Proteins have many roles in the body. Some make up body structures, such as skin and hair; others are hormones or enzymes that control cell activities. Proteins are made from amino acids, of which there are 20 types, according to instructions encoded on the DNA and relayed by messenger RNA (mRNA). An mRNA strand is a copy of a gene, which is a particular section of DNA. mRNA has four bases; three are the same as those of DNA, one is unique to mRNA.
Strands of DNA separate along a stretch of the molecule. Free bases attach to corresponding bases on one DNA strand to make mRNA. The newly formed mRNA carries the instructions for making a protein and moves into the cytoplasm.
A structure called a ribosome moves along the strand of mRNA three bases at a time. The ribosome brings specific amino acids into place according to the sequence of bases in the mRNA triplets.
When the ribosome reaches the end of the mRNA strand, it detaches itself from the assembled chain of amino acids. The chain then folds up to form the newly completed protein.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.