2.26 Ribonucleic Acid (RNA)

Ribonucleic Acid (RNA) is a vital polymer essential for the expression of genetic information. Its structure and function are distinct from DNA→, primarily serving as an intermediary in the synthesis of proteins.

Chemical Composition

Like DNA, RNA is a polymer of nucleotides. Each nucleotide contains three components:

1. Ribose sugar — a pentose (5-carbon) sugar. Unlike the deoxyribose in DNA, ribose has a hydroxyl (–OH) group at the 2' carbon position. This makes RNA chemically less stable than DNA.
2. Phosphate group — links nucleotides together via phosphodiester bonds.
3. Nitrogenous base — one of four bases: Adenine (A), Guanine (G), Cytosine (C), or Uracil (U). RNA uses Uracil instead of the Thymine found in DNA.

Structure

RNA is generally a single-stranded molecule. It does not typically form a stable double helix like DNA.

Secondary Structure: Despite being single-stranded, some regions of an RNA molecule can fold back and pair with complementary regions on the same strand, forming localized double-stranded hairpin structures. This is important for the function of tRNA.

Synthesis

RNA molecules are produced through a process called transcription, in which the enzyme RNA polymerase uses a segment of DNA as a template to synthesise a complementary RNA strand. This is the first step in gene expression.

Function and Types

There are three major classes of RNA. Each class plays a specific and crucial role in the process of protein synthesis (translation), which occurs at ribosomes in the Cytoplasm and Organelles→.

1. Messenger RNA (mRNA)

• Carries the genetic code from DNA in the nucleus to the ribosomes in the cytoplasm.
• The sequence of codons (triplets of bases) on mRNA specifies the sequence of amino acids in a protein.
• Its length is proportional to the size of the protein it encodes.

2. Transfer RNA (tRNA)

• Transports specific amino acids to the ribosome during protein synthesis.
• Has a characteristic clover-leaf secondary structure formed by internal base pairing.
• Contains an anticodon loop that base-pairs with the complementary codon on mRNA, ensuring the correct amino acid is added.

3. Ribosomal RNA (rRNA)

• Together with ribosomal proteins, forms the structural and catalytic core of ribosomes.
• rRNA is the most abundant type of RNA in the cell (~80%).
• It catalyses the formation of peptide bonds between amino acids during translation.

Comparison: RNA vs DNA