Epigenetics and Inheritance
Introduction
In biology, epigenetics is the study of mitotically and/or meiotically heritable changes in gene function that cannot be explained by changes to the DNA sequence. Epigenetics normally involves change that is not erased by cell division and that also affects the regulation of gene expression. Epigenetics reflects our understanding that, despite the complement of DNA being fundamentally identical in all somatic cells of an organism, patterns in gene expression can vary greatly among different cell types. Additionally, these patterns have the potential for clonal inheritance.
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Role of DNA Methylation
Holliday, Pugh, and Riggs proposed the idea that DNA methylation was palindromic, acted as an epigenetic mark, and that distinct enzymes were responsible for methylation of both modified and unmodified DNA already methylnated on one strand. This idea was in response to Ohno et al. 1959; Lyon 1961, where X chromosome inactivation in mice was studied. A silenced X chromosome was randomly selected and clonally inherited in somatic cells, even though no DNA sequence changes were present. The second methylation was less likely to occur after the first DNA methylation. However, the complementary strand would be modified at the same palindromic site shortly after. This was a key insight, as it suggested that a DNA methylation mark on one strand would be copied to the daughter strand after DNA replication, thus allowing for transmission of the methylated state from one generation to the next.
Role of Chromatin
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Appendix
Palindromic Sequence
In DNA or RNA, a palindromic sequence is where one strand is identical to a complimentary strand in the same direction. i.e. one segment of nucleotides that reads the same in both directions on complementary strands. For example, the sequence 5'-GAATTC-3' is palindromic as its complementary strand reads 3'-CTTAAG-5', which is also 5'-GAATTC-3' when read in the 5' to 3' direction.