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DNA Structure Questions
A right-handed double helix with antiparallel strands
A parallel double helix
Explanation:
Watson and Crick (1953) proposed the classic model: a right-handed double helix with antiparallel strands and complementary base pairing.
Q.
How many base pairs are there per complete turn of the B-DNA helix?
10
8
12
20
Explanation:
In standard B-DNA, one full helical turn contains approximately 10 base pairs.
Q.
The vertical rise per base pair in B-DNA is approximately:
0.34 nm
2.0 nm
0.1 nm
3.4 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.
Q.
The diameter of the B-DNA double helix is approximately:
10 nm
0.2 nm
2 nm
20 nm
Explanation:
The B-DNA helix is roughly 2 nm (20 Å) wide — a constant width due to purine-pyrimidine pairing.
Q.
The two strands of a DNA double helix are described as:
Covalently linked end-to-end
Parallel
Antiparallel
Randomly oriented
Explanation:
One strand runs 5'→3' while the other runs 3'→5'. This antiparallel orientation is essential for complementary base pairing and replication.
Q.
The two strands of DNA are held together primarily by:
Ionic bonds between sugars
Peptide bonds between nucleotides
Hydrogen bonds between complementary bases
Phosphodiester bonds between bases
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.
Q.
In B-DNA, one complete turn of the helix is approximately how long?
0.34 nm
34 nm
3.4 nm
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.
Q.
Which groove of the B-DNA helix is wider and more accessible to proteins?
The major groove
Both grooves equally
The minor groove
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.
Q.
Z-DNA differs from B-DNA in that Z-DNA is:
A single-stranded structure
A right-handed helix with 12 bp/turn
A left-handed helix
A triple-stranded helix
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.
Q.
Heating a DNA solution to separate the two strands is known as:
Hybridization
Denaturation
Annealing
Renaturation
Explanation:
Denaturation (or melting) breaks the hydrogen bonds between strands, converting the double helix into two single strands.
Q.
The temperature at which 50% of a DNA sample is denatured is called:
Melting temperature (Tm)
Hybridization temperature
Extension temperature
Annealing 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.
Q.
A DNA molecule with higher GC content will have:
A lower melting temperature
A higher melting temperature
No change in melting temperature
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.
Q.
When double-stranded DNA denatures, its absorbance at 260 nm:
Becomes zero
Decreases (hypochromic effect)
Increases (hyperchromic effect)
Remains unchanged
Explanation:
Unstacking of bases upon strand separation increases UV absorbance — the hyperchromic effect, commonly used to monitor DNA melting.
Q.
When separated DNA strands re-anneal, their UV absorbance:
Increases (hyperchromic effect)
Remains constant
Decreases (hypochromic effect)
Becomes infinite
Explanation:
Renaturation restores base stacking, decreasing UV absorbance — the hypochromic effect.
Q.
Negatively supercoiled DNA is characterized by:
Over-winding that resists strand separation
Formation of triple helices
Under-winding that facilitates strand separation
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.
Q.
The enzyme that relieves supercoiling by transiently cutting the DNA is:
Primase
Ligase
Helicase
Topoisomerase
Explanation:
Topoisomerases cleave and reseal DNA strands to relieve torsional stress generated during replication, transcription, and other processes.
Q.
The fundamental repeating unit of chromatin is the:
Nucleosome
Centromere
Nucleoid
Solenoid
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.
Q.
The core of a nucleosome consists of a histone octamer containing:
Four copies of H1
Two copies each of H2A, H2B, H3, and H4
Two copies each of H1, H2A, H2B, and H3
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.
Q.
Which histone is NOT part of the nucleosome core particle?
H2A
H3
H4
H1
Explanation:
Histone H1 is a linker histone that binds the DNA entering and exiting the nucleosome core. It is not part of the octamer.
Q.
Loosely packed, transcriptionally active chromatin is called:
Euchromatin
Heterochromatin
Satellite chromatin
Facultative chromatin
Explanation:
Euchromatin is decondensed and gene-rich, permitting transcription factor access. In contrast, heterochromatin is condensed and largely silent.
Q.
Tightly packed, transcriptionally inactive chromatin is called:
Nucleolus
Solenoid
Heterochromatin
Euchromatin
Explanation:
Heterochromatin remains condensed throughout the cell cycle and includes regions such as centromeres, telomeres, and inactivated X chromosomes.
Q.
Telomeres are found at:
The ends of linear chromosomes
At origins of replication
The centromere region
Within the nucleolus
Explanation:
Telomeres are specialized DNA–protein complexes at the ends of linear eukaryotic chromosomes that prevent end-to-end fusions and degradation.
Q.
The centromere is the chromosomal region where:
DNA replication initiates
Ribosomes are assembled
Telomerase extends chromosome ends
Sister chromatids attach to the spindle during cell division
Explanation:
The centromere assembles the kinetochore, which attaches to spindle microtubules during mitosis and meiosis, ensuring accurate chromosome segregation.
Q.
Satellite DNA is best characterized as:
Protein-coding genes
Single-copy housekeeping genes
Mitochondrial DNA only
Highly repetitive non-coding sequences often near centromeres
Explanation:
Satellite DNA consists of highly repetitive, non-coding sequences, particularly concentrated near centromeres and in heterochromatic regions.