Download Homeotic Genes

Homeotic Genes
&
Gene Cascades
•Ever wondered why so many organisms have two
eyes, two ears or a set of fore and rear limbs in
approximately the same place?
•Ever wondered how the various parts of a fetus
manage to grow at just the right time in relation to
other parts?
•Imagine a human fetus that grew arms where its
legs should be, or developed fully formed and
functioning newborn ears on a 10 week old fetus.
•There are a series of genes, called homeotic
genes, and their role it is to control the genes that
play a function in early development.
Homeotic Genes
•Homeotic genes are a kind of master gene that
controls the early development of organisms.
•These genes play an important role in the time and
place when certain genes are switched on or off during
embryonic development.
•Due to their ability to control other genes, a mutation in
a homeotic gene can have devastating effects.
•Studying mutations in homeotic genes assisted
scientists in understanding their role.
Homeotic Genes
Normal fly with antennae
in the correct position.
Homeotic genes
functioning normally.
Normal fly with legs in
the place of antennae.
Homeotic genes have
been mutated.
http://www.esb.utexas.edu/palmer/bio303/group25/DROSOPHILA/homeotic_genes.htm
•Both the fruit fly embryo
and the mouse embryo
show similar genes
controlling the
development of various
body parts.
•In the fruit fly the
homeotic (HOM) genes
are on chromosome 3.
•In mammals, the
homeotic genes (HOX1,
HOX2, HOX3, HOX4) are
found on four different
chromosomes.
http://www.people.virginia.edu/~rjh9u/homeo.html
Homeotic Genes
•There is little doubt that this well preserved section of DNA
appeared very early on in the evolution of living things.
•It was an important tool because it let the cells know where they
were during the early stage of embryonic development so that they
could then go on and differentiate in order to develop a certain part
of the organism.
•Once an early cell at the anterior end of the early embryo could
identify that it was actually at the anterior end, then it could go on
with safely and accurately growing the head of a normal fetus.
•Homeotic genes work in modules or levels. Once one module or
level has developed, further modules/levels are formed within it to
take on more and more specialised jobs.
•For example limb buds develop at one level before fingers or toes
which develop at a later level.
Gene Cascades •The protein made by each
gene triggers one or more
genes into either turning on
or off.
•Genes that perform similar
functions are grouped
together
•If the genes do not become
active at a particular time
there will be a major
disruption of the
developmental process.
http://www.biology.pomona.edu/martinez/zoo-Hom.htm
•Disruption of many of these
genes results in the death of
the embryo.
Homeobox Genes
•A common exon sequence, discovered in 1984 and
found in all homeotic genes studied so far.
•Scientists believe that all organisms have identical – or
at least very similar homeobox sequences. They identify
the sequence using a laboratory prepared DNA probe
that binds to similar sequences.
•Homeobox genes are 180 base pairs in length and
code for a series of amino acids that are positively
charged.
•As DNA is negatively charged the positively charged
section of the protein is attracted to the DNA. This is
how the homeotic gene binds to DNA.
http://www.mun.ca/biology/desmid/brian/BIOL2060_W2003/CellBiol21/CB21_30.html
The End…
the rest we will leave
for the experts…