What builds proteins in a cell?

Protein synthesis, the production of proteins, takes place at the ribosomes, which are situated in the cytoplasm of the cell. DNA is too large, and also too important, to leave the nucleus of the cell, so it replicates small sections of itself to produce mRNA (messenger Ribonucleic Acid) which can diffuse through the cytoplasm to reach the ribosomes. The sections of itself which it copies are individual genes, which code for proteins.

DNA is a double-helical structure, made up of nucleotides which form two strands. Joining the two strands together are the ends of the nucleotides, which each take the form of one of four nitrogenous bases - adenine (A), thymine (T), cytosine (C) and guanine (G). The bases only join together in certain combinations, known as complimentary base pairs: A bonds with T, and C bonds with G. This is the reason why DNA replication is possible: the two strands separate, then gain the appropriate complimentary bases to build-up two complete double-helical DNA molecules, which are identical (unless mistakes are made, which are called mutations).

The mRNA, the copy of a protein gene, therefore contains a 'base sequence', a very long string of bases of a specific order and length - this determines the size and shape of the protein. Each set of three consecutive bases, a triplet, codes for one amino acid, the building-blocks of proteins. Most amino acids can be produced by the body, but some have to be specifically obtained from the diet - these are called essential amino acids. First-class proteins provide all the essential amino acids, and are obtained from meats, whereas second-class proteins only provide certain combinations of them, and come from plants. This is why vegetarians and vegans have to carefully choose what they eat in a meal so that between the various second-class proteins, the combinations of essential amino acids compliment each other to provide the full set overall.

The ribosomes follow the 'genetic instructions' laid out by the triplet code, and assemble the coded-for amino acids to produce the relevant protein.