Q.39 Which of the following is the most common form of a DNA?
1. Z-DNA
2. A-DNA
3. B-DNA
4. C-DNA
B-DNA is the most common form of DNA found in cells under physiological conditions. This right-handed double helix dominates in vivo, serving as the standard structure for genetic storage and processes like replication.
Why B-DNA?
B-DNA features ~10.5 base pairs per helical turn, a 2.0 nm diameter, and 3.4 Å rise per base pair, with antiparallel strands stabilized by hydrogen bonds (A-T, G-C) and base stacking. It prevails at neutral pH and moderate humidity, exposing bases in major/minor grooves for protein binding during transcription and replication. Watson and Crick’s model depicts this canonical form, essential for life’s genetic machinery.
Option Analysis
| Option | Key Traits and Context | Most Common? |
|---|---|---|
| Z-DNA | Left-handed helix; ~12 bp/turn; favored by high salt, GC-rich sequences; rare, linked to gene regulation. | No |
| A-DNA | Right-handed, squat/wide (~11 bp/turn, 2.3 nm diameter); forms in low humidity/RNA-DNA hybrids; dehydrated state. | No |
| B-DNA | Right-handed, elongated (~10.5 bp/turn, 2.0 nm diameter); physiological norm in aqueous cellular environments. | Yes |
| C-DNA | Right-handed, elongated (~9.3 bp/turn); low humidity artifact; least studied, not prevalent in nature. | No |
