Chromosome Banding

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Chromosome Banding
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Introduction

• the earliest techniques stained chromosomes
  uniformly, and only allowed a few chromosomes of
  unusual size or shape to be identified
  unequivocally.
• it is not possible to distinguish between the p
  and q arms of metacentric chromosome.

Mitotic chromosomes (metaphase) of incense cedar
(Calocedrus decurrens) stained by aceto-carmine
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Chromatin

• Chromosomes are made of chromatin.
• there are two forms of chromatin
• heterochromatin
• euchromatin
• these two forms of chromatin can be
  distinguished by intensity of staining.

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Heterochromatin

• condensed chromatin and genetically inactive
  (limited transcription) during interphase.
• it consists of repetitive DNA sequences which
  are relatively rich in AT base pairs and is late
  replicating in the cell cycle.
• heterochromatic regions of chromosomes are
  generally referred to as positively
  heteropycnotic (pycnotic dense) by cytologist.
• is darkly stained.

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Heterochromatin

• heterochromatin is usually localized to the
  periphery of the nucleus.

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Heterochromatin

• Heterochromatin is believed to serve several
  functions, from gene regulation to the protection
  of the integrity of chromosomes.
• there are two types of heterochromatin
  constitutive heterochromatin and facultative
  heterochromatin.

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Constitutive Heterochromatin

• constitutive heterochromatin is present at
  identical positions on all chromosomes in all
  cell types of an organism.
• any genes contained within the constitutive
  heterochromatin will be poorly expressed.
• all human chromosomes 1, 9, 16, and the Y
  chromosome contain large regions of constitutive
  heterochromatin.
• in most organisms, constitutive heterochromatin
  occurs around the chromosome centromere and near
  telomeres.

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Facultative Heterochromatin

• facultative heterochromatin is variable in its
  expression. It varies with the cell type and may
  be manifested as condensed, or heavily stained,
  chromatin in only certain differentiated somatic
  cells in the same organism.
• an example of facultative heterochromatin is
  X-chromosome inactivation in female mammals one
  X chromosome is packaged in facultative
  heterochromatin and silenced, while the other X
  chromosome is packaged in euchromatin and
  expressed.

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Euchromatin

• chromatin that is not belonged to
  heterochromatin is euchromatin.
• is lightly stained due to the less compact
  structure.
• early-replicating and GC rich region.
• it should be noted that in prokaryotes,
  euchromatin is the only form of chromatin
  present.
• euchromatin participates in the active
  transcription of DNA to mRNA products.

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Chromosome Banding

• chromosome banding is developed based on the
  presence of heterochromatin and euchromatin.
• heterochromatin is darkly stained whereas
  euchromatin is lightly stained during chromosome
  staining.
• a band is defined as that part of a chromosome
  which is clearly distinguishable from its
  adjacent segments by appearing darker or brighter
  with one or more banding techniques.
• there are a few types of chromosome banding
  G-banding, C-banding, Q-banding, R-banding etc.

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G-Banding

• G-banding is obtained with Giemsa stain
  following digestion of chromosomes with enzyme
  trypsin.
• Giemsa stain, named after Gustav Giemsa, an
  early malariologist, is used for the
  histopathological diagnosis of malaria and other
  parasites. It is a mixture of methylene blue and
  eosin. It is specific for the phosphate groups of
  DNA and attaches itself to regions of DNA where
  there are high amounts of adenine-thymine
  bonding.
• it yields a series of lightly and darkly stained
  bands - the dark regions tend to be
  heterochromatic, late-replicating and AT rich.
  The light regions tend to be euchromatic,
  early-replicating and GC rich .

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G-Banding
G-banding of human female metaphase chromosomes
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Q-Banding

• Q-banding is a fluorescent pattern obtained
  using quinacrine for staining. The pattern of
  bands is very similar to that seen in G-banding.
• Quinacrine banding (Q-banding) was the first
  staining method used to produce specific banding
  patterns for mammalian chromosomes.
• It is especially useful for distinguishing the Y
  chromosome.
• This method requires a fluorescence microscope
  (quinacrine fluoresces strongly in the
  ultraviolet) and is no longer as widely used as
  G-banding.

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Q-Banding
Q-banding of human male metaphase chromosomes
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R-Banding

• R-banding is the reverse of G-banding (the R
  stands for "reverse").
• the dark regions are euchromatic
  (guanine-cytosine rich regions) and the bright
  regions are heterochromatic (thymine-adenine rich
  regions).
• telomeres are stained well by this procedure.
• Reverse banding (R-banding) requires heat
  treatment and reverses the usual white and black
  pattern that is seen in G-bands and Q-bands.

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R-Banding
R-banding of human female metaphase chromosomes
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C-Banding

• C-banding stains the constitutive
  heterochromatin, which usually lies near the
  centromere.

Chromosomes of mouse
Chromosomes of human female