雜文: The Structure of Eukaryotic Chromosomes

http://www.cbs.dtu.dk/staff/dave/roanoke/genetics980218.html

Biology 210
GENETICS
18 February, 1998

Chapter 6c

The Structure of Eukaryotic Chromosomes

Part 2: DNA Compaction
in chromatin and chromosomes

A Brief Outline

6.4a Introduction.

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.

6.4a Introduction

Figure 6_0 from Griffiths et al. - Lampbrush 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.

Figure 16-7 from Griffiths et al., 1996.

6.4b The Nucleosome is the Basic Structure Unit in Chromatin

The first level of compaction is where the DNA wraps around nucleosomes.
Griffiths et al., 1996

On the Histone family of proteins:

There are 4 histone components of the nucleosome:

histone	evolutionary conservation
H2A	moderately conserved
H2B	moderately conserved
H3	very conserved
H4	nearly 100% conserved

Most organisms contain many copies of the histone genes, in tandem repeats scattered through the chromosomes.
Figure 16-23 from Griffiths et al., 1996

6.4c Nucleosome Core Particles

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).
Figure 6-8 from Hartl & Jones, 1998

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.

Figure 6_11 from Hartl & Jones, 1998

Figure 16_5b from Griffiths et al., 1996

This 30 nm fiber is then further compacted:
Figure 16-8 from Griffiths et al., 1996

The chromosome condensation is a dynamic process, as can be seen by the following drawing of chromosomes in meiotic prophase from a protozoan:
Figure 16_6 from Griffiths et al., 1996

Figure 16_6 from Griffiths et al., 1996

6.4x DNA sequences affecting chromosome condensation

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 disease.

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.

Expansion 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
repeat sequences found adjacent to the nucleosomal dyad. Expansion of these CGG repeats abolishes the preferential nucleosome assembly due to methylation. Thus, the expansion and methylation of these triplet repeats can alter the functional organization of chromatin, which may contribute to alterations in the expression of the FMR1 gene and the disease phenotype.

PMID: 8798682, UI: 96394576

6.5 Polytene Chromosomes