well, read your books..don’t mean to offend or not wanting to help, but i think those are questions that you can find easily enough from the books, or simply search from the internet.

there are 4 bases that builds the nucleic acid, comprises purine (adenine and guanine) and pyrimidine (thymine (in DNA)/uracil (in RNA), and cytosine).

it’s the gene expression. every cell that builds the body is differentiated and specialized in the body development process. so this differentiation also includes the control of genes or the gene expression; which genes are to be expressed and which are not. some genes are activated and some genes are inactivated. from your example: the gene that codes for the production of insulin in a liver cell is "on", while in your ear cells, the gene is "off". 🙂