Nucleotides and Nucleic Acids

Nucleotides

• A nucleotide consists of three parts: sugar, base, and phosphate backbone.
• Nucleosides are nucleotides without a phosphate group.

Nucleic Acids

• DNA and RNA are polymers of nucleotides.
• DNA and RNA have different functions:
  • Storage of genetic information (DNA)
  • Transmission of genetic information (mRNA)
  • Processing of genetic information (ribozymes)
  • Protein synthesis (tRNA and rRNA)
  • Regulation of expression (siRNA and miRNA)

Functions of Nucleotides and Nucleic Acids

• Nucleotides have various functions:
  • Energy for metabolism (ATP)
  • Enzyme cofactors (NAD+)
  • Signal transduction (cAMP)
• Nucleic acids have various functions:
  • Storage of genetic information (DNA)
  • Transmission of genetic information (mRNA)
  • Processing of genetic information (ribozymes)
  • Protein synthesis (tRNA and rRNA)
  • Regulation of expression (siRNA and miRNA)

Helical Structure of DNA

• DNA consists of two chains of nucleotides coiled around each other in a right-handed double helix.
• Sugar-phosphate backbones of the two strands spiral around the outside of the helix.
• Nitrogenous bases extend into the centre at right angles to the axes of the helix.

Nucleotide Linkage

• Nucleotide subunits are linked together by phosphodiester bonds.

Hydrogen Bonding Between Base Pairs

• Hydrogen bonding is a non-covalent attraction that aids in maintaining the double helix structure.
• Adenine forms 2 H bonds with thymine (A=T).
• Cytosine forms 3 H bonds with guanine (G=C).
• This H bonding pattern is called base pairing.

RNA Types Involved in Protein Synthesis

• Ribosomal RNA (rRNA): 80%, range: 120-5070 nucleotides.
• Transfer RNA (tRNA): 15%, 75 nucleotides.
• Messenger RNA (mRNA): varies, varies.

RNA Structure

• RNA is intrinsically single-stranded.
• RNA cannot form a B-form helix due to a bulky 2'-OH.
• When helical, RNA adopts an A-form geometry.
• RNA has a deep and narrow major groove and a wide minor groove.
• Secondary structure is observed in rRNA and tRNA, and assumed to be in mRNA.

Messenger RNA (mRNA)

• mRNA carries the instructions for building a protein.

Types of RNA Molecules

• Messenger RNAs (mRNAs): intermediates that carry genetic information from DNA to the ribosomes.
• Transfer RNAs (tRNAs): adaptors between amino acids and the codons in mRNA.
• Ribosomal RNAs (rRNAs): structural and catalytic components of ribosomes.
• Small interfering RNAs (siRNA): RNA interference (RNAi).
• Long non-coding RNAs (long ncRNAs): transcription.
• MicroRNAs (miRNAs): RNA interference (RNAi).
• Ribozymes: RNA enzymes.

Eukaryotic mRNA

• Eukaryotic mRNA is capped at the 5' end.
• The 3' end is polyadenylated.
• PolyA tail is not coding.
• It is added after transcription.
• It stabilizes mRNA.
• Removal of polyA degrades RNA and inhibits translation.

Ribosomes

• Bacterial ribosome: Mr 2.7 x 106, 50S and 30S subunits.
• Eukaryotic ribosome: Mr 4.2 x 106, 60S and 40S subunits.

Ribosomal RNA (rRNA)

• rRNA is complex and makes up the ribosomes.
• Ribosomes are the "protein factories" and are composed of many proteins and rRNA molecules.

Transfer RNA (tRNA)

• tRNA acts as a translator between the nucleic acid and protein worlds.
• tRNA structure includes:
  • Acceptor stem
  • Anticodon stem
  • 5' end
  • 3' end
  • Anticodon
  • Amino acid