ABSTRACT
Genetic information generally flows in a one-way direction: DNA is decoded to make RNA, and then coding RNA is used to make polypeptides that subsequently form proteins. Quite often a very simple chemical group is added to the side chain of the amino acid, but sometimes a large carbohydrate, lipid, or even another protein is joined to the side chain, as described. The canonical double helix is portrayed as a rigid, rather uniform structure, but it can undergo local changes in conformation and alternative types of base pairing. The variation between cells happens because of differences in gene expression, primarily at the level of transcription: different genes are transcribed in different cells according to the needs of the cells. Some genes, known as housekeeping genes, need to be expressed in essentially all cells, but other genes show tissue-specific gene expression or they may be expressed at specific times.
