Chemosensory%20receptors

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Chemosensory receptors

• Chemoreceptors include olfactory receptors.
  They all have 7 transmembrane domains and form
  the largest super gene family in animals.
• No. of chemoreceptor genes
• Nematode 1220
• Fruitfly 170

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Homeotic Genes

• Genes in which a mutation can transform one
  body segment into another segment
• Commonly known as Hox genes
• They are transcription factors

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Homeobox-containing protein bound to DNA
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Cis- and trans-regulators of gene expression
Transcription factor
Transcription
Exon 1
Exon 2
Exon 3
cis-element (6-10 nucleotides)
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Hox genes and Drosophila body plan
Sean Carroll
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Sean Carrolls lab
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Observations

• The vertebrate genes homologous to fly homeotic
  genes are found in four clusters, tightly linked.
• Vertebrate Hox genes are expressed in the same
  anterior-posterior order along the body axis as
  in flies

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Implications

• The homeobox regulatory gene clusters date to
  ancestors that predate the arthropod-vertebrate
  divergence 600 million years ago
• Humans and mice arent so different from flies in
  body-plan development

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Pax Genes
Defined by the presence of a conserved paired-box
that codes for a 128-amino-acid paired domain, a
DNA binding domain
Encode nuclear transcription factors involved in
developmental control, notably the central
nervous system
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Human and mouse
9 Pax genes Pax-1 to -9
Drosophila
Paired, gooseberry, gooseberry neuro, pox meso,
pox neuro, eyeless, sparkling
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Paired domain
Octapeptide
Homeodomain
Pax-1 Pax-9
Pax-2 Pax-5 Pax-8
Pax-3 Pax-7
Pax-4 Pax-6
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Expression Patterns
Pax-1 vertebral column and thymus Pax-9
vertebral column
Pax-2 neural tube and hind brain,
excretory system ear and eye Pax-5
brain, neural tube, liver cells giving
rise to B lymphocyte Pax-8 neural
tube, hind brain, excretory
system, thyroid
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Expression Patterns
Pax-3 neural tube, dermomytome and
limb bud, cranio-facial
structure Pax-7 brain, neural tube,
dermomyotome
Pax-4 pancreas Pax-6 brain, neural tube,
nose, eye, pancreas
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Mutations in mouse can cause
Pax-2 defects in kidney
optic nerve, retina
Pax-3 neural crest defects
Pax-6 small eye
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An additional eye on the antenna induced by
targeted expression of mouse Pax 6
Halder, G., Callaerts, P. and Gehring, W.J.
(1995). Induction of ectopic eyes by targeted
expression of the eyeless gene in Drosophila.
Science 267, 1788-1792.
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Pax 6 in mammals vs. eyeless in
fruitflies

• The two proteins have highly similar paired
  domains.
• Mouse Pax 6 gene can induce eye development in
  Drosophila, despite more than 600 million years
  of separation!
• Despite great differences in eye type,
  Drosophila and human use Pax 6 to control eye
  development!

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Loss of duplicate genes

• A duplicate gene can become nonfunctional (a
  pseudogene) if there is no advantage to keep it.
• Indeed, there are pseudogenes in every eukaryotic
  genome studied.
• Example
• Globin pseudogenes in human

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Many chemoreceptor pseudogenes

• In nematode 1220 functional genes but 419
  pseduogenes
• In mouse 1000 olfactory receptor (OR) genes
• In human only 400 OR genes
• Reason There are many more OR pseudogenes in
  human than in mouse.

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Conclusions (1)

• A gene duplicate can become nonfunctional and
  eventually disappear from the genome.

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Conclusions Gene duplication can

• Meet a large dosage requirement
• Lead to fine-tuning of cell physiology
• Provide functional expansion
• Provide functional diversification
• Provide raw materials for the emergence of novel
  function
• Has led to evolution of body plan in animals
• Has led to the evolution of the central nervous
  system in animals

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Thanks!