Biology Reference
In-Depth Information
scientific ideas are provisional because they may be changed in the future as the
result of new discoveries.
Some of the experimental evidence in support of this new view is shown in the
lower left-hand side of Figure 5.5. Genetic engineering techniques allow scientists
to insert genes into the eggs of Drosophila so that they are expressed in tissues where
they are normally switched off. If the eyeless gene is expressed in the cells that give
rise to legs, eyes develop on these legs. Structures that appear in an abnormal loca-
tion are called “ectopic”. Ectopic eyes have also been made to appear on antennae
and wings of Drosophila by targeting the expression of the inserted eyeless gene to
the precursors of these organs. But a more amazing discovery is that the Pax6 gene
from the mouse will also cause ectopic eyes to appear on the antenna of Drosophila.
This eye is a typical fly eye, not a mouse eye, because it requires over 2000 other
genes to make an eye and in this experiment only one of these genes, the Pax6 gene,
is derived from a mouse.
How is the Pax6 gene thought to work? This gene encodes a protein that acts as
a transcription factor. You will recall from Chapter 4 that transcription factors are
proteins that bind to regions of DNA that control the expression of other genes. The
binding of a transcription protein onto a particular sequence of bases in the DNA
either switches the gene(s) on or switches it off. The latter gene(s) may also encode
another transcription factor(s) and so regulates the expression of yet more genes. In
this way, a hierarchical cascade of events can be initiated by one master regulatory
gene. This mechanism of gene regulation was discovered in the 1950s by French
scientists working on how bacteria adapt their metabolism to utilise different nutri-
ents they encounter in their environment, but it is now clearly established that the
same principle underlies the development of specialised organs in the most complex
organisms on the planet. All organisms are related to one another, from bacteria to
humans.
Further Reading
1.
Animal Eyes . Michael F. Land and Dan-Eric Nilsson. Oxford Animal Biology Series. Published
by Oxford University Press, 2002. ISBN 0-19-850 9685.
2.
Master Control Genes in Development and Evolution: Walter J. Gehring, Yale University Press,
1998. ISBN 9780300074093.
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