Crash Course in Genetics: What is a gene?

It at first seems to be a simple question … what is a gene?

The NIH (US National Institute of Health) defines it as follows:

“A gene is the basic physical and functional unit of heredity”

It goes on to say that a gene is made up of DNA sections that are between several hundred and several million base pairs long. The Human Genome Project (HPG) that was completed in 2003 identified just over 20,500 different genes in humans. These genes encode all the information that makes you you by holding the instructions for different proteins and other regulatory functions.

But to understand how they do this, and why the current definition of a gene is being actively contested, we need to look at the foundation and histories of what a gene is.

 

What is DNA?

genecelldna
Genes and DNA inside your body, fromĀ ASHG

As mentioned above, a gene is a section of DNA that encodes the instructions for a protein or regulatory function. DNA (deoxyribonucleic acid) is found in cell nuclei and is often compared to a binary computer code because it encodes instructions in a series of 4 bases (denoted by the letters A, T, G and C) similar to the series of 1s and 0s in computer code. For protein-coding genes, a set of 3 base pairs can go through a copying process and order a single amino acid into a new protein (this article has far more detail on this process if you’re interested).

A gene has between several hundred and 2 million separate base pairs that work together for their specific function (making their protein or regulating a function). Normally a single gene doesn’t change major features like your height, eye colour, hair type, etc… but these are instead a combination of multiple genes working together. A notable exception is in the case of single-gene genetic disorders, where a mutation prevents function of that specific protein, causing incredible impact on the individual.

 

History of genes

The history of genetics is normally traced back to Gregor Mendel, fondly remembered as the “Father of Genetics”, for working out that some ‘factors’ communicated hereditary information. It wasn’t until 1883 that Wilhem Roux proposed that chromosomes (coiled DNA) might carry genetic information. The name gene was coined in 1909 by Wilhelm Johannsen. In 1910 and 1911, it was discovered that genes reside on specific chromosomes and work was begun on mapping fruit fly genes. 1953 marked the discovery of DNA’s structure and the later discovery of how information was encoded. By 1972, scientists started to sequence genes, determining the order of base pairs on specific proteins.

At this point, the commonly held belief was that there were a defined number of genes in each species and that genes themselves were set sections of DNA with defined beginnings and ends.

This view changed, perhaps irreparably between the years of 1990 and 2003 while the Human Genome Project was being completed as scientists’ understanding of genes developed. In fact it stills leaves us in a dilemma today about what a ‘gene’ really is, so stick around to next week to read more.


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