7.8: Summary

Proteins are made within cells. The blueprints to make proteins are encoded in the structure of DNA, the genetic material that lies in the center— the nucleus—of cells. Human DNA contains about 20,000 blueprints to make proteins. Each blueprint—the directions to make a particular protein—is called a gene.

DNA is a long double-stranded chain of chemical units, each of which contains a substance called a base. There are four kinds of bases found in DNA, and it’s the sequence of these bases along the chain of DNA that encodes the directions to make protein.

The bases are “read” in groups of three. This provides 64 possible combinations of the four bases—more than enough to code for the 20 kinds of amino acids needed to make protein.

A particular sequence of three bases (triplet) is the genetic code for a particular amino acid. With this genetic code in hand, one can determine the sequence of amino acids in a protein by “reading”—three at a time—the sequence of bases in the protein’s blueprint.

One kind of base will join only with one other kind of base. In this way, a complementary copy of a precise sequence of these bases is made.

The first step in making a protein is to obtain such a copy of the particular sequence of bases in DNA that encodes the directions to make that protein. This copy is made in the nucleus of the cell and is known as messenger RNA.

The messenger RNA moves out of the nucleus into the cytoplasm, and attaches itself to a ribosome, the site of protein synthesis. As the triplets of bases are “read” on the messenger RNA, the corresponding amino acids are delivered via transfer RNA, and the amino acids are connected. When the entire messenger RNA is read, the newly-formed protein is released from the ribosome.

Our dietary protein requirement provides the amino acids necessary for protein synthesis. For adults, the dietary protein requirement is generally that amount needed to maintain the body’s proteins. Relative to body weight, children and pregnant women need more protein because they are adding—rather than just maintaining—body proteins. Nursing mothers also need additional protein in order to make milk protein.

Athletes do need a little more protein, but typically the protein in their diets alone far exceeds their need. When protein intake is excessive, the excess isn’t stored. Rather, the amino acids are converted to fat or carbohydrate-like compounds, and are then broken down to produce energy or are stored as body fat. Because protein-rich foods are generally expensive, such uses of protein are economically wasteful.

The RDA for protein, like the RDA for other nutrients, is generous. The average protein intake in the United States is well above the RDA.

Many of the vegetarians in the U.S. include dairy products and/or egg in their diet (lacto and/ or ovo vegetarians), so can easily get adequate amounts of vitamin B₁₂, calcium, and high-quality protein. Strict vegetarians (vegans) have to be more vigilant, especially in getting vitamin B₁₂, because B₁₂ isn’t found naturally in plant foods.

In many developing countries, severe protein deficiency is common. The protein deficiency is usually accompanied by deficiencies of other nutrients and inadequate calories. Inadequate calories makes the protein deficiency worse, because the body’s first priority is fulfilling energy needs—the body will break down amino acids to meet the need for energy rather than using the amino acids to make protein. This manifests itself in large numbers of children with the ills of protein-energy malnutrition.

Breastfeeding can be a matter of life or death for the infants—breast milk can be their only dependable source of high quality protein and other essential nutrients.

Moderation is a key to good nutrition. Protein intake is no exception. Animal sources of protein are generally high in protein, but have no fiber, and are generally high in fat. Since the typical American diet is high in protein and fat and low in fiber, our diet could be improved by eating smaller portions of meats and high-fat dairy products, and eating larger portions of whole grains, vegetables, and fruit.