Epigenetics

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Environmental conditions can affect DNA indirectly by modifying epigenetic factors. Epigenetic factors are compounds that attach to, or "mark" DNA. These factors interact with genetic material, but do not change the underlying DNA sequence. Instead, they act as chemical tags, indicating what, where, and when genes should be "turned on" or expressed.

Some epigenetic factors come from natural sources (such as the food we eat), or are even encoded in the DNA, and are a normal part of gene regulation. That is, the epigenome helps control which genes are active in a particular cell, and therefore, which proteins are transcribed locally. For example, epigenetic factors tell brain cells to act like brain cells, and skin cells to behave like skin cells. In the absence of a normal epigenome, disease can occur. These factors are also increasingly implicated in social and behavioral traits.

The epigenome is important because it is a primary location of gene-environment interactions; it can be altered by exposure to certain environmental stimuli. Environmental effects may be direct or indirect. For example:

  • An environmental agent, such as a chemical toxin, may enter the cells of a tissue and interface directly with the genetic material.
  • Alternatively, an environmental condition, such as chronic stress, may stimulate the body to produce its own intrinsic epigenetic factors.

These environmental modifications leave the DNA sequence unchanged, but interfere with the expression of that sequence (the transcription and translation processes). Scientists are still investigating the way that the epigenome affects gene regulation. However, we know that epigenetic factors may act on DNA, RNA, and proteins at any stage of the gene-to-protein pathway:

Pre-transcription

Some epigenetic factors appear to act on the processes that pack and wind DNA into chromosomes. DNA that is packed tightly into a chromosome cannot be 'read' in the transcription process. Alternatively, DNA that is kept out of chromosome form is always available for transcription. Two main DNA packing mechanisms have been described:

DNA methylation: the addition of a biochemical structure called a methyl group to a piece of DNA, which holds the DNA inits condensed chromosome form, effectively inactivating the sequence at that location. Abnormal methylation has been implicated in depression, drug use, and memory.

Histone modification: In the formation of the chromosome, DNA winds around proteins called histones. Modifications of the protein affect the ability of DNA to come in and out of chromosome form.

Post-transcription

Other epigenetic factors interfere between transcription and translation. This could block, enhance, or change the message carried by RNA to the protein-making machinery of the cell. For example, RNA interference (RNAi) refers to the binding of specialized RNA molecules to messenger RNA, inhibiting or facilitating translation.

Post-translation

Some epigenetic factors may exert their effect on the protein after translation. For example, a protein's shape may be modified, or it may be disabled.

    The epigenome can be passed from cell to cell within an organism, and in some cases from parent to child. Furthermore, environmental exposures during fetal life can alter the epigenome. Epidemiologists have begun investigating how environmental conditions experienced by a parent can influence health and other traits in children and even grandchildren.

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