Transcript Lesson_6_Genes_and_body_plansx

Genes and body plans
• Explain that the
genes that control
development of
body plans are
similar in plants,
animals and fungi,
with reference to
homeobox
sequences.
• How do all the cells, tissues and organs that
form an organism develop in the right places?
• This is an area of biology about which much
is still unknown but it is known that part of
the process involves homeobox genes.
• Homeobox genes control how an organism’s
body develops as it grows from a zygote into
a complete organism. They determine the
body plan (including the polarity and
positioning of organs).
• All animals have homeobox genes that are
similar (homologous).
• These genes are highly conserved.
• This implies that their activity is
fundamental to the development of an animal.
• A mutation in a homeobox gene is so
disastrous that the organism usually doesn’t
survive.
• A way of studying homeobox genes is to
study what happens when they go wrong.
Drosophila (fruit fly)
• Body plan of typical insect –
head thorax and abdomen.
• Thorax is made up of 3
segments (T1, T2, T3).
• A pair of legs grows from each
segment.
• A pair of wings grows from T2.
• A pair of halteres (used for balance when
flying) grows from T3.
• Drosophila has a homeobox gene called Ubx –
this stops wings forming in T3.
• If the fly has a mutation in both copies of Ubx,
then wings grow in T3 instead of halteres.
• The fly has 2 pairs of wings and cannot fly.
Normal adult fly
Bithorax mutant
• A homeobox gene called Antp is usually turned on in
the legs where it causes legs to develop.
• It is turned off in the head.
• In some mutant flies the Antp gene is switched on
in the head producing legs
instead of antennae.
Mutants
Wild-type
• Homeobox genes code for the production of
proteins called transcription factors.
• These can bind to a particular region of DNA
and cause it to be transcribed.
• So, a single homeobox gene can switch on a
whole collection of other genes.
• The normal Antp gene, switches on all the
genes involved in leg production.
• The normal Ubx gene switches off all genes
involved in wing production.
• This is similar in other organisms including
humans and mice.
Now use p114-115 to answer the
following questions
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How many Hox genes are there in fruit flies, mice and humans? How do
scientists know all these carriers of Hox genes arose from a common
ancestor?
The fruit fly (Drosophila) and mouse homeobox gene sequences do not
look exactly alike. Can you guess why?
What do homeobox genes code for in animals?
Why are homeobox (also called homeotic) genes called the "master
control genes" or "master switches"?
What other organisms besides fruit flies have homeobox genes?
What does the presence of related groups of homeobox genes in fruit
flies and mice indicate about their evolution?
How does Saint-Hilaire's "ridiculous" idea that vertebrates have the
body plan of an upside-down insect have a sound basis in evolutionary
genetics?
What do homeobox genes suggest about the evolution of different eyes
in different organisms?