BI2233: Establishment of the body plan 4 Synopsis Continuation of
• Homeotic (or Hox) genes are a family of transcriptor genes, which exhibit unusual
properties. They serve as the “master control” genes, which direct the development of
specific segments or structures in the body. There are 39 of these hox genes, which are
organized into four clusters (A-D) on four different Chromosomes 1-13.
• The segment identity in Drosophila (the fruit fly) is specified by genes of the Antennapedia (a
hox gene, first seen in the fruit fly) and bithorax (located in the right-arm of chromosome 3)
complexes.
• The Hox genes of the bithorax complex are responsible for the diversification of posterior
(back side) segments, while the antennapedia complex controls specification of anterior
regions (these control the formation of the legs). Mutations can involve gain of function or
loss of function.
• Hox genes play a critical role in the development of embryos. This role involves
transcriptional regulation, through which gene position is determined by the time and
places that genetic information is expressed in during development.
• Polycomb, Trithorax and chromatin are group-genes, which are present in the fruit fly, and
which regulate gene expression. Polycomb and Trithorax have opposite affects on the
expression of Hox genes, with one protecting expression and the other responsible for
suppression.
• Hox gene cluster “co-linearity” refers to the gene order inside of chromosomes, which is
defined by the serial activation of genes in ontogenetic units along the A/P axis. (Telomeric
to Centrometric).
• Hox gene targets are specific objects that a hox gene binds to. Few of these have been
identified, yet the ones that have played an important role in cell function and identity.
Examples include: Six2; p53; Progesterone receptors; Pleiotrophin…)
• Patterning the vertebrate body plan. (Spatiotemporal(space-time) gene expressions play a
major role in giving specific identity to anatomical regions on the vertebral column. HoxPG9,
especially may play a critical role in the formation of the lateral plate responses.
• Identity of somites (blocks of muscle tissue which can contribute to the development of
trunks and tails) along the A/P (anterior/posterior) axis is specified by Hox genes. Somites
can produce retinoic acid. Some mutations truncate somites Embryos elongate during
somitogenesis.
,Homeotic Mutations
These mutations can cause different segments of the body to take on new roles.
If a mutation leads to the expression of the Antennapedia gene in a fruit fly’s head, it can grow
legs instead of antennae in this place. Mutations in the thorax, meanwhile, can cause fruit flies
to develop a second pair of wings behind the first.
Hox Genes in the Bithorax Complex are Responsible for the diversification of posterior
segments
The Bithorax Complex is one of the two Drosophila melanogaster homeotic gene complexes
that are found in the right arm of chromosome three.
In wild type diversification involves the expression of the three Hox genes of this bithorax
complex and the parasegment identity. The removal of all three genes in the bithorax complex
,converts parasegments 5-13 into nine parasegments. Different kinds of mutations and
developments in the bithorax complex lead to diversity in the fruit fly’s posterior segments.
Below are possible combinations:
1. Ultrabithorax, abdominal-A and Abdominal B Absent: No Ubx, abd-A, Abd-B
2. Ultrabithorax, added back: Add back Ubx
3. Ultra Bithorax and abdominal-A added back: Add back Ubx + abd-A
4. Ultrabithorax only absent: No Ubx.
Homeotic Mutation
Wild type vs. Antennapedia
,
• Homeotic (or Hox) genes are a family of transcriptor genes, which exhibit unusual
properties. They serve as the “master control” genes, which direct the development of
specific segments or structures in the body. There are 39 of these hox genes, which are
organized into four clusters (A-D) on four different Chromosomes 1-13.
• The segment identity in Drosophila (the fruit fly) is specified by genes of the Antennapedia (a
hox gene, first seen in the fruit fly) and bithorax (located in the right-arm of chromosome 3)
complexes.
• The Hox genes of the bithorax complex are responsible for the diversification of posterior
(back side) segments, while the antennapedia complex controls specification of anterior
regions (these control the formation of the legs). Mutations can involve gain of function or
loss of function.
• Hox genes play a critical role in the development of embryos. This role involves
transcriptional regulation, through which gene position is determined by the time and
places that genetic information is expressed in during development.
• Polycomb, Trithorax and chromatin are group-genes, which are present in the fruit fly, and
which regulate gene expression. Polycomb and Trithorax have opposite affects on the
expression of Hox genes, with one protecting expression and the other responsible for
suppression.
• Hox gene cluster “co-linearity” refers to the gene order inside of chromosomes, which is
defined by the serial activation of genes in ontogenetic units along the A/P axis. (Telomeric
to Centrometric).
• Hox gene targets are specific objects that a hox gene binds to. Few of these have been
identified, yet the ones that have played an important role in cell function and identity.
Examples include: Six2; p53; Progesterone receptors; Pleiotrophin…)
• Patterning the vertebrate body plan. (Spatiotemporal(space-time) gene expressions play a
major role in giving specific identity to anatomical regions on the vertebral column. HoxPG9,
especially may play a critical role in the formation of the lateral plate responses.
• Identity of somites (blocks of muscle tissue which can contribute to the development of
trunks and tails) along the A/P (anterior/posterior) axis is specified by Hox genes. Somites
can produce retinoic acid. Some mutations truncate somites Embryos elongate during
somitogenesis.
,Homeotic Mutations
These mutations can cause different segments of the body to take on new roles.
If a mutation leads to the expression of the Antennapedia gene in a fruit fly’s head, it can grow
legs instead of antennae in this place. Mutations in the thorax, meanwhile, can cause fruit flies
to develop a second pair of wings behind the first.
Hox Genes in the Bithorax Complex are Responsible for the diversification of posterior
segments
The Bithorax Complex is one of the two Drosophila melanogaster homeotic gene complexes
that are found in the right arm of chromosome three.
In wild type diversification involves the expression of the three Hox genes of this bithorax
complex and the parasegment identity. The removal of all three genes in the bithorax complex
,converts parasegments 5-13 into nine parasegments. Different kinds of mutations and
developments in the bithorax complex lead to diversity in the fruit fly’s posterior segments.
Below are possible combinations:
1. Ultrabithorax, abdominal-A and Abdominal B Absent: No Ubx, abd-A, Abd-B
2. Ultrabithorax, added back: Add back Ubx
3. Ultra Bithorax and abdominal-A added back: Add back Ubx + abd-A
4. Ultrabithorax only absent: No Ubx.
Homeotic Mutation
Wild type vs. Antennapedia
,
