These changes in gene activity may stay for the remainder of the cell's life and may also last for many generations of cells, through cell divisions. However, there is no change in the underlying DNA sequence of the organism. Instead, non-hereditary factors cause the organism's genes to behave (express themselves) differently.
The best example of epigenetic changes in eukaryotes is the process of cell differentiation. During morphogenesis, generalised stem cells become the cell lines of the embryo which in turn become fully differentiated cells. In other words, a single fertilized egg cell – the zygote – divides and changes into all the many cell types: neurons, muscle cells, epithelium, blood vessels etc.
As the embryo develops, some genes get switched on, while others are switched off or moderated. This process is called gene regulation. There are many molecules inside the cell nucleus which do the job of adjusting the genes' output.
DNA and histones make up what is called chromatin. Epigenetic modifications to the chromatin are copied during cell division. This produces a line of cells, all of which are alike. This is called a tissue.
Meiosis cancels epigenetic changes, and resets the genome to its baseline state, so the process unfolds in each new generation. There are some exceptions to this rule, but none of these exceptions involve changes to DNA base pair sequences.
This process is different from mutations of the DNA. Genetic mutations change the primary DNA sequence, and mutations can happen in any cell. However, only mutations in cells involved in reproduction can affect the offspring.
References[change | change source]
- By "DNA sequence" we mean the sequence of nucleotide base pairs in an exon, which is the part of a gene which determines the sequence of amino acids in the coded protein.
- King R.C. Stansfield W.D. & Mulligan P.K. 2006. A dictionary of genetics, 7th ed. Oxford. p146
- Adrian Bird (2007). "Perceptions of epigenetics". Nature 447 (7143): 396–398. . .PMID 17522671
- Special report: 'What genes remember' by Philip Hunter | Prospect Magazine May 2008 issue 146
- Reik, Wolf (2007). "Stability and flexibility of epigenetic gene regulation in mammalian development". Nature 447: 425–432. . http://www.nature.com/nature/journal/v447/n7143/full/nature05918.html. Retrieved 2008-04-05.