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How does the cell know what parts of the information to access in order to produce what it needs?

November 2012

Written by Alysia L. vandenBerg, PhD

Imagine that you are on a journey, visiting the nucleus of a cell of the human body. We can think of the nucleus as the "inner city". In terms of activity, the inner city resembles an ant colony on caffeine—it's incredibly busy. Access to the inner city is guarded; only certain materials are let through to the information and production headquarters housed there. About 40 miles of information, coded onto fine thread is crammed inside the inner city, but it's only about the size of a tennis ball. The knowledge of how to produce about 23,000 products (proteins) is encompassed within this coded information (DNA), but the city only really produces around 10-20% of that for day-to-day business. How does the inner city know what parts of the information to access in order to produce what it needs? And how is this knowledge then remembered or maintained, from day to day, year to year, in order to avoid chaos? Researchers studying epigenetics are interested in exactly this type of question: how does a cell know which parts of the genetic information that's encoded in a cell's DNA to access in order to produce what it needs, and once these decisions are made how can they be perpetuated, or else changed, if necessary?

The roots of the term epigenetics go all the way back to Aristotle, who coined the term epigenesis. Epigenesis was used in opposition to the theory of preformation, which posited that all living organisms existed in miniature inside of the sperm (or the egg), and merely expanded in size over time [link]. In contrast, the theory of epigenesis proposed that embryos began as an undefined mass with new parts added during development (1). Conrad Waddington, described as a "renaissance biologist" (2), was a 20th century scientist-philosopher who built on Aristotle's early notions about animal development. He is credited with using the term epigenetics in the context of development, describing it as, "the branch of biology which studies the causal interactions between genes and their products, which bring the phenotype into being" (2). This was a rather prescient observation, considering that the structure of DNA was not even known at that time.

References:

1. J. M. Slack, Conrad Hal Waddington: the last Renaissance biologist? Nat Rev Genet3, 889 (Nov, 2002)

2. C. H. Waddington, Canalization of development and genetic assimilation of acquired characters. Nature 183, 1654 (Jun 13, 1959)

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