Biology Reference
In-Depth Information
Figure 3.1
DNA is condensed in eukaryotes by being packaged in nucleosomes, the first level of
condensation. A nucleosome consists of core DNA wound around two molecules each of the his-
tones H2A, H2B, H3, and H4. These eight histone molecules are called an octamer. Nucleosomes
are connected to other nucleosomes by linker DNA. In addition, a single molecule of the histone H1
(not shown) binds in the linker and helps to condense the nucleosome. Nucleosomes are organized
into structures called 30-nm fibers (see text and
Figure 3.2
).
by restriction enzymes, but the linker DNA is vulnerable to these enzymes.
The core DNA is wound
≈
1.75 times around a histone octamer, consisting of
two molecules each of H2A, H2B, H3, and H4. A single molecule of histone H1
is associated with the linker DNA and is apparently helpful in compacting the
nucleosome. Linker DNA lengths vary from 20 to 100 nucleotide (nt) pairs in dif-
ferent species and in different cell types within the same organism.
Origins of DNA replication in higher eukaryotes are larger and more complex
than in prokaryotes (see Chapter 1). However, the underlying molecular pro-
cesses are conserved (
Benbow et al. 1992
).
3.7 Packaging Long, Thin DNA Molecules into Tiny Spaces
Eukaryotes have to solve a serious packaging problem in their nuclei. If eukary-
otic chromosomes were simply linear DNA molecules, the average length might
be
≈
5cm. If these long, thin chromosomes were tangled together inside the
nucleus, replication would be difficult and separation of the intertwined chro-
mosomes during mitosis could result in breakage of the chromosomes and
subsequent loss of essential genetic information. Therefore, DNA needs to be
condensed, yet packaged so that both DNA replication and transcription can
occur without loss or damage. DNA packaging is achieved by a highly organized
and hierarchical condensation scheme (
Figure 3.2
).
Eukaryotic DNA is
supercoiled
, i.e., the double helix is twisted around itself,
which begins the condensation process. The next level of compaction is achieved
by organizing the DNA into nucleosomes. Nucleosomes occur in a regular pat-
tern, with linker or intervening DNA between each nucleosome (
Figure 3.2B
).
