A primer on molecular biology for imagers: II. Transcription and gene expression

The process of gene expression is complex and highly regulated to ensure that the right gene is expressed at the right place, at the right time, and in regulated amounts. The cell has multiple levels at which it controls the expression of a transcript including gene expression, alternate splicing, and stability of the transcript. Alternate splicing to generate different RNA species from a given gene and DNA rearrangements where genes are rearranged during cellular differentiation (eg, immunoglobulin genes) are additional mechanisms used to generate diversity in complex organisms. Epigenetic mechanisms such as methylation where CpG-rich islands in the promoter region depending on their methylation status can also modulate gene expression. The reader is requested to refer to the books, review articles, and web sites for additional information (2-13).