What is a Gene?


A gene has been defined in the early 20th century as the unit of heredity. The physical nature of a gene was not known then, but conceptually explained the 'laws' of inheritance of traits as first described by Gregor Mendel. This 'unit of heredity' is a powerful and suggestive idea and without doubt has been shaped by the concurrent discovery of the atom by physicists - that nature is organized into smallest units that form building blocks of matter. Organized in the right way, these building blocks are the foundation of every substance and life form known to man.

'The Human Genome 2012: 18,451 RNA genes, 11,224 pseudogenes, 20,687 protein coding genes, an average of 6.3 splice variants per gene, gene sequences cover 2.94% of the genome, exon sequences cover 1.22% of the genome' (ENCODE, Nature.com)

So what do we know about genes today? They are best identified as units of DNA responsible for the synthesis of proteins and RNA components of cells (see also 'Molecular Biology of the Gene'). In molecular biology, the gene has a clearly defined meaning of a DNA sequence at a physical location (locus) on a chromosome. There it forms a molecular entity that can be used as a tool, cutout, manipulated, recombined, reinserted and changed and the changes are reflected in the protein and RNA molecules that they code for. In short, molecular biology has proven that the gene is more than a metaphor. Gene- and biotechnology are based on those molecular pieces of DNA called a gene. Although a gene clearly codes for a cellular component (e.g. a protein), it is not an independent entity, but part of a well organized chromosomal structure, which itself is part of a well organized cell. The chromosomal structure and its importance (i.e. function) for the viability of cells is currently one of the hottest fields in biology and medicine. The genome projects with the goal of sequencing the entire genetic information of organisms (for an update on genome projects see NCBI's Genomic Biology) are revealing the organization of DNA information on chromosomes. These studies are the foundations of two new sciences -- genomics and proteomics - with the goal of looking not just at individual building blocks, or parts of cells, but how these parts interact and function together. Are all parts necessary? Can parts be changed to improve the workings of a cell? How can the interaction of parts explain the occurrence of diseases? And how can this insight result in the rational design of novel drugs, for instance by introducing a small molecule that interferes with the misguided interaction between two components?

Need a definition of what a gene is?

The gene has become the focus of everything biological, including evolution. Yet, the definition of a gene turns out to be unfinished business, as biologists keep discovering new functions associated to genes, how genes are organized on chromosomes, how they are expressed, how their expression is coordinated, and how a single gene can give rise to various gene products (splicing), particularly in higher eukaryotic organisms. Many of these functional aspects of genes are now highlighting the importance of cellular structures; how DNA is stored, accessed, copied and transcribed by proteins, and that both the DNA and these DNA binding proteins can be chemically modified by additional proteins. These modifications change how DNA and proteins interact with each other. The big consequence? The cell not only contains DNA with genetic information stored on it, but also cellular components that actively access and modify instructions of when, how and in what order genetic information is used by the cell.

These sets of activities that change the way DNA can be used in cells is called epigentics. Epigenetic programs are responsible for the incredible process of development in higher organisms, where a single fertilized zygote (a single cell) starts dividing and daughter cells with different properties and structures appear (e.g. neurons, red blood cells, skin cells, muscle cells). This occurs not by changing DNA content, but access to DNA content. The human body, to give an idea about the cellular diversity established by epigenetic programs, contains over 250 different cell types, all having the same genetic information, but using it in different ways. As a result, these cell types have different metabolism, shape, and physiology.

For a thought provoking account of this new science of epigenetics and non-DNA inheritance systems read the book 'Evolution in Four Dimensions' by Eva Jablonka and Marion Lamb.

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