Nucleic Acid Structures Overview

Nucleic Acid Structures

• This lecture covers the structures of DNA and RNA, outlining their differences and the diverse types of RNA and their functional relationships.

DNA Structure

• DNA is a double helix composed of two antiparallel polynucleotide strands.
• The strands are held together by hydrogen bonds.
• The bases are paired, with adenine pairing with thymine and guanine with cytosine.
• The sugar-phosphate backbone forms the outside of the double helix.
• The helix is 20 Å wide, with one nucleotide every 3.4 Å (0.34 nm). There are 10 nucleotides per turn of the helix.
• Key models show pyrimidine: pyrimidine pairings are < 20 Å. Purine: purine pairings give a helix > 20 Å. Pyrimidine: purine pairings form a double helix of about 20 Å.
• DNA is highly stable due to the stacking arrangement of bases and the presence of counter-ions to shield the negative charges.

RNA Structure and Types

• RNA, unlike DNA, is typically a single-stranded molecule.
• The extra hydroxyl group on the ribose sugar in RNA prevents stable double-helix formation.
• RNA can adopt secondary structures like hairpins, stem-loops and tertiary structures such as pseudoknot.
• Some RNA types have different conformations, like non-canonical base pairings and non-Watson-Crick bonding; these structures widen the major groove and enhance accessibility to ligands or proteins.
• There are at least seven types of RNA.
• Ribosomal RNA (rRNA) is a structural and enzymatic component of ribosomes (80% of total cellular RNA).
• Messenger RNA (mRNA) carries genetic information from DNA to the cytoplasm for protein synthesis.
• Transfer RNA (tRNA) acts as an adapter molecule, carrying amino acids to ribosomes during translation (5% of total RNA).
• Small nuclear RNA (snRNA), signal recognition particle RNA (SRP RNA),microRNA (miRNA), piwi-interacting RNA (piRNA), and short interfering RNA (siRNA) are involved in various cellular processes, including pre-mRNA processing, protein synthesis regulation, and gene silencing.

DNA Replication and Protein Synthesis

• The process of supercoiling is essential to allow DNA to be stored efficiently but also allows access to the genetic information necessary for transcription and replication.
• The negative supercoiling is more energetically favorable in most organisms, helping maintain the structure and accessibility for protein production.

Other Relevant Concepts

• X-ray diffraction was instrumental in determining the 3D structure of DNA, which was crucial to the development of models of DNA and the discovery of the double helix.
• Rosalind Franklin using X-ray diffraction significantly contribute to the discovery of DNA structure.
• Chargaff's Rules state that the amount of adenine equals the amount of thymine, and the amount of guanine equals the amount of cytosine in a genome.