BIOL 104 Lecture Notes - Lecture 19: Hox Gene, Ultrabithorax, Somite

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13 May 2018
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Somites are blocks of tissue that reside to the left and right sides of the developing neural tube. The
number of somites varies by organism, but is always the same for that organism. These blocks of tissue
eventually become the bones and cartilage of the vertebral column and ribs as well as the muscles of
the ribs, limbs, body wall, and back.
Somites form from anterior (head) to posterior (tail) as loks of tissue are shaped ito ues. The left
and the right somites are not connected to each other, but form synchronously (at the same time).
Furthermore, each somite forms every fixed amount of time that is characteristic of the organism. For
example, each somite forms every 30 minutes in a zebrafish and every 330 minutes in humans.
This suggests that there is a lok or a osillator that is read  ells to for a soite. I 1997, a
version of the fruitfly hairy gene showed that its expression is turned on and off and on and off and on
ad off as eah soite is fored….
It starts out in the posterior region of the embryo and moves like a wave toward the new somite that
forms. Expression remains in the newly formed somite.
Mutations in human genes that encode proteins involved in the segmentation clock gives rise to human
skeletal deformities such as congenital scoliosis.
How do somites know what to become? What somites become along the A/P position is partially
determined by Hox Genes. They were originally discovered in flies where mutations caused
transformations of segments in the fly. Ultrabithorax and Antennapedia are two Hox gene examples.
Ultrabithorax mutations cause a segment that should not have wings to be transformed into a segment
that does. Antennapedia transforms the segment that should make the antennae into a segment that
makes legs instead.
Hox genes are lined up on the chromosome in a particular order. This order is similar to the order of
segments along the A/P axis.
Hox genes contain a sequence of DNA called a homeobox that codes for a sequence of amino acids in
the Hox protein called a homeodomain. This sequence is a DNA binding sequence, and therefore, make
all hox proteins transcription factors. Combinations of different hox proteins in a segment determine
the fate of that segment. This is referred to as the Hox Code.
Like fruitflies, humans and other vertebrates are segmented. We have vertebral bones that are
different based on the A/P position. For example, our thoracic (upper back) vertebrae are attached to
rib bones while our lumbar (lower back) vertebrae are not. Our embryos must tell these somites in the
upper back that they will become thoracic vertebral bones. Hox genes dictate this.
While fruitflies have one set of Hox genes, humans and mice have four sets. These have duplicated over
the course of evolution. We do have all of the same hox genes as the fly. Human Hox genes are lined up
on the chromosome in a particular order. This order is similar to the order of segments along the A/P
axis.
Shifts in Hox gene expression are big problems in human embryos. Shifts can lead to transformations of
vertebral bones (and many other things in the embryo). We discussed shifts from lumbar into thoracic
and from sacral (even lower back) into lumbar.
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Document Summary

Somites are blocks of tissue that reside to the left and right sides of the developing neural tube. The number of somites varies by organism, but is always the same for that organism. These blocks of tissue eventually become the bones and cartilage of the vertebral column and ribs as well as the muscles of the ribs, limbs, body wall, and back. Somites form from anterior (head) to posterior (tail) as (cid:271)lo(cid:272)ks of tissue are (cid:862)shaped(cid:863) i(cid:374)to (cid:272)u(cid:271)es. The left and the right somites are not connected to each other, but form synchronously (at the same time). Furthermore, each somite forms every fixed amount of time that is characteristic of the organism. For example, each somite forms every 30 minutes in a zebrafish and every 330 minutes in humans. This suggests that there is a (cid:272)lo(cid:272)k or a(cid:374) os(cid:272)illator that is (cid:862)read(cid:863) (cid:271)(cid:455) (cid:272)ells to for(cid:373) a so(cid:373)ite.

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