Download Histone depleted metaphase chromosomes Scaffold Attachment

Centromeres
Heterochromatin
Kinetochore - spindle fiber attachment
No universal DNA sequences
Repeat sequences - CENs - 5 to 170 bp –
humans, alphoid satellite repeats
Specific associated proteins
Telomeres
Telomeres
•About 15,000 bp in humans
•Lagging strand problem
•Telomere shortening
•Hayflick – and then we die
•Cancer, AIDs, stem cells
The structure and organization of
chromosomes change during the
course of the cell cycle.
Nonrandom chromosome positioning
• Gene rich chromosomes toward center
• Gene poor chromosomes toward
periphery
• Centromeres are not the determining
factor
• Chromosomes with adjacent positions
more likely to interact cytolologically
How porous are territories?
• Previously it was predicted that
active genes would be located at the
surface of chromosome territories.
• Now, it appears that they are also in
the interior
• Some are also found on loops
outside of the territory
In order for DNA to physically fit into a
nucleus, several levels of packing are
necessary.
How is the DNA packaged in this way and
how is it available for replication and
transcription if it is tightly packed?
The 4 nm DNA filament
Histones
• folding and coiling chromosomes
• 45% of the total mass
• 60 million molecules of each type per
cell
• Histones are highly conserved across all
eukaryotic organisms
• Histones are small basic proteins (102-135 aa)
rich in lysine and arginine
• Each histone contains a region that folds in a
characteristic structure called the histone fold
and a tail region
• Tail region is post translationally modified in
various ways to control many aspects of
chromatin structure
Core particle - Approximately 1.8 turns of DNA
(consisting of 146 bp) wound around the
outside of the histone octamer.
Histone “tails” stick out between the coil of DNA
Post-translational chemical modification of the tails controls function.
Modification patterns comprise the “histone code”.
The 10 nm nucleoprotein fibril
Nucleosomes + 60 (approx) bp DNA
A eukaryotic gene of 10,000 bp will be
associated with about 50 nucleosomes. A
human cell contains 3 x 107 nucleosomes.
One molecule of histone H1 and the
nucleososome is sometimes referred to as the
chromatosome.
Note that other
chromatin modifying
complexes include
kinases, methylases
and ubiquitin
conjugating proteins.
Acetylation typically
correlates with
transcriptional
activation while
deacetylation
correlates with
repression.
• Non-histone proteins (NHPs, acidic
proteins, nonhistone chromosomal
proteins, NHC proteins)
– help regulate DNA transcription and
replication
– at least 30 types
“Chromatin remodeling complexes” and “Chromatin
modifying complexes” are important for transcriptional
activation
Chromatin
modifying
complex
Chromatin decondensation appears to require two types
of protein complexes each made of several polypeptide
subunits:
1. Histone acetylase complexes.
These are often referred to as HATS for histone
acetylases.
2. Chromatin remodeling factors.
These are often refered to as Swi/Snf factors
because they were first identified as yeast mutants
defective in mating type switching and in the ability to
metabolize sucrose (sucrose non-fermenting).
Chromatin remodeling factors use energy from
ATP hydrolysis to rearrange the packing of
nucleosomes in higher order chromatin structures
There are several different chromatin remodeling
complexes in cells.
Some of these bind to activation domains and
de-condense the associated chromatin.
Some bind to repression domains and condense
the associated chromatin.
Most interphase chromatin is condensed into 30nm coils.
Most interphase chromatin
is condensed into 30nm coils.
A model for the structure of
an interphase chromosome
Lampbrush chromosomes (amphibian oocyte, immature eggs)
In mitotic chromosomes, a scaffold is formed, and
matrix attachment sites correspond to sites of
chromatin attachment to the scaffold.
Histone depleted metaphase chromosomes
Scaffold Attachment Regions (SARs)
Regions of the chromosomes with sequences specific for
topoisomerase, HMG protein, and histone H1 binding
Found only in untranscribed regions of the eukaryotic chromosomes
Spaced along the chromosomes, with the intervening
regions containing one or more genes?
Highly AT rich (65%) and may be several hundred bp
long
Discrete loops of DNA appear to be connected
at both ends to the scaffold
Quaternary structure:
final folding into chromosome shape