The genetic control of segmentation involves a cascade of gene regulation'

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• The genetic control of segmentation involves a
  cascade of gene regulation.
• bicoid protein gradient switches hunchback on at
  a high concentration.
• hunchback activates and represses other gap genes
  like krüppel, knirps, giant.
• Gap gene products and gap genes interact,
  sharpening expression boundaries.
• Axis is divided into unique domains containing
  different combinations of transcription factors
  pair-rule genes (even-skipped ,fushi tarazu).

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• The genetic control of segmentation involves a
  cascade of gene regulation.
• bicoid protein gradient switches hunchback on at
  a high concentration.
• hunchback activates and represses other gap genes
  like krüppel, knirps, giant.
• Gap gene products and gap genes interact,
  sharpening expression boundaries.
• Axis is divided into unique domains containing
  different combinations of transcription factors
  pair-rule genes (even-skipped ,fushi tarazu).

3

• The genetic control of segmentation involves a
  cascade of gene regulation.
• bicoid protein gradient switches hunchback on at
  a high concentration.
• hunchback activates and represses other gap genes
  like krüppel, knirps, giant.
• Gap gene products and gap genes interact,
  sharpening expression boundaries.
• Axis is divided into unique domains containing
  different combinations of transcription factors
  pair-rule genes (even-skipped ,fushi tarazu).

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Fly Gene interactions
Alternate expression by vertical lines of nuclei
result in the 'zebra stripe' pattern of the
embryo in which all nuclei of the stripe express
the same pair-rule genes. Primary pair-rule genes
are essential for the formation of the periodic
striped pattern, and they are controlled directly
by the gap gene proteins, which activate the
transcription of some pair-rule genes while
repressing the transcription of others. For
example, the hairy gene is expressed in seven
stripes across the embryo. Its regulatory region
can be separated in elements regulating each of
the 7 stripes. For stripe 6, knirps and caudal
are activators, while Krüppel and Hunchback are
repressors. Similarly, in the even-skipped gene,
stripe 2 is activated by Bicoid and Hunchback and
is repressed by Giant and Krüppel. Interactions
between primary pair-rule genes become
self-stabilized once inititated, and they are
responsible for allowing or inhibiting the
secondary pair-rule genes. Pair-rule and gap
genes also interact to regulate the homeotic
genes that determine the identity of each segment.
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Fly Gene interactions
How do you work with lethal mutants? Heat
shock Promoters of heat shock genes possess
several regulatory elements that are required for
transcriptional activation under heat stress. The
ability of heat shock promoters to sense heat is
credited to the presence of a consensus sequence
located in the upstream promoter region. This
consensus sequence is referred to as the heat
shock element (HSE) and consists of
pentanuclotide repeats with a core sequence of 5'
nGAAn3' or its inverse complement, 5'nTTCn3'
(Pelham, 1982). These repeat core elements are
arranged in alternating orientations separated by
two nucleotides.
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• Clone a piece of DNA that contains the site to
  which the tx factor binds.
• Label one end of the DNA molecules with a
  radioactive molecule, e.g. radioactive ATP.
• Digest the DNA with DNase I.
• DNase I cuts DNA molecules randomly.
• Choose conditions that most molecules will be cut
  only once.
• The result will be a mixture of radioactive
  fragments of varying length.
• Separate the fragments by electrophoresis.

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• Clone a piece of DNA that contains the site to
  which the tx factor binds.
• Label one end of the DNA molecules with a
  radioactive molecule, e.g. radioactive ATP.
• Digest the DNA with DNase I.
• DNase I cuts DNA molecules randomly.
• Choose conditions that most molecules will be cut
  only once.
• The result will be a mixture of radioactive
  fragments of varying length.
• Separate the fragments by electrophoresis.

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Homeotic selector genes
Homeotic genes were discovered because of the
fact that mutations in these genes can cause
unusual phenotypes. In homeotic mutants,
correctly formed body parts arise in ectopic
places. Just as segmentation genes have the
function to divide the body into repeated units,
the homeotic genes are responsible for rendering
these units different from each other.
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The Homeotic selector genes are responsible for
the identity of each segment. Homeotic genes
regulate, for example, whether the segment
develops a haltere or a wing The mutations were
genetically mapped and shown to fall in two gene
complexes, the Antennapedia Complex (ANTC) and
the Bithorax Complex (BX-C).
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Each segment has an unique identity. Homeotic
selector genes specify each segment to control
other genes and maintain segment identity. Two
complexes or a split complex (Bithorax and
Antennapedia the HOM genes), together are
homologous to the HOX gene complexes of
vertebrates. First identified by homeotic genes,
mutations in which cause homeosis, the
transformation of one structure into another
structure. Antenna to leg (Antennapedia) or
haltere to wing (Bithorax)
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The homeotic genes (here called "HOM" genes) in
Drosophila are clustered close to each other in
the DNA, and they set a ground plan for embryonic
development in the fly. Vertebrates also contain
homeotic (HOX) genes. Here, we find four
clusters, or complexes, of homeotic genes. These
genes are closely related to the insect genes,
their order in the DNA is the same, and their
action during embryonal development follows
the same order in time and space in the fly.
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Homeotic genes of the bithorax complex (BX-C) are
responsible for the posterior segments
Expression is controlled by gap pair-rule
genes. Antennapedia complex controls
specification of anterior regions Antennapedia
complex (Antp-C) consists of 5 homeobox genes.
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Segment identity of mammals. The expression
boundary is sharp at the upper end of the segment

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Segmentation of the back part of the primitive
brain is visible in the nine rhombomeres. The
HOX-B 2 gene is expressed in rhombomere 4, the
HOX-B 3 gene in rhombomeres 5 and 6 and the HOX-B
4 gene in rhombomeres 7 and 8.
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Synpolydactyly (SPD) is a dominantly inherited
congenital limb malformation. Typical cases have
3/4 finger and 4/5 toe syndactyly, with a
duplicated digit. The condition has recently been
shown to be caused by expansions of a
trinucleotide repeat encoding a 15-residue
polyalanine tract in HOXD13.
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FIG. 5. The limb phenotype in pedigree Q.
Photograph (A) and x-ray (B) of right hand of
II.1 at age 5 years and 18 months, respectively,
showing severe SPD (duplicated third finger with
soft tissue syndactyly as far as distal phalanx,
bifid third metacarpal, fifth finger
camptodactyly, and middle phalanx hypoplasia of
all fingers),
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Fig. 1. Examples of polydactylous limbs. Human
preaxialpolydactyly of the hand (left) and
chicken mutant diplopodia-3 foot (right). Both
exhibit an additional digit which morphologically
closely resembles the neighboring digit 1 (an
extra thumb).
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Fig. 1. Examples of polydactylous limbs. Human
preaxialpolydactyly of the hand (left) and
chicken mutant diplopodia-3 foot (right). Both
exhibit an additional digit which morphologically
closely resembles the neighboring digit 1 (an
extra thumb).
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Dominant or recessive?