Nucleic Acid Structure and Function

Nucleic Acid Structure and Function - Gene Scene Investigators

• Nucleic Acids: DNA & RNA; polymers of nucleotides.
• Building Blocks:
  • Nucleotide: Nitrogenous Base + Pentose Sugar (Deoxyribose in DNA, Ribose in RNA) + Phosphate group(s).
  • Nucleoside: Nitrogenous Base + Pentose Sugar.
• Nitrogenous Bases:
  • Purines (double ring): Adenine (A), Guanine (G). 📌 Pure As Gold.
  • Pyrimidines (single ring): Cytosine (C), Thymine (T, in DNA), Uracil (U, in RNA). 📌 CUT the Pye.
• DNA Structure: Double helix; two antiparallel strands. Complementary base pairing: A=T (2 H-bonds), G≡C (3 H-bonds).
• RNA Structure: Mostly single-stranded. Key types: mRNA, tRNA, rRNA.
• Functions: DNA stores genetic information; RNA is crucial for protein synthesis and gene regulation.

⭐ Chargaff's rules for dsDNA: Amount of Adenine (A) = Thymine (T), and Guanine (G) = Cytosine (C). Thus, total Purines (A+G) = total Pyrimidines (C+T).

Nucleic Acid Structure and Function - The Helix Blueprint

• DNA Double Helix: Two polynucleotide chains coiled.
  • Antiparallel strands (5'→3' & 3'→5').
  • B-form: common, right-handed.
• Base Pairing (Chargaff's Rules):
  • Adenine (A) pairs with Thymine (T) via 2 H-bonds ($A=T$).
  • Guanine (G) pairs with Cytosine (C) via 3 H-bonds ($G \equiv C$).
  • Purines (A,G) pair with Pyrimidines (T,C).
• Forms of DNA:
  • B-DNA: Common, right-handed, 10.5 bp/turn.
  • A-DNA: Right-handed, wider, shorter (found in dehydrated conditions).
  • Z-DNA: Left-handed, zig-zag backbone, G-C rich regions.
• Grooves: Major & Minor grooves; sites for protein binding.
• Packaging: DNA + histones → nucleosomes → chromatin.

⭐ Z-DNA is a left-handed helix, often formed by alternating purine-pyrimidine sequences (e.g., GCGCGC), and may play a role in gene regulation.

Nucleic Acid Structure and Function - The Versatile Transcript

• RNA vs. DNA:
  • Ribose sugar (2'-OH).
  • Uracil (U) for Thymine (T).
  • Usually single-stranded; forms secondary structures (hairpins).
• Types of RNA & Key Functions:
  • mRNA (Messenger RNA): Template for protein synthesis.
    • Eukaryotic: Monocistronic; 5' cap (7-methylguanosine), 3' poly-A tail (stability, translation).
    • Prokaryotic: Polycistronic; Shine-Dalgarno sequence (ribosome binding).
  • tRNA (Transfer RNA): Carries specific amino acids to ribosome.
    • Cloverleaf structure; anticodon loop (reads mRNA codon); 3'-CCA tail (amino acid attachment).
  • rRNA (Ribosomal RNA): Ribosome's structural & catalytic (ribozyme) core. Most abundant RNA.

    ⭐ Peptidyl transferase activity of 23S rRNA (prokaryotes) / 28S rRNA (eukaryotes) forms peptide bonds.

  • snRNA (Small Nuclear RNA): Pre-mRNA splicing in eukaryotes (component of spliceosome).
  • miRNA/siRNA (Micro/Small Interfering RNA): Regulate gene expression via RNA interference (mRNA degradation or translation inhibition).

Nucleic Acid Structure and Function - Info Flow Highway

• Nucleic Acids: Polymers of nucleotides (pentose sugar + nitrogenous base + phosphate).
  • DNA (Deoxyribonucleic Acid): Genetic blueprint. Double helix (Watson-Crick). Deoxyribose sugar. Bases: Adenine (A), Guanine (G), Cytosine (C), Thymine (T). A pairs with T (2 H-bonds); G pairs with C (3 H-bonds).
  • RNA (Ribonucleic Acid): Involved in protein synthesis, gene regulation. Mostly single-stranded. Ribose sugar. Bases: A, G, C, Uracil (U) instead of T. Types: mRNA (messenger), tRNA (transfer), rRNA (ribosomal).
• Central Dogma: Describes the flow of genetic information.

• Key Processes:
  • Replication: DNA duplication. Semiconservative. Key enzymes: DNA polymerase, helicase, ligase.
  • Transcription: Synthesis of RNA from a DNA template. Key enzyme: RNA polymerase.
  • Translation: Synthesis of protein from mRNA template on ribosomes. Involves tRNA (anticodon) recognizing mRNA codons. ⭐ > The genetic code is triplet, non-overlapping, and degenerate. Start codon: AUG (Methionine). Stop codons: UAA, UAG, UGA (📌 Mnemonic: U Are Away, U Go Away, U Are Gone).

High‑Yield Points - ⚡ Biggest Takeaways

• DNA: double helix, deoxyribose, A,T,G,C; RNA: single-stranded, ribose, Uracil.
• Purines (A, G): two rings; Pyrimidines (C, T/U): one ring.
• A-T pairing (2 H-bonds); G-C pairing (3 H-bonds, stronger).
• Central Dogma: DNA → RNA → Protein (Replication, Transcription, Translation).
• mRNA carries codons; tRNA (anticodon) brings amino acids; rRNA forms ribosomes.
• Genetic code: triplet, degenerate, non-overlapping, nearly universal.
• Okazaki fragments form on lagging strand during DNA replication.