2.27 Roles and Types of RNA

The main biological concepts are the structure, function, and relative abundance of the three primary types of Ribonucleic Acid (RNA) involved in protein synthesis: Messenger RNA (mRNA), Ribosomal RNA (rRNA), and Transfer RNA (tRNA).

1. Messenger RNA (mRNA)

• Structure: A single, uncoiled strand of variable length. Its length is determined by the size of the gene it is transcribed from.
• Codons: The nucleotide sequence of mRNA is read in triplets called codons. Each codon specifies a particular amino acid.
  • Example: A protein with 100 amino acids would be coded by an mRNA molecule with at least 300 nucleotides (100 codons x 3 nucleotides/codon).
• Abundance: Constitutes about 3-4% of the total RNA in a cell.
• Function: To carry the genetic message (code for a protein) from the DNA in the nucleus to the ribosomes in the cytoplasm. This process of synthesizing protein from the mRNA template is called translation.

2. Ribosomal RNA (rRNA)

• Structure: The largest type of RNA molecule. It combines with proteins to form the subunits of a ribosome.
• Abundance: The most abundant type of RNA, making up approximately 80% of a cell's total RNA.
• Function: Acts as a structural component of ribosomes, providing the site or "platform" for protein synthesis to occur. The base sequence is highly conserved across different species, from bacteria to higher organisms.
• Polysome: In many cases, several ribosomes attach to a single mRNA strand to synthesize multiple copies of the same protein simultaneously. This cluster is called a polysome.

3. Transfer RNA (tRNA)

• Structure: The smallest of the RNA molecules, typically 75-90 nucleotides long. It is a single strand that folds into a characteristic cloverleaf shape due to complementary base pairing in certain regions.
  • 5' end: Always terminates with a Guanine (G) base.
  • 3' end: Always terminates with the base sequence CCA. This is the amino acid attachment site.
• Key Regions (Loops):
  • Anticodon Loop: Contains a three-base sequence called the anticodon, which is complementary to a specific mRNA codon.
  • D Loop: Recognizes the specific enzyme (aminoacyl-tRNA synthetase) that attaches the correct amino acid to the tRNA.
  • Theta (Θ) Loop: Recognizes the specific binding site on the ribosome.
• Specificity: There is at least one type of tRNA for each of the 20 amino acids. Human cells contain about 45 different kinds.
• Function: To transport a specific amino acid from the cytoplasm to the ribosome and match it with the correct codon on the mRNA template during protein synthesis.

Comparison of RNA Types