By Dr. David Warmflash
Updated Mar 24, 2022
JVisentin/iStock/GettyImages
Ribonucleic acid (RNA) is a key molecular component found in cells and viruses. In eukaryotic cells, RNA is classified into three main types: ribosomal (rRNA), messenger (mRNA), and transfer (tRNA). While all three participate in ribosomal function, this article focuses on mRNA and tRNA, the molecules that carry genetic information from DNA to the protein‑making machinery of the cell.
Both mRNA and tRNA are polymers composed of RNA nucleotides. Each nucleotide contains a ribose sugar, a phosphate group, and one of four nitrogenous bases. These nucleotides link via phosphate‑sugar bonds, forming a backbone that supports the bases.
RNA uses four bases: adenine (A) and guanine (G) are purines with two rings, while cytosine (C) and uracil (U) are pyrimidines with a single ring.
Transcription generates RNA by pairing nucleotides to a DNA template. In prokaryotes (bacteria and archaea), transcription occurs along a single chromosome. In eukaryotes, it takes place within the nucleus, where DNA is organized into chromosomes. The RNA sequence mirrors the DNA gene that was transcribed, encoding the information necessary for protein synthesis.
mRNA carries the coded message for assembling amino acids into a polypeptide chain. The genetic code links each set of three bases—a codon—to one of the 20 amino acids. Although multiple codons can encode the same amino acid (a property known as degeneracy), each codon specifies only one amino acid, ensuring precise protein construction.
tRNA acts as the translator between the mRNA codons and amino acids. Each tRNA molecule carries a specific amino acid and contains an anticodon that base‑pairs with its complementary mRNA codon, delivering the correct amino acid to the growing polypeptide chain.
Translation is the process by which ribosomes read mRNA codons and assemble the corresponding amino acids into a protein. As the mRNA strand threads through the ribosome, tRNAs match codons to amino acids, adding them sequentially to the nascent protein. Ribosomes are complex structures composed of rRNA and proteins, functioning as the cell’s protein factories.
