A codon consists of 3 consecutive bases in mRNA, with those 3 bases coding for a particular amino acid (or "stop"). Examples of codons would be AGU, AAG, CGU, GCA, etc.
The mRNA is transcribed from DNA, so each codon in mRNA is complementary to a triplet (not a codon, because it is not mRNA) of DNA bases. Complementary just means that the base pairing rules are follows: G bonds with C, and A bonds with U (in RNA) or T (in DNA).
The protein-coding region of a gene (the exons) is composed of triplets.
Okay, so here is the general process. for eukaryotes.
A gene is transcribed into a pre-mRNA. It is then edited, which includes removing the introns (non-coding regions) and splicing together the remaining exons (protein-coding regions). The DNA triplets that make up the protein-coding part of a single gene remain in the complementary mRNA molecule as codons, and the mRNA is exported out of the nucleus. In a complex process that I won't cover in much detail, the order of codons in the mRNA specifies the order that amino acids are added to a growing polypeptide, and thus, specify what protein is made.
What is the relationship between codon and gene.
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