Genetics and epigenetics an update

The magazine « Pour la Science » published a whole issue on areas involved in molecular biology. Named “L’hérédité sans gènes“ published last year. Many subjects were treated and a lot was written on current research on the fields of epigenetics, transcription and translation. I will point out the most interesting subjects discussed.

What is a gene?

It might surprise you but the concept of a gene is an abstract one and has subsequently evolved in parallel of our understanding of molecular biology. A gene was firstly considered as a continuous sequence of nucleotides within a DNA segment that could be translated to a protein until it was realized that proteins were not always expressed from a continuous sequence. From there on the definition was considered incomplete and evolved up to the notion that a gene is a vector of genetic information that that can be interpreted in different ways (one gene can code for more than one protein.)

How many proteins can gene code for?

The notion that a one gene codes for one protein is outdated since the late 70’s as when the first notions of alternative splicing was observed in Eukaryotic cells. Splicing occurs when the “raw” RNA transcribed from the DNA is “refined” into the version used for protein translation. Alternative splicing is the observation different proteins can be made depending on what “useful code” (exon) is remained after the initial “cleanup” phase (removal of introns). The processes of alternative splicing involve 70-95% Eukaryotic cells genes and is an explanation of the great phenotypic plasticity observed in proteins but that the mechanisms involved in that splicing have only been partially deciphered.Equally what was interesting to learn species have different splicing variations for the same proteins that of the same organ.

Project ENCODE vs. Junk DNA 

For those that do not know, Project ENCODE is a consortium of international research groups launched as a follow-up of the “Human Genome Project” with the aim of identifying all functional element of the human genome. As such an important aim was to debunk what Junk DNA was all about. What is admitted is that the project does not invalidate the idea of Junk DNA and that most of DNA is in fact  composed mostly of Transposons and but equally of a bit of pseudogenes (“fossil DNAs that have been deactivated ”). The author of the paper nevertheless argues that the project doesn’t take the latest advances in evolutionary biology as with what has been learned in genetic drift and the different “layers of selection”. Not all biological properties emerge or are removed from evolutionary forces and are more likely to be the fruit of randomness that anything else. And level of selection is crucial into defining if a gene code can be considered active or not.

Imprinted parental genes

Since the 80’s the notion of imprinted parental genes was revealed to the field of genomics. It is the identification that one parental gene out of two (for haploid organisms) compete by other mechanisms that the traditional “recessive”/”dominant” view. These same genes have been since identified as being sensitive to epigenetics forces (affecting primarily CpG islands) and possess numerous similar characteristics. It has been hypothesized that this could follow up the idea that there is a “war between” sex from a genetic point of view and I suspected is the main mechanism of this new technology of “Gene Drive” pointed out in a previous post.

Metabolism and Epigenetics

It has been observed that the same substrates used for epigenetics reactions are the same used for metabolism. Since of this observation this would explain why an interruption or metabolic alteration affects the cellular differentiation. This was clearly illustrated by Prof. Waddington with this metaphor of mountainous landscape would point out that the cell goes through different physiological states before reaching its stable form (please research online for a more detailed overview) and would explain why multicellular organisms could emerge in the first place.

Works Cited

L’HÉRÉDITÉ des gènes. Dossier Pour la Science, p. 120, Décembre 2013

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