Transcript Packaging of DNA in chromosomes

Packaging of DNA in Chromosomes
DNA is about 3 meters long and
it has to be packed in a nucleus,
DNA
which is only a few micrometers in
a diameter.
Nucleosome
Hence highly coiled structure is
required.
First order of packaging:
30 nm
Nucleosomes
Second order of packaging:
Metaphase
chromosome
solenoid fiber
Scaffold
Scaffold fiber
Chromatid
Chromosome
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Nucleosomes: the first order of packaging
• Nucleosomes are the fundamental repeating subunits of all eukaryotic
chromatin (except when packaged in sperm)
• They are made up of DNA and four pairs of protein called histone, and
resemble “beads on a string” when observed with an electron microscope.
• They represent the first order of DNA compaction in the chromosome.
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Evidence for Existence
Purified chromatin
Nuclease treatment –
limiting conditions
Nuclease treatment-non
limiting conditions
Degrade protein and analyze
DNA by gel electrophoresis
Lane 1: DNA marker
Lane 2: Bands of 200 bp, 400 bp etc.
Lane 3: Single Band of 146 bp
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Assembly of Nucleosomes
•
H4
•
H3
H2A
H2B
•
Histone octamer bears N
terminal extension called “tails”
Tails not required for assembly
or DNA binding
They are sites for histone
modification
i.e.
phosphorlylation, acetylation,
methylation etc.
H2A-H2B
dimer
H3-H4
dimer
H3-H4
tetramer
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Histone H1
•
H1 interacts with the linker DNA between
nucleosomes, further tightening the association
of the DNA with nucleosomes
•
Histone H1 has the unusual property of binding
two distinct regions of the DNA duplex.
•
One of two regions bound by H1 is the linker
DNA at one end of the nucleosome. The second
site of DNA bonding is in the middle of the
associated 147 base pair.
•
By binding these two regions of DNA into close
proximity, H1 binding increase the length of
DNA wrapped tightly around histone octamer.
H1 binding produces a more defined angle of
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DNA entry
and exit from the nucleosome.
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30 nm Fiber
•
Binding of H1 stabilizes higher order chromatin structure. 30 nm fiber, represents
the next level of DNA compaction.
•
There are two models for structure of 30 nm fiber:
Solenoid model:
In this model, the flat surfaces on either side of histone octamer discs are adjacent
to each other and DNA surface of nucleosome forms outside accessible surfaces of
super helix. The linker DNA is buried in center of super helix, but it never passes
through the axis of fiber. Rather the linker DNA circles around the central axis as
DNA moves from one nucleosome to the next.
zig zag model:
In this case, 30 nm fiber is compacted form of these Zig zag nucleosome arrays. It
requires the linker DNA to pass through the central axis of the fiber in a relatively
straight form. Thus, longer linker DNA favours this conformation.
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Tails are required for formation of 30 nm
fiber
•
The most likely role of tails is to stabilize the 30 nm fiber by interacting with
adjacent nucleosomes.
•
This model is supported by three dimensional structure of nucleosome,
which shows that amino terminal tails of H2A, H3 and h4 each interact with
adjacent nucleosomes in the crystal lattice
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Additional folding required to fit long
DNA
•
Together packaging of DNA into Nucleosomes and 30 nm fiber results in the compaction
of the linear length of DNA by approximately 40 fold. This is still insufficient to fit 1-2
meters of DNA into a nucleus approximately 10-5 meters across.
•
One popular model proposed that the 30 nm fiber forms loops of 40-90 kb that are held
together at their bases by a proteinaceous structure referred to as nuclear Scaffold.
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•
Two classes of proteins that contribute to nuclear scaffold:
Topoisomerase II: abundant in both scaffold preparations and purified mitotic
chromosomes.
SMC proteins: Abundant component of nuclear scaffold. These proteins are key
components of machinery that condenses and holds daughter chromosomes
together after chromosome duplication. The association of these proteins with
nuclear scaffold may serve to enhance their functions by providing an underlying
foundation for their interactions with chromosomal DNA.
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Metaphase chromosomes
•
When the nucleus divides, the DNA adopts a more compact form of packaging,
resulting in the highly condensed metaphase chromosomes that can be seen with the
light microscope and which have the appearance generally associated with the word
“chromosome”
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• Individual chromosomes can therefore be recognized because of the lengths
of their chromatids and the location of the centromere relative to the
telomeres. Set of chromosomes possessed by an organism can be represented
as karyogram, in which the banded appearance of each one is depicted.
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•
The human karyogram is typical of that of the great majority of eukaryotes, but some
organisms display unusual features not displayed by the human version. These
include:
 Minichromosomes: are relatively short in length but rich in genes. The chicken
genome, for example, is split into 39 chromosomes: 6 macrochromosomes containing
66% of the DNA but only 25% of the genes, and 33 minichromosomes containing the
remaining one–third of the genome and 75% of the genes.
 B chromosomes: are additional chromosomes possessed by some individuals in a
population, but not all. B chromosomes appear to be fragmentary versions of normal
chromosomes that result from unusual events during nuclear division. Some certain
genes, often for rRNAs, but it is not clear if these genes are active.
 Holocentric chromosomes: do not have a single centromere but instead have multiple
structures spread along their length. The nematode Caenorhabtitis elegans has
holocentric chromosomes.
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•
Metaphase chromosome consist of centromere
and a telomere at ends.
•
Centromere bears a kinetochore which acts as a
site for attachment for spindle fibres which
draw divided chromosomes into daughter
nuclei.
•
Special telomeric sequences at ends function in
preventing chromosome shortening.
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QUERIES
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