Question

Define epigenetics.

and a exsample

Answer 1

Epigenetics is the modification of DNA that does not change the sequence of bases in DNA but can affect gene activity. It can be understood as:

If two organisms have the same genotype (same sequence in all of their DNA) their phenotypic traits should be similar but if the two organism are present in different environments, have different food habits, etc. They differ in their phenotypes. This is explained on the bases of epigenetics. Actually it is not only the sequence of the DNA which determines the phenotype of organism but depends upon the expression and repression of this genetic information present in an organism. Which genes are expressed and which genes are repressed. 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.

The two main examples of epigenetics are as:

DNA Methylation: This is the linking of methyl (CH3​ -) group to mostly cytosine residues in the sequence CpG. This Methylation causes repression of the gene, as it prevents the transcription machinery to bind to DNA. Thus if two sequences are similar, one is methylated and another is not, the methylated gene will not be transcribed and non-methylated gene will be transcribed and protein will be synthesized and the two have different phentype but similar sequence of DNA.

Histone Modification: When histones are modified they can influence how chromatin is arranged which, in turn, can determine whether the associated chromosomal DNA will be transcribed or not. If chromatin is not in a compact form (euchromatin), it is active and the associated DNA can be transcribed. Conversely, if chromatin is condensed (heterochromatin), then it is inactive and DNA transcription does not occur.Thus, both have same sequence by give rise to two different phenotypes.

Epigenetics is the reason why a skin cell looks different from a brain cell or a muscle cell though all three cells contain the same DNA, but their genes are expressed differently, which creates the different cell types.