3Expression of Genes
A protein gene is a piece of DNA that determines the amino acid sequence of a protein, and the synthesis of a protein based on genetic information is called gene expression. Specifically, genetic information refers to the nucleotide (base) sequence of DNA strands; mRNA is synthesized using DNA as a template, by which the genetic information of the DNA is transcribed as the sequence of the mRNA. The base sequence of mRNA is defined as a series of genetic codes, and such codes in mRNA are used to synthesize proteins on cytoplasmic granules called ribosomes. One piece of mRNA code corresponds to one amino acid, and these amino acids are linked together following the order of the codes, thus synthesizing proteins. Protein synthesis is called translation, since the mechanism can be compared to a translation from information in one language (i.e., a base sequence) to that in another language (i.e., an amino acid sequence).