Different versions of the sequence of the same gene.
Once the DNA has been sequenced, annotation consists of defining the parts of the sequence which correspond to genes (instructions) in the sequence in silico, and of obtaining an idea about their function. Various resources are utilized, including computerized prediction programmes which are not infallible (this is called ab initio annotion when theses programmes are used alone), comparisons with other genomic sequences, with cDNA sequences or with proteins from the same organism or different organisms. (See the sections on Interpretation of sequences, Comparing genomes, Complementary DNA, Annotation of the human genome).
The variable unit of nucleotides. There are 4 types of bases in DNA: thymine (T), cytosine (C), adenine (A) and Guanine (G). In RNA, thymine is replaced by uracil (U). The order of the bases along the DNA molecule constitutes the sequence. The bases which are opposite each other on each strand of DNA are paired according to the rules, A—T and G---C. These rules make it possible to define the complementary sequence of a specific sequence.