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
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?
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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Copyright © 2000-2012 Lukas K. Buehler