By Timothy Boyer, Molecular Biologist – Updated August 30 , 2022
DNA is a double‑helical polymer composed of two antiparallel polynucleotide strands. Each strand carries a sequence of nucleotides, each with a nitrogenous base: adenine (A), cytosine (C), guanine (G), or thymine (T). The complementary bases pair through hydrogen bonds (A–T, C–G), a feature that permits the strands to separate temporarily during transcription.
During transcription, a segment of one DNA strand is copied by RNA polymerase into messenger RNA (mRNA). In this copy, thymine (T) is replaced by uracil (U), yielding a single‑stranded RNA that preserves the original coding sequence. The mRNA is processed: introns are removed by splicing, and the remaining exons are ligated to form a mature transcript that can be translated.
RNA polymerase reads the template DNA strand and synthesizes a complementary mRNA strand in the 5’→3’ direction. The enzyme pauses at specific sites where splicing factors recognize splice donor and acceptor sites, excising non‑coding introns and joining exons into a continuous coding sequence. The result is a ready‑to‑translate mRNA molecule.
The ribosome attaches to the processed mRNA and reads its nucleotide sequence in sets of three bases—codons—each specifying a single amino acid. Transfer RNA (tRNA) molecules, each carrying a distinct amino acid, match their anticodon to the corresponding codon on the mRNA. This sequential addition of amino acids produces a polypeptide chain that folds into a functional protein.
A single nucleotide change in the DNA can alter a codon, leading to an incorrect amino acid in the polypeptide. Such missense mutations often disrupt the protein’s three‑dimensional structure and impair its biological activity, underscoring the precision required for cellular function.
