How does DNA determines the types of proteins a cell can make?
DNA contains discrete sequences, called genes, each of which holds the code for one or more proteins. But the DNA exists in a unique form that ensures that the codes are used only at the appropriate times. Two sugar-phosphate backbones, connected by “rungs” of nucleotide bases, are wound around each other to form a double helix. They are bound together by chemical bonds between the strands: adenines pair up with thymines, and cytosines with guanines. When a cell needs to make a particular protein, an activation signal stimulates an enzyme (DNA helicase) to unwind the DNA in the region of the appropriate gene. As the two strands of DNA separate, the so-called “coding strand” becomes a template for building a molecule called messenger RNA (mRNA). RNA is similar to DNA, except that it is single-stranded and contains uracil (U) in place of DNA’s thymine. An enzyme called RNA polymerase “transcribes” DNA into mRNA by piecing together complementary mRNA bases along the DNA template. Once the
