Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

FROM DNA TO RNA
331
ribosomal
proteins
made in
cytoplasm
5S rRNA
TRANSCRIPTION
MODIFICATION
AND PROCESSING
OF rRNAs
large
ribonucleoprotein
particle
NUCLEOLUS
loop of chromosomal DNA
rRNA gene
45S rRNA
precursor
snoRNAs
RECYCLING OF RNAs
AND PROTEINS
INVOLVED IN
rRNA PROCESSING
proteins
involved in
processing
of rRNA
Figure 6–45 The function of the
nucleolus in ribosome and other
ribonucleoprotein synthesis. The 45S
precursor rRNA is packaged in a large
ribonucleoprotein particle containing
many ribosomal proteins imported from
the cytoplasm. While this particle remains
at the nucleolus, selected components
are added and others discarded as it is
processed into immature large and small
ribosomal subunits. The two ribosomal
subunits attain their final functional form
only after each is individually transported
through the nuclear pores into the
cytoplasm. Other ribonucleoprotein
complexes, including telomerase shown
here, are also assembled in the nucleolus.
telomerase
proteins
telomerase
telomerase
RNA
immature large
subunit
CYTOPLASM
NUCLEUS
small
subunit
large
subunit
TRANSPORT AND
FINAL ASSEMBLY
OF RIBOSOMES
40S
subunit
60S
subunit
The Nucleus Contains a Variety of Subnuclear Aggregates
Although the nucleolus is the most prominent structure in the nucleus, several
other nuclear bodies have been observed and studied (Figure 6–46). These
include Cajal bodies (named for the scientist who first described them in 1906)
and interchromatin granule clusters (also called “speckles”). Like the nucleolus,
these other nuclear structures lack membranes and are highly dynamic depending
on the needs of the cell. Their assembly is likely mediated by the association of
low complexity protein domains, as described in Chapter 3 (see Figure 3–36). Their
appearance is the result of the tight association of protein and RNA components
involved in the synthesis, assembly, MBoC6 and m6.47/6.45 storage of macromolecules involved in
gene expression. Cajal bodies are sites where the snRNPs and snoRNPs undergo
their final maturation steps, and where the snRNPs are recycled and their RNAs
are “reset” after the rearrangements that occur during splicing (see p. 321). In
contrast, the interchromatin granule clusters have been proposed to be stockpiles
of fully mature snRNPs and other RNA processing components that are ready to
be used in the production of mRNA.
Scientists have had difficulties in working out the function of these small subnuclear
structures, in part because their appearances can change dramatically as
cells traverse the cell cycle or respond to changes in their environment. Moreover,