Transcript lesson 6 Homeobox genes

Genes and Body plans
http://www.pbs.org/wgbh/evolution/library/03/4/real/l_034_04.html
Normal fly
Extra set of wings!
Ubx (Ultrabithorax) gene mutant
Antennae are transformed into legs!
Antp (Antennapedia) gene mutant
Carroll S.B. et al. From DNA to Diversity (2001) Blackwell Science
What caused those body
parts to be in the WRONG
places?
What causes body parts to grow in the
CORRECT places?
Remember that
EVERY CELL of your body contains ALL your
GENES
http://www.pbs.org/wgbh/evolution/library/03
/4/l_034_04.html
• Evolution: Library: Genetic Tool Kit
Homeotic and 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 (front and back part) and positioning
of organs.
• Homeotic genes define a region or position in
the embryo and code for transcription factors
that regulate the activity of other genes.
• Genes in this group contain a conserved
stretch of DNA called the homeobox and are
nearly identical in all species.
• The homeobox is only a portion of each gene
• e.g. if the words below were homeotic genes,
the capital letters would represent the
homeobox:
• togeTHEr / THEoretical / gaTHEring /
boTHEr
• The homeobox is a 180-basepair sequence of
DNA that has been found in many regulatory
genes.
• A particular set of genes, the Hox genes, are
responsible for assigning the head to tail body
pattern in very early embryos.
• Hox genes are a subset of homeotic genes
that contain a homeobox
• Homeobox genes code for the production of
polypeptides about 60 aa long (the
homeodomain) and act as transcription
factors.
• These factors 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.
On your worksheet
• Choose 3 colours and assign a colour each to a, g and i
• Locate the regions labelled a, g and i in the diagrams
of the Drosophila Homeobox gene, the embryo and
the adult
• Use your colour key to colour in these sections in the
Drosophila Homeobox gene, the embryo and the adult
What do you notice?
The pattern of Homeobox expression
Colinearity between
the order of the
homeobox genes and
the expression
pattern
Adult
Eight homeobox
genes regulate the
identity of regions
within the adult and
embryo.
The order of the
genes determines
the body plan
Embryo
Carroll S.B. et al. From DNA to Diversity (2001) Blackwell Science
• All animals have homeobox genes that
are similar (homologous) and highly
conserved.
Evidence that there was colinearity
in homeobox genes between species
Evolution:
Library: Genetic
Tool Kit
http://www.pbs.org/wgb
h/evolution/library/03/4/l
_034_04.html
Hox Genes
Hox genes are a specific family of homeobox genes
that regulate other genes
They are a group of related genes that control the
body plan of an embryo along the anteriorposterior (head-tail) axis.
They are highly conserved
Every Hox gene is a homeobox gene, but not every
homeobox gene is a Hox gene
• This conservation of genes implies that their
activity is fundamental to the development
of an animal.
• Homeosis is a mutation of a homeobox gene
that causes transformation of one area of
the body into another area.
• A mutation in a homeobox gene is so
disastrous that the organism usually doesn’t
survive.
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.
• 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
Mutant fly with 2 wings
Normal adult fly
•Drosophila has a homeobox
gene called Ubx – this stops
wings forming in T3.
Bithorax mutant
•If the fly has a mutation in
both copies of Ubx, wings grow
in T3 instead of halteres.
Hox clusters
• Hox genes are
arranged in clusters.
• their order on the
chromosome is the
same as the order in
which they appear
along the body.
• What this means is that if you look at the
chromosome that contains the Hox genes they
are all lined up in order on the strand of DNA.
• This shown in the drosophila fruit fly below:
• Hox genes are universal in animals.
• Vertebrates have Hox genes, and they have
the same properties: each gene contains a
homeobox, and they are organized on the
chromosome in the order of their expression
from front to back
Larger Hox gene number
More complicated body pattern
Nematodes (round worms) = 1 Hox
cluster
Drosophila = 2
Vertebrates = 4
Drosophila and human Hox genes
Mammals
• In mammals Hox
genes are found
In 4 clusters
Hox
Hox
Hox
Hox
A: Chr. 16
B: Chr. 11
C: Chr 15
D: Chr. 2
Evolution of Hox gene clusters
Hypothetical
common
ancestor
Amphioxus
Haeckel's 1874 version of vertebrate embryonic development.
The top row shows an early stage common to all groups, the second row shows a
middle stage of development, and the bottom row shows a late stage embryo.
The Role of retinoic acid
• Retinoic acid is derived from vitamin A
• It is an activator of HOX genes in
vertebrates
• It activates the genes in the correct
sequence running from head to tail
• It is called a morphogen because it regulates
the pattern of tissue development
• If it is in xs it can however start to switch on
the genes in wrong order resulting in major
birth defects
• Pregnant women should be very careful about
their vitamin A intake
Homework
• Make notes and diagrams on the
development of drosophila and its
genetic control p114