There is no single gene for making vitamin A. There are genes that
encode enzymes that facilitate the chemical steps that build vitamin A.
Humans can either get vitamin A directly from food, or the body can make it from certain other substances found in food, called precursors. They are soluble in cooking oils and they tend to be yellow or orange. The best precursor is beta carotene, because our bodies can make two molecules of vitamin A (retinol) from each molecule of beta carotene. The transgenic rice contains beta carotene. The grains have a noticeably yellow color, which has given it the nickname golden rice.
Rice already has genes which make enzymes that control the chemical pathway for beta carotene synthesis. The plants make beta carotene in the leaves and in the stems, but they make none in the part of the plant which people eat, the endosperm, the white part of the grain. What the plants needed were some genes which could complete the beta carotene synthesis pathway in the endosperm.
Three genes were used, including two from a daffodil, a logical species to choose if you want to make a yellow pigment like beta carotene.
For an explanation of molecular diagrams, click here.
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