18 February, 1998
6.4b The Nucleosome is the Basic Structure Unit in Chromatin.
6.4c Nucleosome Core Particles.
6.4d The Arrangement of Chromatin Fibers in a Chromosome.
6.4e DNA sequences affecting chromosome condensation.
6.5 Polytene Chromosomes.
Just like in bacteria, the DNA in eukaryotes is highly compacted (roughly 7000x in mitotic chromosomes). However, unlike bacteria, in most eukaryotes, the DNA forms stable protein complexes. Here's a picture of a "lampbrush chromosome", which is thought to reflect the underlying orgainsation of all chromosomes, with a central scaffold (here stained brightly) and projecting lateral loops (stained red) formed by a folded continuous strand of DNA associated with histone proteins.
If you were to extract the DNA from a single, linear chromosome, and stretch it out, it would be one very long molecule (more than 2 cm for human chromosome 1).
The level of chromosome condensation can be monitored using electron microscopy. It is possible to control the condensation by varying the ionic strength of the solution.
Here is a very famous picture in which the chromosome (see the "X" in the middle) has been carefully manipulated such that all of the histone proteins have been removed - you can see all the DNA make loops to and from the central scaffold. There is an enormous amount of DNA in the chromosomes.
On the Histone family of proteins:
There are 4 histone components of the nucleosome:
|H4||nearly 100% conserved|
Most organisms contain many copies of the histone genes, in tandem repeats
scattered through the chromosomes.
The organisation of nucleosomes. The DNA molecule is wrapped around
the nucleosome about 2 times. The nucleosome actually consists of
a histone octamer, with two copies each of histones H2A, H2B, H3, and H4.
This is Figure 6.8 from you text (page 231).
6.4d The Arrangement of Chromatin Fibers in a Chromosome
Figure 6_10 from Hartl & Jones. The DNA is wrapped around the nucleosome core particles, which are then condensed into a 30 nm fiber. This then folds into a larger 300 nm fiber, which coils up into a 700 nm fiber, which then makes up the 1400 nm chromatid arm.
This 30 nm fiber is then further compacted:
The chromosome condensation is a dynamic process, as can be seen by
the following drawing of chromosomes in meiotic prophase from a protozoan:
Remember from last week, there are certain DNA sequences that can be
quite rigid, and some sequences can be flexible. There are some DNA
sequences which can facilitate chromatin condensation, and amplification
of these sequences results in large regions of chromatin that is very condensed.
One such motif is the CGG triplet repeat, which, when amplified causes
the DNA in the chromosome to form what is called a "fragile site", because
when viewed under the electron microscope, this region looks like it might
break off quite easily. In fact, chromosome breakage at this point
DOES occur quite often, and it is associated with many different types
of genetic diseases in humans. For example, "fragile X" syndrome
is the most common genetic form of mental retardation in humans - and the
molecular basis of this disease has been found to be due to the amplification
of a triplet repeat of the CGG sequence from about 20 copies (e.g., 60
bp long) to a repeat of up to several THOUSAND base pairs in length.
When the DNA gets amplified, the resulting change in chromosome structure
provides a "fragile site". Amplification of triplet repeats is also
responsible for more than 40 other diseases in humans, including Huntington
J Biol Chem 1996 Oct 4;271(40):24325-24328
Nucleosome assembly on methylated CGG triplet repeats in the fragile X mental retardation gene 1 promoter.
Godde JS, Kass SU, Hirst MC, Wolffe AP
Laboratory of Molecular Embryology, NICHHD, National Institutes of Health, Bethesda, Maryland 20892-5430, USA.
and methylation of CGG repeat sequences is associated with Fragile X syndrome
in humans. We have examined the consequences of CGG repeat expansion and
methylation for nucleosome assembly and positioning on the Fragile X Mental
Retardation gene 1 (FMR1) gene. Short unmethylated CGG repeats are not
particularly favored in terms of affinity for the histone octamer or for
positioning of the reconstituted nucleosome. However, upon methylation
their affinity for the histone octamer increases and a highly positioned
nucleosome assembles with the
PMID: 8798682, UI: 96394576
Last modified on: 2 February, 2000 by Dave Ussery