Download Body plans Homeobox

Body plans Homeobox
Body Plans and Homeobox Sequences
 In any multicellular organism, development is controlled
and coordinated:
 cells end up where they are meant to be
 development follows a body plan and is under genetic
control
 The genes which control the body plan are called
homeobox genes.
 Homeobox (Hox) genes are 180 base pairs long.
 They code for small protein transcription factors (60
amino acids long).
 Transcription factors bind to DNA allowing it to be
transcribed and ‘switch on’ many genes in a cascade
effect.
 Homeobox (or Hox) genes have been
discovered in:
 fungi
 plants
 animals
 from fruit fly to humans
 The following outline shows how they are
important in controlling the body plan of
fruit fly.
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Thorax
Abdomen
Head
Pair of wings on T2
Compound eye
Pair of balance organs on T3
Antennae
Paired legs on T1, T2 and T3
Hox genes of the fruit fly, Drosophila melanogaster
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 The fruit fly has the body plan of a typical insect, i.e.
the adult has a segmented body.
 It has:
 head section
 thorax (three segments T1, T2 and T3)
 abdomen
 Each of the thorax sections has a pair of legs, there is a
pair of wings on T2 and a pair of balance organs on T3.
 Hox genes control:
 polarity of the body
 polarity of the segments
 development of individual segments
 The Antp (antennapedia)
homeobox gene codes for a
transcription factor that
switches on all of the genes
that allow leg development.
 It is switched on in thorax
sections 1, 2 and 3 and
usually switched off in the
head segment.
 If switched on in the head
segment, a pair of legs
develop instead of
antennae.
 It produces the
antennapedia condition.
Wild type and mutant
Drosophila with antennapedia
mutation.
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 Similarly, the Ubx (ultrabithorax) homeobox gene
usually stops development of wing production on
thorax segment 3.
 If mutated, an extra pair of wings is grown instead
of the usual balance organs.
 Animations showing the role of homeobox genes in
Drosophila melanogaster (the fruit fly) :
 http://www.dnalc.org/view/16760-Animation-37Master-genes-control-basic-body-plans.html
 The situation is similar in
humans where Hox A11 and
Hox D11 switch on the genes
for development of the
forelimb.
 It has been suggested that
thalidomide may have switched
off the homeobox genes for
limb development.
 This caused the birth defects
typical of thalidomide use.
 Thalidomide can certainly
insert itself into DNA and
inhibits production of new
blood vessels in limb buds.
Baby born to a mother who had
taken thalidomide during pregnancy.
Note the malformation of the foot
and the right arm.