Common Misunderstandings of Genetics - Genes As Words

Genes As Words

It is popularly supposed that a gene is "a linear sequence of nucleotides along a segment of DNA that provides the coded instructions for synthesis of RNA" and even some current medical dictionaries define a gene as "a hereditary unit that occupies a specific location on a chromosome, determines a particular characteristic in an organism by directing the formation of a specific protein, and is capable of replicating itself at each cell division."

In fact, as the diagram illustrates schematically, genes are much more complicated and elusive concepts. A reasonable modern definition of a gene is "a locatable region of genomic sequence, corresponding to a unit of inheritance, which is associated with regulatory regions, transcribed regions and/or other functional sequence regions." One of the major complicating factors is that the exons which code for the proteins are often separated by many introns, which used to be called "junk DNA" but appear to have various as-yet-ill-understood purposes. The exons can be combined in different orders (splice variants) to produce different proteins. For example the gene called Dscam in Drosophila has 110 introns and therefore tens of thousands of possible splice variants.

This kind of misperception is perpetuated when mainstream media report that an organism's genome has been "deciphered" when they mean that it has simply been sequenced.

A related misconception is that the sole function of genes is to code for proteins, with the non-coding remainder being "junk DNA". However, it now appears that, although protein-coding DNA makes up barely 2% of the human genome, about 80% of the bases in the genome may be being expressed, so the term "junk DNA" may be a misnomer.