​Which one of the following nucleic acids, with the same concentrations in water, will form a stable stem-loop structure upon annealing by heating and flash cooling on ice?

Correct option is B

​A stem-loop structure (also called a hairpin loop) is a secondary structure that forms when a single-stranded nucleic acid folds back on itself, creating a complementary double-stranded stem region and an unpaired loop region.

Key factors influencing stem-loop stability include:

1. Complementary Base Pairing: Strong base-pairing between regions forms a stable stem.
2. GC Content: G≡C pairs form three hydrogen bonds, making them more stable than A=U pairs in RNA.
3. Loop Length: Too short or too long loops reduce stability.

• (a) 5’ – GGCUUAUUUUCUUCGG – 3’
• The potential stem: GGCU.....UCGG
• The loop is too long due to excess U’s, reducing stability.
• (b) 5’ – CCGAACUUUUAUUCGG – 3’ (Correct Answer)
• The potential stem: CCGA.....UCGG
• A strong GC-rich stem with a balanced loop makes this structure stable.
• (c) 5’ – AUGCCAUUUUCGGCUU – 3’
• The potential stem: AUGCC.....CGCUU
• The stem is less GC-rich, making it weaker compared to (b).
• (d) 5’ – AGAGCGUUUUAUUCGG – 3’
• The potential stem: AGAGC.....CGG
• The loop is large, leading to an unstable structure.

Thus, option (b) forms the most stable stem-loop structure due to a well-balanced GC-rich stem and an optimal loop size.

Information Booster:

• Stem-loop structures are critical in RNA folding, affecting stability and function.
• RNA hairpins are found in tRNA, ribosomal RNA, and regulatory non-coding RNAs.
• GC-rich stems contribute to high thermal stability due to three hydrogen bonds per pair.
• Optimal loop size (4–10 nucleotides) enhances stability; very short or long loops destabilize the structure.
• Flash cooling after heating helps stabilize the correct conformation by rapidly locking in base-pairing interactions.

Additional Knowledge:

(a) 5’ – GGCUUAUUUUCUUCGG – 3’

• The presence of too many uracils (U’s) in the loop increases flexibility but reduces stem stability.
• GGCU at the start can form a partial GC-paired stem, but the rest is weak.

(b) 5’ – CCGAACUUUUAUUCGG – 3’(Most Stable)

• The stem (CCGA...UCGG) contains strong GC pairing, ensuring high stability.
• The loop contains UUUUUAUU, which is an ideal size for stable formation.

(c) 5’ – AUGCCAUUUUCGGCUU – 3’

• The stem contains AUGCC...GGCUU, but weaker AU pairs reduce stability.
• Loop formation is possible but less stable than (b).

(d) 5’ – AGAGCGUUUUAUUCGG – 3’

• The AGAGC...CGG stem is possible, but the loop is longer, making it unstable.
• The presence of multiple purines (G, A) in the loop affects folding efficiency.