Conformation and stability of Nucleic Acids

🧬 Molecular Biology: Premium Study Notes

IN-DEPTH THEORY | CSIR-NET | GATE | DBT-JRF

1. Chargaff’s Rule & Base Composition

Erwin Chargaff established the chemical rules that govern the base composition of DNA, which directly led to the discovery of the double helix. Theory concept: The geometry of the DNA backbone requires a bulky Purine (two rings) to always pair with a smaller Pyrimidine (one ring) to maintain a constant 20 Å diameter.

• First Parity Rule: In double-stranded DNA, the sum of purines equals the sum of pyrimidines. [A + G = T + C].
• Molar Equivalence: The molar amount of Adenine equals Thymine (A=T), and Guanine equals Cytosine (G=C).
• Species Specificity: The ratio of (A+T)/(G+C) is constant for a given species but varies wildly between different organisms.

🔥 CSIR-NET TRICK: Chargaff's rule applies ONLY to dsDNA. If an exam question states A=25% and T=35%, the genetic material is definitely single-stranded (e.g., ssDNA in φX174 bacteriophage).

2. DNA Structural Polymorphism (A, B, Z Forms)

DNA is not a static molecule. Depending on relative humidity, salt concentration, and nucleotide sequence, it adopts different helical conformations.

• B-DNA: Right-handed, ~10.5 bp/turn. It has a wide and deep major groove perfectly sized for alpha-helices of transcription factors to bind.
• A-DNA: Right-handed, ~11 bp/turn. Shorter and wider. Theory point: DNA-RNA hybrids and dsRNA are forced into the A-form because the 2'-OH group on the RNA ribose sterically prevents the B-form structure.
• Z-DNA: Left-handed, ~12 bp/turn. Favored by alternating GC sequences. Biological role includes relieving torsional strain (negative supercoiling) behind active RNA polymerase.

3. Stability of DNA

The double helix is surprisingly robust. Theory concept: Hydrogen bonds only provide specificity (A pairs with T). The actual stability comes from Base Stacking Interactions (hydrophobic forces and van der Waals interactions between the flat aromatic rings).

• GC Content: Higher GC content increases Melting Temp (T_m) because G-C pairs stack more tightly than A-T pairs.
• Hyperchromic Effect: When DNA denatures (melts into ssDNA), the unstacked bases absorb ~37-40% MORE UV light at 260 nm.

4. Renaturation (C₀t Curve)

Renaturation kinetics depend on sequence complexity. The C₀t_1/2 value defines how long it takes for half the DNA to snap back together.

• Low C₀t (Fast): Highly repetitive DNA (centromeres, telomeres). They easily find their matching partner.
• High C₀t (Slow): Unique, single-copy structural genes. They take a long time to bump into their exact complement in a solution.

5. Functional RNAs: tRNA & miRNA

🧬 tRNA (Transfer RNA)

tRNA acts as the physical link between mRNA and the amino acid sequence of proteins.

• Acceptor Stem: The 3' end strictly terminates with 5'-CCA-3'. Amino acids are covalently attached to the 3'-OH of the Adenine.
• D-Loop: Contains modified Dihydrouridine; acts as a recognition site for aminoacyl-tRNA synthetase.
• Wobble Hypothesis: The 1st base of the anticodon (5' end) is flexible. If it's Inosine (I), it can pair with U, C, or A on the mRNA.

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🧬 miRNA (Micro RNA) & Gene Silencing

Endogenous ~21-23 nucleotide non-coding RNAs that regulate gene expression post-transcriptionally.

• Biogenesis Pathway: Transcribed as pri-miRNA → Cleaved by Drosha (in Nucleus) into pre-miRNA → Exported via Exportin-5 → Cleaved by Dicer (in Cytoplasm) into mature miRNA.
• Action (RISC Complex): Loaded into Argonaute (AGO) proteins.
  - If pairing is imperfect (common in animals) → Translational Repression.
  - If pairing is perfect (common in plants) → mRNA Cleavage / Degradation.