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Chapter 8
Chromosome and chromatin structure
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Dr. S Hosseini-Asl; Radiology; ARUMS; 1392
DNA bases
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Nucleotide
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dNTPs
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ds DNA
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DNA
vs.
RNA
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Histones
Histones are highly alkaline proteins found
in eukaryotic cell nuclei that package and
order the DNA into structural units called
nucleosomes. They are the chief protein
components of chromatin, acting as spools
around which DNA winds, and play a role
in gene regulation. Without histones, the
unwound DNA in chromosomes would be
very long (a length to width ratio of more
than 10 million to one in human DNA). For
example, each human cell has about 1.8
meters of DNA, but wound on the histones
it has about 90 micrometers (0.09 mm) of
chromatin, which, when duplicated and
condensed during mitosis, result in about
120 micrometers of chromosomes.
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Human Chromosome
Telomeres
Petit
Centromere
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Centromere
 The centromere is the part of a chromosome that links
sister chromatids. During mitosis, spindle fibers attach to
the centromere via the kinetochore. Centromeres were
first defined as genetic loci that direct the behavior of
chromosomes. Their physical role is to act as the site of
assembly of the kinetochore - a highly complex
multiprotein structure that is responsible for the actual
events of chromosome segregation - e.g. binding
microtubules and signaling to the cell cycle machinery
when all chromosomes have adopted correct attachments
to the spindle, so that it is safe for cell division to proceed
to completion (i.e. for cells to enter anaphase).
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Telomere
A telomere is a region of repetitive nucleotide
sequences at each end of a chromosome which
protects the end of the chromosome from
deterioration or from fusion with neighboring
chromosomes. Its name is derived from the Greek
nouns telos (τέλος) 'end' and merοs (μέρος, root:
μερ-) 'part.' Telomere regions deter the degradation
of genes near the ends of chromosomes by allowing
chromosome ends to shorten, which necessarily
occurs during chromosome replication. Over time,
due to each cell division, the telomere ends become
shorter.
During cell division, enzymes that duplicate DNA
cannot continue their duplication all the way to the
end of chromosomes. If cells divided without
telomeres, they would lose the ends of their
chromosomes, and the necessary information they
contain. The telomeres are disposable buffers
blocking the ends of the chromosomes, are
consumed during cell division, and are replenished
by an enzyme, telomerase reverse transcriptase.
Telomere shortening and aging
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Chromosome types
 Metacentric
p=q
 Submetacentric p<q
 Acrocentric
p<<q
 Telocentric
p=0
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Human Chromosome groups
 A: 1-3, MC (2=SMC)
 B: 4,5, SMC
 C: 6-12,X , SMC
 D: 13-15 , AC
 E: 16-18 , SMC
 F: 19,20 , MC
 G: 21,22,Y , AC
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Acrocentric chromosomes
NOR
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Nucleolus
NOR: Nucleolus Organizing Region
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Techniques for chromosome
studies
1. Routine cytogenetic
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Solid staining
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G-banding
G-banding is a technique used in cytogenetics to produce a
visible karyotype by staining condensed chromosomes. It is
useful for identifying genetic diseases through the
photographic representation of the entire chromosome
complement. The metaphase chromosomes are treated with
trypsin (to partially digest the chromosome) and stained with
Giemsa. Dark bands that take up the stain are strongly A,T
rich (gene poor). The reverse of G-bands is obtained in
R-banding. Banding can be used to identify chromosomal
abnormalities, such as translocations, because there is a
unique pattern of light and dark bands for each chromosome.
It is difficult to identify and group chromosomes based on
simple staining because the uniform color of the structures
makes it difficult to differentiate between the different
chromosomes. Therefore, techniques like G-banding were
developed that made "bands" appear on the chromosomes.
These bands were the same in appearance on the homologous
chromosomes, thus, identification became easier and more
accurate.The acid-saline-Giemsa protocol reveals G-bands.
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R-, Q-, C-banding
Different chromosomal staining techniques reveal variations in chromosome structure. Cytogeneticists use
these patterns to recognize the differences between chromosomes and enable them to link different disease
phenotypes to chromosomal abnormalities. Giemsa banding (a), Q-banding (b), R-banding (c) and Cbanding (d)
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Techniques for chromosome
studies
2. Molecular cytogenetic
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ISH (In Situ Hybridization)
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In situ hybridization (ISH) is a type of hybridization that uses a labeled complementary DNA or RNA strand (i.e., probe)
to localize a specific DNA or RNA sequence in a portion or section of tissue (in situ), or, if the tissue is small enough (e.g.
plant seeds, Drosophila embryos), in the entire tissue (whole mount ISH), in cells and in circulating tumor cells (CTCs).
This is distinct from immunohistochemistry, which usually localizes proteins in tissue sections. DNA ISH can be used to
determine the structure of chromosomes.
FISH (Fluorescent ISH)
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SKY (Spectral Karyotyping)
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Spectral karyotyping is a molecular cytogenetic technique used
to simultaneously visualize all the pairs of chromosomes in an
organism in different colors. Fluorescently labeled probes for
each chromosome are made by labeling chromosome-specific
DNA with different fluorophores. Because there are a limited
number of spectrally-distinct fluorophores, a combinatorial
labeling method is used to generate many different colors.
Spectral differences generated by combinatorial labeling are
captured and analyzed by using an interferometer attached to a
fluorescence microscope. Image processing software then
assigns a pseudo color to each spectrally different combination,
allowing the visualization of the individually colored
chromosomes.
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SKY
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CGH (Comparative Genomic Hybridization)
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Comparative genomic hybridization (CGH) or
Chromosomal Microarray Analysis (CMA) is a molecularcytogenetic method for the analysis of copy number changes
(gains/losses) in the DNA content of a given subject's DNA
and often in tumor cells.
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CGH will detect only unbalanced chromosomal changes.
Structural chromosome aberrations such as balanced
reciprocal translocations or inversions cannot be detected,
as they do not change the copy number.
Region
1p13.2
Band
Sub-band
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Spermatogenesis
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Oogenesis
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