Deoxyribonucleic acid, or DNA, is the blueprint for life in all known organisms. This molecule contains instructions that guide an organism’s development, functioning, growth, and reproduction. These instructions direct the construction of proteins, complex molecules performing many functions within cells. DNA information is read in units, allowing cells to translate this genetic code into proteins for biological processes.
Understanding Codons and Amino Acids
Genetic instructions for building proteins are carried by messenger RNA (mRNA) in units called codons. A codon is a sequence of three nucleotides, the building blocks of RNA. Each codon specifies a single amino acid or signals the termination of protein synthesis. Amino acids are organic molecules that are the building blocks of proteins. Proteins are primarily constructed from 20 common amino acids.
The Genetic Code’s Rules
The genetic code uses a “triplet code,” where three consecutive nucleotides form each codon. This three-base system provides 64 possible codon combinations, sufficient to specify the 20 common amino acids. This leads to degeneracy or redundancy in the genetic code. Degeneracy means multiple codons can code for the same amino acid, protecting against mutations. For example, the amino acid phenylalanine can be coded by both UUU and UUC codons.
Codons also act as signals for cellular machinery. The codon AUG serves as the “start” codon, signaling where protein synthesis begins. It also codes for the amino acid methionine. Conversely, three “stop” codons—UAA, UAG, and UGA—do not code for any amino acid but signal the end of protein synthesis. The genetic code is consistent across nearly all life forms, highlighting its shared evolutionary history.
The Role in Protein Building
The sequence of codons in mRNA dictates the order in which amino acids link to form proteins, a process known as translation. This process occurs in ribosomes, cellular structures that read mRNA codons. Transfer RNA (tRNA) molecules carry amino acids to the ribosome, matching their anticodon sequence to the mRNA codon. As the ribosome moves along the mRNA, amino acids are sequentially added, forming a long chain called a polypeptide.
The sequence of amino acids in this polypeptide chain determines how it folds into a three-dimensional structure. This folded shape dictates the protein’s biological function. Therefore, the genetic code, through codons and their corresponding amino acids, provides instructions for cells to produce proteins that perform most functions in living organisms.