DNA Structure
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All Questions for: DNA Structure
Question 1: The Watson-Crick model describes B-DNA as:
- A A right-handed double helix with antiparallel strands
- B A left-handed single helix
- C A parallel double helix
- D A triple helix
Explanation: Watson and Crick (1953) proposed the classic model: a right-handed double helix with antiparallel strands and complementary base pairing.
Question 2: How many base pairs are there per complete turn of the B-DNA helix?
- A 10
- B 12
- C 8
- D 20
Explanation: In standard B-DNA, one full helical turn contains approximately 10 base pairs.
Question 3: The vertical rise per base pair in B-DNA is approximately:
- A 0.34 nm
- B 3.4 nm
- C 0.1 nm
- D 2.0 nm
Explanation: Each base pair adds ~0.34 nm (3.4 Å) to the helix length, so a full 10-bp turn is 3.4 nm.
Question 4: The diameter of the B-DNA double helix is approximately:
- A 2 nm
- B 0.2 nm
- C 10 nm
- D 20 nm
Explanation: The B-DNA helix is roughly 2 nm (20 Å) wide — a constant width due to purine-pyrimidine pairing.
Question 5: The two strands of a DNA double helix are described as:
- A Antiparallel
- B Parallel
- C Randomly oriented
- D Covalently linked end-to-end
Explanation: One strand runs 5'→3' while the other runs 3'→5'. This antiparallel orientation is essential for complementary base pairing and replication.
Question 6: The two strands of DNA are held together primarily by:
- A Hydrogen bonds between complementary bases
- B Phosphodiester bonds between bases
- C Peptide bonds between nucleotides
- D Ionic bonds between sugars
Explanation: Hydrogen bonds between complementary bases (A-T: 2 bonds; G-C: 3 bonds) are the main non-covalent interactions between strands. Base-stacking forces also contribute to stability.
Question 7: In B-DNA, one complete turn of the helix is approximately how long?
- A 3.4 nm
- B 0.34 nm
- C 34 nm
- D 0.034 nm
Explanation: With 10 bp per turn and 0.34 nm per bp, the pitch of B-DNA is about 3.4 nm.
Question 8: Which groove of the B-DNA helix is wider and more accessible to proteins?
- A The major groove
- B The minor groove
- C Both grooves equally
- D Neither groove is accessible
Explanation: The major groove exposes base-pair edges in a way that allows sequence-specific recognition by transcription factors and other DNA-binding proteins.
Question 9: Z-DNA differs from B-DNA in that Z-DNA is:
- A A left-handed helix
- B A right-handed helix with 12 bp/turn
- C A triple-stranded helix
- D A single-stranded structure
Explanation: Z-DNA is a left-handed double helix with a zigzag sugar-phosphate backbone. It forms in GC-rich alternating sequences and may play roles in gene regulation.
Question 10: Heating a DNA solution to separate the two strands is known as:
- A Denaturation
- B Renaturation
- C Annealing
- D Hybridization
Explanation: Denaturation (or melting) breaks the hydrogen bonds between strands, converting the double helix into two single strands.
Question 11: The temperature at which 50% of a DNA sample is denatured is called:
- A Melting temperature (Tm)
- B Annealing temperature
- C Extension temperature
- D Hybridization temperature
Explanation: The melting temperature (Tm) reflects the stability of the duplex and increases with GC content because G-C pairs have three hydrogen bonds.
Question 12: A DNA molecule with higher GC content will have:
- A A higher melting temperature
- B A lower melting temperature
- C No change in melting temperature
- D A more negative charge
Explanation: Because G-C pairs form three hydrogen bonds versus two for A-T, higher GC content increases duplex stability and raises the melting temperature.
Question 13: When double-stranded DNA denatures, its absorbance at 260 nm:
- A Increases (hyperchromic effect)
- B Decreases (hypochromic effect)
- C Remains unchanged
- D Becomes zero
Explanation: Unstacking of bases upon strand separation increases UV absorbance — the hyperchromic effect, commonly used to monitor DNA melting.
Question 14: When separated DNA strands re-anneal, their UV absorbance:
- A Decreases (hypochromic effect)
- B Increases (hyperchromic effect)
- C Remains constant
- D Becomes infinite
Explanation: Renaturation restores base stacking, decreasing UV absorbance — the hypochromic effect.
Question 15: Negatively supercoiled DNA is characterized by:
- A Under-winding that facilitates strand separation
- B Over-winding that resists strand separation
- C Formation of triple helices
- D Complete strand separation
Explanation: Negative supercoiling denotes under-winding of the duplex (fewer helical turns than relaxed). It favors strand separation and is the biologically predominant state in cells.
Question 16: The enzyme that relieves supercoiling by transiently cutting the DNA is:
- A Topoisomerase
- B Helicase
- C Ligase
- D Primase
Explanation: Topoisomerases cleave and reseal DNA strands to relieve torsional stress generated during replication, transcription, and other processes.
Question 17: The fundamental repeating unit of chromatin is the:
- A Nucleosome
- B Solenoid
- C Nucleoid
- D Centromere
Explanation: A nucleosome consists of ~147 bp of DNA wrapped 1.65 times around a histone octamer; it is the basic packaging unit of eukaryotic chromatin.
Question 18: The core of a nucleosome consists of a histone octamer containing:
- A Two copies each of H2A, H2B, H3, and H4
- B Four copies of H1
- C Two copies each of H1, H2A, H2B, and H3
- D One copy of each of H1–H4
Explanation: The histone octamer consists of two copies each of H2A, H2B, H3, and H4. Histone H1 binds linker DNA and promotes higher-order compaction.
Question 19: Which histone is NOT part of the nucleosome core particle?
- A H1
- B H2A
- C H3
- D H4
Explanation: Histone H1 is a linker histone that binds the DNA entering and exiting the nucleosome core. It is not part of the octamer.
Question 20: Loosely packed, transcriptionally active chromatin is called:
- A Euchromatin
- B Heterochromatin
- C Facultative chromatin
- D Satellite chromatin
Explanation: Euchromatin is decondensed and gene-rich, permitting transcription factor access. In contrast, heterochromatin is condensed and largely silent.
Question 21: Tightly packed, transcriptionally inactive chromatin is called:
- A Heterochromatin
- B Euchromatin
- C Nucleolus
- D Solenoid
Explanation: Heterochromatin remains condensed throughout the cell cycle and includes regions such as centromeres, telomeres, and inactivated X chromosomes.
Question 22: Telomeres are found at:
- A The ends of linear chromosomes
- B The centromere region
- C Within the nucleolus
- D At origins of replication
Explanation: Telomeres are specialized DNA–protein complexes at the ends of linear eukaryotic chromosomes that prevent end-to-end fusions and degradation.
Question 23: The centromere is the chromosomal region where:
- A Sister chromatids attach to the spindle during cell division
- B Telomerase extends chromosome ends
- C Ribosomes are assembled
- D DNA replication initiates
Explanation: The centromere assembles the kinetochore, which attaches to spindle microtubules during mitosis and meiosis, ensuring accurate chromosome segregation.
Question 24: Satellite DNA is best characterized as:
- A Highly repetitive non-coding sequences often near centromeres
- B Protein-coding genes
- C Mitochondrial DNA only
- D Single-copy housekeeping genes
Explanation: Satellite DNA consists of highly repetitive, non-coding sequences, particularly concentrated near centromeres and in heterochromatic regions.