This is a preliminary and incomplete version of the glossary for a series of articles entitled "On Making the Genome Whole". Readers are invited to submit suggestions for improvement.
acetylation. Attachment of an acetyl group to a molecule, which is then said to be "acetylated". Certain enzymes (known as acetyltransferases) can do this. See also histone modifications. Deacetylation is the corresponding removal of an acetyl group, accomplished by histone deacetylases.
acetyl group. A small chemical group with the formula, -COCH3. See acetylation.
acetyltransferase. An enzyme that can attach a acetyl group to another molecule. In an epigenetic context, the term generally refers to the attachment of a acetyl group to a histone (by a histone acetyltransferase).
actin. A very common protein that forms filaments. It provides a kind of cellular "skeleton" in the cytoplasm of all cells, and also plays a major role in muscle contraction. It has been found to be present in the cell nucleus as well, and to be required for certain chromosomal movements.
activator. A protein transcription factor with a positive effect on gene expression. (Compare repressor.) An activator may work in conjunction with one or more co-activators. With or without the help of co-activators, the activator commonly plays a role in bringing RNA polymerase (the transcribing enzyme) to the gene promoter. The DNA sequence bound by the activator is called an enhancer, and it may lie in the promoter region immediately adjacent to the gene being activated, but it can also be far distant on the chromosome - or even on an altogether different chromosome. It must, however, be brought near to the promoter in order to recruit RNA polymerase. Note: the term "activate" can be used more generally to refer to any process that helps to bring a gene to expression.
adenine. See nucleotide base.