Chapter 19
DNA Organization in Chromosomes
In comparison with eukaryotes, the
chromosomes of viruses and bacteria are much
less complicated.
Usually consist of a single nucleic acid molecule,
largely devoid of associated proteins, and
contain relatively little genetic information
Chromosomes of viruses consist of either DNA
or RNA and can be either single or double
stranded.
They may exist as circular structures (closed
loops) or they may take the form of linear
molecules.
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The chromosomes of bacteria are also
relatively simple.
Always consist of a double-stranded DNA
molecule compacted into a structure sometimes
referred to as the “nucleoid”.
DNA in bacterial chromosomes is found to be
associated with several types of “DNA-binding
proteins”.
Two such proteins, HU and H, are small but
abundant in the cell and contain a high
percentage of positively charged amino acids
that can bond ionically to the negative charges
of the phosphate groups in DNA.
These proteins are structurally similar to
molecules called “histones” that are associated
with eukaryotic DNA.
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DNA is organized into chromatin in eukaryotes -Structure and organization of DNA in
eukaryotes is more complex than viruses and
bacteria due to (1) greater amount of DNA per
chromosome and (2) larger number of proteins
associated with eukaryotes.
During interphase, the genetic material and
associated proteins are uncoiled and dispersed
throughout the nucleus as “chromatin”.
A substantial amount of protein is associated
with the chromosomal DNA in all phases of
the eukaryotic cell cycle.
The associated proteins are divided into two types:
1. Positively charged HISTONES
2. Less positively charged NONHISTONES
Of these 2 types of proteins, histones play the
more important role
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Histones contain large amounts of the
positively charged amino acids lysine and
arginine which allows them to bond to the
negatively charged phosphate groups of
nucleotides.
Basic model for chromatin structure:
1. Digestion of chromatin by certain
endonucleases yields DNA fragments
of about 200 bp in length or multiples
thereof.
2. Electron microscopy revealed that
chromatin fibers are composed of linear
arrays of spherical particles. These are
now called nucleosomes and confirm the
repeating structure.
3. Studies of interactions of histone molecules
and DNA in nucleosomes show that histones
H2A, H2B, H3, and H4 occur as two types of
tetramers: (H2A)2 -- (H2B)2 & (H3)2 -- (H4)2
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Each repeating nucleosome unit consists of
of of each tetramer in association with about
200 bp of DNA.
4. When nuclease digestion is extended, some of
the 200 bp of DNA are removed from the
nucleosome, creating a “nucleosome core
particle” consisting of 196 bp. This number
is consistent in all organisms examined.
This DNA is responsible for linking
nucleosomes together and is associated with
the fifth histone, H1.
5. Thus, the 196 bp DNA core is coiled around
an octamer of histones in a left-handed
superhelix, which completes about 1.7 turns
per nucleosome
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Why is this packaging of DNA necessary?
Nucleus of a human cell is 5 - 10 µm in diameter.
Haploid genome contains 3.2 X 109 bp of DNA
distributed among 23 chromosomes.
The diploid genome contains 2X this amount.
At 0.34 nm per bp, this equals almost 2 meters
of DNA!!!
One estimate suggests about 25 X 106
nucleosomes per nucleus.
[FIGURE 19.12]
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