11.3 Nuclear Bodies Flashcards
() compartmentalize specific processes within the nucleus
Nuclear bodies
similarity between cytoplasmic organelles and nuclear bodies
they compartmentalize the nucleus and serve to concentrate proteins and RNAs that function in specific nuclear processes
main difference between nuclear bodies and cytoplasmic organelles
nuclear bodies are not enclosed by membranes; instead, they are maintained by protein-protein and protein-RNA interactions
The () is the site of rRNA transcription, rRNA processing, and ribosome assembly
nucleolus
The nucleolus is organized around
nucleolar organizing regions
nucleolar organizing regions are the chromosomal regions that contain the genes for the
5.8S, 18S, and 28S rRNA
the transcription of () leads to the fusion of small prenucleoloar bodies that contain processing factors and other components of the nucleolus
45S pre-rRNA
morphologically, nucleoli consist of 3 regions, and in each region:
- fibrillar center - where rRNA transcription occurs
- dense fibrillar component - where rRNA processing occurs
- granular component - where ribosome assembly occurs
the size of the nucleolus depends on the () of the cell
metabolic activity
large nucleoli are found in cells actively engaged in
protein synthesis
variation in nucleolus size is mainly due to the differences in the size of the (1), reflecting the levels of (2)
- granular component
- ribosome assembly
nucleoli contain more than 300 proteins and about 200 () that function in pre-rRNA cleavage, ribose methylation, and pseudouridylation
small nucleolar RNAs (snoRNAs)
overview of pre-rRNA processing
- genes that encode ribosomal proteins are transcribed outside the nucleolus by RNA polymerase II, yielding mRNAs that are translated on cytoplasmic ribosomes
- the ribosomal proteins are then transported from the cytoplasm to the nucleolus, where they are assembled with rRNAs (5S, 5.8S, 18S, and 28S) to form pre-ribosomal particles
- the pre-ribosomal particles are then exported to the cytoplasm, forming the active 40S and 60S subunits of eukaryotic ribosomes
clustered sites of gene repression
Polycomb bodies
proteins that are responsible for transcriptional repression of a variety of genes
Polycomb proteins
how do Polycomb proteins repress transcription
methylation of histone H3 lysine 27 residues
Polycomb proteins are composed of 2 complexes:
- a complex that binds to methylated H3 lysine 27 residues
- an enzyme that methylates adjacent H3 lysine 27 residues
methylation of H3 lysine 27 residues can () to generate domains of repressed chromatin
spread to adjacent nucleosomes
domains in which polycomb proteins in the nucleus are localized; they are frequently associated with heterochromatin, consistent with their role in gene repression
Polycomb bodies
small nuclear bodies that are involved in assembly of snRNPs and other RNA-protein complexes; this is also where snRNPs are concentrated in once they are imported to the nucleus
Cajal bodies
Cajal bodies are where the final stages of snRNP maturation occur; this includes
modification of snRNAs (ribose methylation and pseudouridylation)
the enzyme responsible for RNA methylation () is concentration in Cajal bodies as well as in nucleoli
fibrillarin
(), like snoRNAs serve as guide RNAs to direct the ribose methylation and pseudouridylation of snRNAs in Cajal bodies
Cajal-specific RNAs (scaRNAs),
how are Cajal bodies said to have a role in telomerase assembly
they may promote the assembly of the RNA-protein telomerase complex as well as facilitate its delivery to telomeres
discrete nuclear bodies containing components of the mRNA splicing machinery; this is where matured snRNPs are transferred to
nuclear speckles
