Prokaryotes have small genomes,
usually less than 10 7 bp. They get along just
fine carrying out replication, transcription and translation
simultaneously.
Eukaryotic genomes are huge,
ranging from 10 7
bp to 10 12
bp. As we saw in the
exercise, the eukaryotic nucleus has to pack and unpack a
tremendous quantity of chromatin once every cell cycle. That's
a lot of spaghetti to keep organized! For this reason,
eukaryotes need the elaborate mechanisms of mitosis and
meiosis.
The solution to this conflict is the eukaryotic chromosome.
Chromatin is DNA complexed with proteins. The chromosome
is a single DNA molecule complexed with proteins. It is organized
to allow for a hierarchal packing scheme.
Summary of Chromosome Structure
While there are multiple proteins that complex with DNA, histones are by far the predominant class of chromatin proteins. Histone proteins are responsible for helping DNA coil, as well as regulating transcriptionally "open" and "closed" genes. As they have an overall positive charge, they attract negatively-charged DNA. This charge attraction alone is sufficient for nucleosomes to form. There are five main histone proteins you should know:
Histone Protein | Molecular Weight (kD) | Major Amino Acids | Function |
---|---|---|---|
H1 | 21 | Lys++ | Linker histone |
H2A | 13.8 | Lys | Core histone |
H2B | 13.8 | Lys | Core histone |
H3 | 15.4 | Arg | Core histone |
H4 | 11.4 | Arg | Core histone |
MICROCOCCAL NUCLEASE DIGESTION OF
CHROMATIN [Spiker et al. (1983) PNAS 80:815 Fig. 4] (Note: this figure compares 3 alternative methods for chromatin isolation) Fig. 4. Micrococcal nuclease digestion of wheat embryo nuclei and chromatin. Nuclei and chromatin substrates were adjusted to 1µg of DNA/ml and digested with micrococcal nuclease at 50 units/ml at 37°C. Time course of digestion experiments were carried out for all samples. The DNA fragments were then purified and separated electrophoretically on agarose gels (pH8.1). A HaeIII-digested phi-X174DNA. B chromatin isolated by the method of Simon and Becker. A 6min. digest is shown; this extent of digestion and quantitiy of DNA applied to the gel showas a nucleosome pattern better than that found under any other conditions although the faint nucleosome pattern is barely noticeable against the background of variable length fragments produced by micrococcal nuclease. C chromatin isolated by the modified method of Bonner et al.; a 10 min. digest is shown. D nuclei isolated as described in the text; a 2-min. digest is shown. E HaeIII-digested Lambda phage DNA. |
The histone core particle is an octamer, and
contains 2 copies each of H2A, H2B, H3, and H4. H1 is a
linker histone that has three distinct regions: a basic (+),
random coil at the N-terminus, a central globular region,
and a highly conserved, highly basic C-terminus. DNA coils
around histone octamers to form nucleosomes.
Nucleosomes are the first stage of the coiling process, and
contain the histone octamer and two turns of DNA. Figure from http://en.wikipedia.org/wiki/Nucleosome |
The nucleosome structure includes
the core histone octomer plus 2 turns of DNA. Two rotated
views of the nucleosome are shown in ball-and-stick
representation. The two strands of the DNA double helix
are shown in light blue and cream color. The eight histone
proteins in the nucleosome core are each colored
differently. Wrapped
around these particles is two-turns of a DNA
helix. These are left-handed turns. They take up,
not surprisingly, 146 bp. These figure were produced from PDB entry 2CV5, using the NCBI Cn3D viewer. |
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Complementarity between
positive charges on the core protein and the DNA
phosphodiester backbones. The (H3-H4)2
is white and the H2A-H2B dimers are blue. the C-alpha
atoms of lysines and arginines on the cylindrical surface
are indicated by red, and the atoms marking the positions
of the positive end of helix dipoles are indicated by
orange. Atoms of the DNA backbone are "undersized" so as
to allow visualization of the protein surface. Nucleosomes can form spontaneously in-vitro The positive charges of the histone proteins and the negative charges of the DNA backbone are sufficient to drive nucleosome formation. In chromatin reconstitution experiments, histones H2a, H2b, H3 and H4 are added to DNA, resulting in nucleosome formation. Therefore, the process of nucleosome formation is solely due to charge. From: Moudreinakis, EN and Arents, G (1993) Structure of the Histone Octamer Core of the Nucleosome and Its Potential Interactions with DNA. in Cold Spring Harbor Symposia on Quantitative Biology Vol. LVIII DNA and Chromosomes.pp. 273-279. Cold Spring Harbor Press 1993. Fig. 6 |
NUCLEOSOMES:
"BEADS
ON A STRING" If you isolate nuclei (eg. Drosophila) and extract chromatin at low ionic strength (<100mM salt), you see "beads on a string" images in electron microscopy (a). These beads are nucleosomes (b). An interpretation of the structure is shown below: Two turns of DNA are coiled around each nucleosome, with a short region of naked DNA linking each nucleosome. Since the width of a nucleosome is about 10nm, these are called "10nm fibers". (In contrast, a DNA double helix is 2nm wide.)
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But is the 30 nm fiber
actually there in-vivo? For many years, it was thought that the 30 nm solenoid was the default structure for chromatin in the nucleus and in the mitotic chromosome. However, that assumption was based on the in-vitro evidence described above. There are new reports that indicate that in-vivo, the 30 nm fiber is probably not the predominant structure. Fussner et al. report that 3D images generated by combining electron spectroscopic imaging with 3D tomography find no evidence for 30 nm solenoides in mouse embryonic fibroblasts. Rather, all chromatin seemed to be in the 10 nm fiber configuration Further evidence from cryo-EM and X-ray scattering, indicate strong evidence for chromatin at periodicities both 6 nm and 10 nm, but no evidence for a periodicity of 30 nm, in both interphase chromatin and mitotic chromosomes. Tentative conclusion: The 30 nm fiber is most likely an artifact of salt concentrations used in chromatin reconstitution experiments. The current favored model is that the default state of chromatin in the nucleus, and in the mitotic chromatin, is the 10 nm fiber. |
Alternative model: Irregular folding of 10-nm
fibers. Maeshima, K., Imai, R., Tamura, S., & Nozaki, T. (2014). Chromatin as dynamic 10-nm fibers. Chromosoma, 123(3), 225–237. http://doi.org/10.1007/s00412-014-0460-2 Since the 30-nm fiber appears not to form at physiological salt conditions, one model proposes that 10-nm fibers associate in an unorganized fashion, governed by hydrophobic and hydrophilic interactions. The result is irregularly folded bundles of 10-nm fibers of chromatin. One advantage of this model is that it predicts that chromatin bundles will be less stable than the highly-organized 30-nm fibers would be. That means that bundles will be expected to open and close, to fold and unfold, giving transcriptional enzymes and DNA binding proteins better access to genes. Thus, transcription is easier to initiate on irregularly-folded chromatin, than it would be in 30-nm fibers. |
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Linker histones are important to coiling of
nucleosomes. Histone 5 is also a linker histone, that is
seen more often in transcriptionally active chromatin.
Histone H1 tends to be seen more in inactive chromatin. The
individual nucleosomes can be linked in three different
ways, as per the diagram at right. |
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In mamalian spermatids, the nuclear volume is
about 5% that of the somatic cell nucleus. To achieve this
level of packaging, an arginine- and cysteine-rich class of
proteins called protamines replace
histones. As illustrated in the figure, a round of
transcription occurs in haploid spermatids, producing
copious quantities of protamines, which displace histones in
mature spermatids. In mamalian spermatids, protamines do not
directly displace histones. Mamalian spermatids first
displace histones with basic "transition proteins", which
are in turn displaced by protamines. |
In this discussion about histone proteins and nucleosomes, you may have been wondering: if DNA is complexed with these proteins, won't that make it hard to transcribe DNA? The enzymes of the eukaryotic transcriptional apparatus are adapted to the presence of chromatin proteins. This is the price eukaryotic cells pay for having such large genomes. You can't have a large genome without having the extra overhead of organizing it, which is done by the chromatin proteins. Like everything else in eukaryotic cells, gene expression and replication are very deliberate and carefully orchestrated processes.
So, for gene expression to occur, the DNA can be complexed with proteins, but it has to be in a transcriptionally "open" position. How can we detect this "open" chromatin structure?
We can compare chromatin structure around the beta-globin gene of chick embryo erythroblasts (precursors to red blood cells), which do express globin genes, and an undifferentiated cell line MSB, which does not express globin genes. In erythroblasts, the DNA of the globin gene is preferentially sensitive to DNAseI digestion. This suggests that the structure of the chromatin is looser in cells expressing globin. |
Note: These treatments are done using isolated nuclei,
not naked DNA
Procedure: |
In the autoradiogram, we see that even at the highest DNAseI concentration, the 4.6kb fragment is relatively insensitive to digestion in nuclei from a cell line that does not express the globin gene. In the erythroblasts, however, specific degredation of this sequence can be seen to occur even at the lowest concentration. The interpretation of this is that the globin gene is in a more open chromatin configuration, which provides greater access to DNAseI. This phenomenon is referred to as general nuclease sensitivity, because the whole gene appears to be digested. However, more detailed studies of certain genes can detect specific sites within transcriptionally active genes that are hypersensitive to nuclease digestion. | From Lodish et al. Molecualr Cell Biology http://www.ncbi.nlm.nih.gov/books/bookres.fcgi/mcb/ch9f32b.gif |
The Adh1 (alcohol dehydrogenase) gene in maize can be induced (ie. made to produce mRNA) by anaerobic conditions. This helps the plant detoxify the end products of anaerobic respiration. In cell culture, anaerobic conditions can be simulated by bubbling Ar into cell suspensions for 2hr. This system permits the comparison of chromatin structure of the Adh1 gene between anaerobic and aerobic conditions. Below is a restriction map of the region near the Adh1 gene in maize. (Start of transcription is +1). We can map hypersensitive sites by indirect end-labeling. Note that in the general sensitivity experiment, we used a probe that overlapped the entire gene in question. Since it is possible that many sites in a given gene would be digested simultaneously by DNAseI, the Bam fragment above was chopped into pieces too small to detect on an agarose gel. But with indirect end labeling, we use a short probe that lays outside the region we wish to examine, from +1 to +210.
M is a
marker lane, with a mixture of DNAs generated by
digesting this gene with either BamH1, Xba1, Pst1
or Alu1, and then mixing all of these DNAs together.Now, when the DNAse-ed
DNA is digested with Hind3, the +1 to +210 probe will only
detect fragments upstream from the Hind3 site. If we do a
limiting digestion with DNAseI, in any given
fragment, only one of several possible hypersensitive
sites will be digested in a given molecule. Thus, we
get a "ladder", in which each rung represents a different
cut site. Note that there are two constitutive
hypersensitive sites, even in uninduced (aerobic) nuclei,
whereas in induced nuclei, we see 8 distinct
hypersensitive sites.
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from Anna-Lisa Paul, Vimla Vasil, Indra K. Vasil, and Robert J. Ferl. Constitutive and anaerobically induced DNase-I-hypersensitive sites in the 5′ region of the maize Adh1 gene. PNAS 84:799-803 |
So, we can identify regions that are in a more transcriptionally "open" position. But what does that "open" configuration look like with the histones? We know from electron micrograph data that the 10nm beads on a string configuration is present during transcription. |
What's happening here? Let's start with two hypotheses.
As a means of detecting whether or not DNA is bound up with histones, crosslinking mediated by psoralen was used to covalently crosslink any DNA that was accessible when cells were treated with UV. The spacer DNA between nucleosomes is more accessible to crosslinking, resulting covalent bonds between the two strands. When chromatin is isolated for Electron Microscopy, the proteins are lost, and only the DNA remains on the grid. The crosslinked spacer regions remain as doublestranded helix. The DNA that was complexed with histones becomes single-stranded, giving appearance of bubbles on the circular chromosome. Thus, each bubble tells us which DNA was complexed into nucleosomes. Since we see bubbles on the entire SV40 genome, and we also see an RNA transcript coming off of the circle, with nucleosomes on either side of the transcription site, we can conclude that nucleosome structure remains intact even during transcription. |
Nucleosomes seem to protect little bubbles of DNA in transcribing genes from treatment with DNA crosslinking reagents (eg. 6-methyl psoralen). The bubbles can be shown to be roughly 200bp apart. Only the inter-nucleosome DNA is crosslinked. The long "tail" on the circle is the growing mRNA transcript.
So how does the DNA become accessible to RNA polymerase while
staying complexed with histone proteins? The answer is a process
called chromatin remodeling.
Chromatin
remodeling:
insights and intrigue from single-molecule studies
Bradley R Cairns. Nature Structural & Molecular Biology 14, 989 - 996 (2007) Published online: 5 November 2007 doi:10.1038/nsmb1333 The precise details of chromatin remodeling are still under investigation. However, the figure at right illustrates one model for chromatin remodeling. In (d), D and Tr domains of the ocatmer "walk" along the double helix by rotating around the Hinge domain. Thus, while the DNA is more accessible to the solvent, and hence, more sensitive to nucleases, the nucleosome never completely dissociates from the DNA helix. Image displayed by hypertext link to https://media.springernature.com/lw685/springer-static/image/art%3A10.1038%2Fnsmb1333/MediaObjects/41594_2007_Article_BFnsmb1333_Fig3_HTML.gif?as=webp |