Chromatin organization
Nuclei
and other membrane-bound organelles are absent in prokaryotic cells. The
nucleoid is the region of a prokaryotic cell in which the chromosomal DNA is present.
This arrangement is not simple, however, especially considering that chromosome
of E. coli is much larger than the cell itself.
Most
prokaryotes do not contain histones (except those species in the domain
Archaea). Thus, supercoiling is one way through which prokaryotes compress
their DNA into smaller spaces.
Genomes
can be negatively supercoiled. It means that the DNA is twisted in the opposite
direction of the double helix. Or genomes can also be positively supercoiled. It
means that the DNA is twisted in the same direction as the double helix. Majority
of bacterial genomes are negatively supercoiled during normal growth.
Multiple
proteins work together in order to fold and condense prokaryotic DNA. One
protein called HU, the most abundant protein in the nucleoid. It works with an
enzyme called topoisomerase I in order to bind DNA and introduce sharp bends in
the chromosome, generating the tension essential for negative supercoiling.
— Studies have also revealed that other proteins,
including integration host factor (IHF), can bind to specific
sequences within the genome and introduce additional curves.
— The
folded DNA is then arranged into different conformations that are supercoiled
and wound around tetramers of the HU protein, much like eukaryotic chromosomes
are wrapped around histones.
Eukaryotic DNA Packaging
— The
human genome comprises of all the DNA present in the cell. It can be categorized into the nuclear genome
and the mitochondrial genome (16.6 kb).
— If the double helices consisting of all 46 chromosomes
in a human cell could be placed end-to end, they would reach just about 2
meters (6 feet); but the nucleus, which contains the DNA, is only about 10 µm
in size
— The specialized proteins which bind to and fold the
DNA, generating a series of coils and loops that produce increasingly higher
levels of organization, restricting the DNA from becoming an unmanageable
tangle.
— the DNA present in the nucleus is divided between a
set of distinct chromosomes. For example, the human genome which is approximately
3.2 x 10e 9 nucleotides is distributed over 24 different chromosomes
— Chromosome contain single, very long linear DNA molecule
associated with proteins that fold and pack the fine DNA thread into a more
compact structure
Nucleosome- solenoid model of chromatin fibers
— In 1974, this model was proposed by Kornberg
and Thomas and is extensively accepted model.
— Chromatin is complex of histone and non-histone protein
with the nuclear DNA of eukaryotic cells.
— According to this model, chromatin fibers comprise of
discrete particles called nucleosomes which produce a bead like
structure. The thread is composed of the DNA molecule; wound around the beads.
— Nucleosome is the basic structure of chromatin fiber comprise of
DNA wound around an “octamer” of histone proteins.
— The Linker or Spacer DNA is stretch of the DNA between
two nucleosomes.
— Nucleosome comprises of ~ 200bp DNA associated with
histone octamer. Histone octamer composed of the histone H2A, H2B, H3, H4. the
associated DNA and histone octamer together constitute the core particle.
Chromatin organization| Nucleosome- solenoid model of chromatin fibers, Eukaryotic DNA Packaging
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