Stopping and starting
The body's tools for activating and deactivating genes come in a variety of forms and probably have not all been identified. To understand how they work – and the role they may play in cancer – it helps to know how a cell's DNA is used as a blueprint for producing proteins.
Within the cell nucleus, DNA is not a free-floating wanderer, but is double-wrapped around "pillars" of proteins known as histones. The maypole-like combination of DNA and histones forms a structure called chromatin. (This complicated packaging makes it possible to fit the many strings of DNA into the tiny confines of the nucleus. If all the DNA in a single cell were uncoiled, it would stretch almost 10 feet.)
When a gene is activated, the portion of its DNA that holds the blueprint for a specific protein is copied onto an RNA molecule, a process called transcription. RNA then carries the information to another portion of the cell, where the RNA provides the instructions for assembling a protein.
One of the ways the body quiets an active gene is by attaching tags known as methyl groups (compounds of carbon and hydrogen) to DNA in the chromatin, a process known as methylation. This causes the chromatin to fold more tightly, essentially sealing off the genes from contact with specific proteins and thereby shutting down the transcription process.
A related system of gene control involves the histones. When a methyl group, acetyl group (which contains carbon, hydrogen, and oxygen), or other compound fastens onto the histones, gene activity goes up or down depending on the nature of the compound and its location on the histones.
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