Epigenetics is the reason why different parts of a plant changes to stem, leaf, root or flowers cells, though they all have the same DNA, but their genes are expressed differently (turned "on" or "off"), and creates the different cell types. Epigenetics literally means "above" or "on top of" genetics. It refers to external modifications to DNA that turn genes "on" or "off." These modifications do not change the DNA sequence, but instead, they affect how cells "read" genes.In other words they are the non-genetic factors cause the organism's genes to behave (or "express themselves") differently.
Epigenetics most often involves changes that affect gene activity and expression, but the term can also be used to describe any heritable phenotypic change. Such effects may be the result of environmental factors and cause cellular and, physiological or phenotypic characters. These processes do not involve modification of genome, but formed due to selection pressures of the changes in environment. They act as a buffer and help acclamatisation resulting in long term genetic (adaptive) responses. Due to these changes the organism will be able to respond well to environment changes.
Causes of Epigenetic modifications
There are a few mechanisms such as DNA methylation, changes in chromatin structure and DNA damage which change the DNA behaviour which cause epigenetic modifications
1. DNA methylation.
Environmental factors can cause addition of methyl groups to nuclear bases in DNA. A methylation of cytosine will result in production of 5-methyl cytosine. If 5-methyl cytosine is present at CG or CpG sites(the regions where guanine follows in 5’-3’ direction) in the promoter or enhancer regions of genes, the gene actions are often suppressed.. These are because the methylated cytosine inhibits transcription factors from binding. Presence of 5-methyl cytosine at a promoter region can cause gene silencing. But if metylated cytosine is present in transcription site (but not at the site from which the transcription begins), expression of gene is often enhanced
2. Changes in chromatin
Acetylation of lysine,methylation of lysine and arginine methylation, phosphorylation of serine and threonine, and ubiquitination (addition of ubiquitin to a substrate protein) is and sumoylation of lysine cause changes in folding pattern of histone proteins and change the shape of histone in nucleosomes. This causes incomplete unwinding of DNA. At times these histones may act as templates, initiating the surrounding new histones to be shaped in the new manner. These processes change the transcribing pattern and produce epigenetic changes
3. DNA damage (endogenous damage by free radicals formed from metabolic byproducts or caused by radiation (UV, X-ray, gamma), hydrolysis, plant toxins, and viruses) can also cause epigenetic changes. DNA damage is very frequent, occurring on average about 60,000 times a day per cell of the human body
Certain foods like Soybean can reduce DNA damage. Soy isoflavones are found to decrease DNA damage due to oxidative stress.
Ecological speciationalso can be attributed to epigenetic changes. I have already noted (in My article on Red Algae) that small genomes of Red Algae were resulted due to drastic reduction in genome size, pruning introns, non-coding DNA, and superfluous genes from the genomecaused by the excruciating environment of acidic hot water.
Sources:
Articles on Epigenetics from Google searches
Mammen Daniel
