Is it all genetic? By Lilian Spork
Originally it was speculated that proteins determined the heritability of an organism, and that deoxyribonucleic acid (DNA) and its subunits were not varied enough to contain the genetic information essential for life. Since then however, scientists have established that it is in fact DNA that constitutes the genetic code. Nevertheless this development has not excluded proteins from geneticists’ minds. Instead proteins are believed to influence genetic expression. This specific area of research –known as Epigenetics- is not a recent field; rather it is making advancements worthy of more attention. This area of research studies phenotypic discordances caused by different molecules such as proteins
(Bio and gene technology glossary, 2011). It is these proteins
that can ultimately change an individual’s phenotype.
Essentially it is known that cells can express different traits in order to specialise. This is evident in cell differentiation, whereby genes are turned on and others are repressed to form specialised cells such as skin, liver or muscle. However these patterns of regulation also occur outside of cell specialisation. Within a chromatin there is both a double helix DNA strand as well as specialised proteins. The DNA is tightly wrapped around these proteins known as histones
(Epigenetics – beyond genes: Glossary, 2006). It is
the expression and activity of these histones that represses or allows for the
expression of specific genes. Methylation and histone deacetylation are two
processes that can influence these proteins. Throughout specialisation of cells
‘a methyl group is added to a histone’, this in turn silences genes (HD Glossary, 2008). Research
has shown that these histone placements can be passed on to future generations,
however these changes to the chromatin, unlike genetic mutations, can be
reversed and/or altered at any time (Mario F. Fraga, 2005).
This epigenetic development is exemplified by the article: “Epigenetic differences arise during the lifetime of monozygotic twins” published in 2005. The subjects of this study were volunteer monozygotic twins (MZ). Eighty pairs were studied to ensure reliable and corroborating results. It is a general understanding that these monozygotic twins are the result of a single fertilised egg that has cleaved to form two individual zygotes. The two individuals are genetically identical; however they develop ‘phenotypic discordances’, making them perfect candidates in the study of Epigenetics. The twins illustrate the effects of time and environment on independent phenotypes. By monitoring the differences between the pair scientists can identify possible causes of these discordances.
After comparing the histone differences in the twins it was found that epigenetic modifications in MZ twin pairs diverge as they become older. This aging process associated with different phenotypes is called ‘‘epigenetic drift’’. This ‘drift’ is significantly influenced by environmental factors such as “smoking habits, physical activity, or diet, among others”
(Mario F. Fraga, 2005). There was a
distinct relationship between the age and proximity of the twins and their
phenotypic discordances. The longer the pair had lived apart and the older they
were meant that they showed the ‘greatest differences in levels of acetylation
of histone H3 and H4’, ultimately having more phenotypic discordances. It is
conclusive that genetics are not the only determining factors in the phenotype
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Mario F. Fraga, E. B. (2005). Epigenetic differences arise during the lifetime. Proceedings of the National Academy of Sciences of the United States of America , 10604–10609.
Princeton University, T. T. (2012, January 6). WordNet Search: phenotype. Retrieved 03 11, 2012, from WordNet: http://wordnetweb.princeton.edu/perl/webwn?s=phenotype
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