The DNA in a nucleus can be used over and over again in the process of making proteins because it serves merely as a template from which hundreds of thousands of copies of messenger RNA (mRNA) can be made. mRNA is a complementary copy of a gene's nucleic acid sequence.

Making mRNA begins with the unraveling of small regions of the DNA by the enzyme helicase and the dissociation of the double strands (Figure 3-A). A group of proteins containing the enzyme RNA polymerase attaches to the unwound DNA at a specific sequence of bases called the promoter , and begins to make mRNA by matching each DNA base with its complementary RNA base (3-B). As this protein complex moves down the DNA template, polymerase adds RNA bases until it reaches a stop signal, detaches from the DNA, releases the mRNA (3-C), and the mRNA migrates out of the nucleus.

After transport out of the nucleus, mRNA undergoes a number of post-transcriptional modifications before it is translated into a protein. For example, the complete DNA sequence of a gene does not all code for protein. Stretches of non-coding DNA, called intervening sequences or introns, must be removed from the gene prior to translation. Scientists speculate that introns help to maintain DNA conformation, or serve as physical spacers to allow interaction with proteins that regulate gene expression. Other modifications to mRNA include adding a string of adenine bases to the end of the molecule. This poly-A tail plays a role in anchoring to the mRNA to the ribosome. When the mRNA is ready, it becomes incorporated into the ribosome and translation begins.

The ribosome is the protein factory of the cell. It is an amazing conglomeration of enzymes, supporting proteins, and RNA molecules that mediate the chemical reactions required to build a protein. Using the code within the mRNA, a special type of RNA called transfer RNA (tRNA) shuttles amino acids from the cytoplasm to the growing protein chain.

A mRNA is anchored to a ribosome at the mRNA's translation start signal, ATG, the codon for the amino acid methionine (Figure 4-A). The protein chain is extended as tRNA's bring in amino acids. Each amino acid is carried by its own tRNA molecule; one side of the tRNA, the anticodon , is configured to match the corresponding three-base codon of the mRNA (4-B). This process continues until a stop codon (UAG, UGA, or UAA) is reached and protein synthesis terminates.

Proteins can undergo a variety of post-translational changes, including chemical modifications such as the addition of phosphate or lipid groups, and clipping to achieve the final, active version of the protein. Such modifications affect enzymatic activity, binding to other proteins in the cell, so that the correct protein complexes form, and direct the newly made proteins to their proper location within the cell.