (T) ✓ Correct
C. Uracil (U)
D. Guanine (G)
Solution: By complementary base pairing, adenine pairs with thymine (A-T) and guanine pairs with cytosine (G-C). Uracil replaces thymine only in RNA, not in DNA.
Q14 — DNA/RNA Structure · medium · numerical
In a sample of double-stranded DNA, adenine makes up 30% of the total bases. According to Chargaff's rule, what percentage of the bases is cytosine?
A. 70%
B. 20% ✓ Correct
C. 40%
D. 30%
Solution: Chargaff's rule: A = T and G = C. If A = 30%, then T = 30%, so A + T = 60%. The remaining 40% is shared equally between G and C, giving C = 20%.
Q15 — DNA/RNA Structure · medium · theory
Which nitrogenous base is found in RNA but never in DNA?
A. Adenine
B. Guanine
C. Thymine
D. Uracil ✓ Correct
Solution: RNA uses uracil (U) in place of thymine. Adenine, guanine and cytosine occur in both DNA and RNA, while thymine is found only in DNA and uracil only in RNA.
Q16 — DNA/RNA Structure · hard · numerical
A double-stranded DNA molecule has 200 base pairs, of which 60 are adenine-thymine (A-T) pairs. Each A-T pair has 2 hydrogen bonds and each guanine-cytosine (G-C) pair has 3. How many hydrogen bonds hold the whole molecule together?
A. 600
B. 400
C. 460
D. 540 ✓ Correct
Solution: A-T pairs = 60, giving 60 × 2 = 120 bonds. G-C pairs = 200 - 60 = 140, giving 140 × 3 = 420 bonds. Total = 120 + 420 = 540 hydrogen bonds.
Q17 — DNA/RNA Structure · hard · theory
In the DNA double helix a purine always pairs with a pyrimidine (A with T, G with C). What is the main structural reason for this specific pairing?
A. It removes the need for hydrogen bonds
B. It makes the two strands run in the same direction
C. It keeps the width of the double helix uniform along its length ✓ Correct
D. It converts deoxyribose into ribose
Solution: A purine (larger, two rings) pairs with a pyrimidine (smaller, one ring). This always gives the same combined width, so the two strands stay a constant distance apart and the helix has a uniform diameter throughout.