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Inheritance: Not Just DNA

2015-04-18

Recent research has shown a second aspect to the cell’s system of genetic control.  Most of us have heard of DNA– and know that it contains “the code of life”, the instructions for all the proteins that the body makes, in every cell.

What most of us have not, until now, heard about, are histone proteins. These proteins carry the strands of DNA like spools holding coils of wire; the six or so feet of DNA contained in a person’s each and every cell (except for red blood cells) is completely wound around thousands of these histone proteins so that it fits inside a cell that is too small to see with the naked eye.  Variations in the histone proteins control the activity of the genes, so that some will not be transcribed to RNA to be manufactured into proteins, while others will actively translate into RNA and then protein.  There are many variations, and it seems that it some cases changes in a single amino acid will change the character of the entire histone protein, enabling highly discrete changes in translation activity and protein production.

The changes in histone proteins cause inheritable changes during an individual’s lifetime, unlike DNA, which is unable to respond in this way.  In a sense, it verifies a previously discredited theory of evolution that was known as LaMarckian evolution and was party doctrine in the Soviet Union for a long time (called Lysenkoism.)  The epigenetic changes behave exactly as a Lysenkoist  theoretician would predict.

So in fact, evolution proceeds by two pathways: the set of instructions, and the code for turning them on and off.  Changes in histone proteins cause changes in the expression of DNA.  Unlike DNA, histone proteins can change a cell’s behavior during the lifetime of an individual organism.  DNA is changed by mutations, which occur almost randomly in single cells.  The change is not passed on to an individual’s descendants unless the DNA in sperm and ova is changed.

Changes in histone proteins seem to occur in tandem in all cells of an individual’s body, in response to changes in the environment.  Histone changes cause changes in the amount of protein that the cells produce, not in the character of the protein that is produced.

Histone changes also seem to cause cells to differentiate into the types of cells that make up different organs.  All the cells in the body carry essentially the same DNA, but cells in different organs produce different types and quantities of proteins.  This organ specificity of protein production seems to be controlled by differences in histones.  Early in the growth of the embryo, cells differentiate into distinct types that eventually go on to produce different organs; this starts with the distinction of front from back and top from bottom, then proceeds to inside from outside, solid organ from muscle from nerve cell from skin cell from intestinal surface cell, and so on.  All these differences appear to be caused by changes in histones that are responsive to the location of the cell within the embryo.

Other histone changes relate to responses to the environment of the individual as a whole; some appear to respond to the level of stress that is placed on the organism, for example.  People and animals who are exposed to various types and degrees of stress during early development appear to respond with permanent changes in nerve responsiveness as well as changes in the levels of stress hormones in the blood.  These changes are inheritable, a contradiction to earlier, simpler “Darwinian” theories of evolution.

More advances in biology are on the horizon.  There are still many mysteries that we have not solved.  But we are making progress.  Now, if we can just avoid slipping backwards, we may eventually get somewhere.

Next: genetically modified organisms.

 

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