11.3 Nuclear Bodies

Question 1

() compartmentalize specific processes within the nucleus

Answer 1

Nuclear bodies

Question 2

similarity between cytoplasmic organelles and nuclear bodies

Answer 2

they compartmentalize the nucleus and serve to concentrate proteins and RNAs that function in specific nuclear processes

Question 3

main difference between nuclear bodies and cytoplasmic organelles

Answer 3

nuclear bodies are not enclosed by membranes; instead, they are maintained by protein-protein and protein-RNA interactions

Question 4

The () is the site of rRNA transcription, rRNA processing, and ribosome assembly

Answer 4

nucleolus

Question 5

The nucleolus is organized around

Answer 5

nucleolar organizing regions

Question 6

nucleolar organizing regions are the chromosomal regions that contain the genes for the

Answer 6

5.8S, 18S, and 28S rRNA

Question 7

the transcription of () leads to the fusion of small prenucleoloar bodies that contain processing factors and other components of the nucleolus

Answer 7

45S pre-rRNA

Question 8

morphologically, nucleoli consist of 3 regions, and in each region:

Answer 8

1. fibrillar center - where rRNA transcription occurs
2. dense fibrillar component - where rRNA processing occurs
3. granular component - where ribosome assembly occurs

Question 9

the size of the nucleolus depends on the () of the cell

Answer 9

metabolic activity

Question 10

large nucleoli are found in cells actively engaged in

Answer 10

protein synthesis

Question 11

variation in nucleolus size is mainly due to the differences in the size of the (1), reflecting the levels of (2)

Answer 11

1. granular component
2. ribosome assembly

Question 12

nucleoli contain more than 300 proteins and about 200 () that function in pre-rRNA cleavage, ribose methylation, and pseudouridylation

Answer 12

small nucleolar RNAs (snoRNAs)

Question 13

overview of pre-rRNA processing

Answer 13

1. genes that encode ribosomal proteins are transcribed outside the nucleolus by RNA polymerase II, yielding mRNAs that are translated on cytoplasmic ribosomes
2. 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
3. the pre-ribosomal particles are then exported to the cytoplasm, forming the active 40S and 60S subunits of eukaryotic ribosomes

Question 14

clustered sites of gene repression

Answer 14

Polycomb bodies

Question 15

proteins that are responsible for transcriptional repression of a variety of genes

Answer 15

Polycomb proteins

Question 16

how do Polycomb proteins repress transcription

Answer 16

methylation of histone H3 lysine 27 residues

Question 17

Polycomb proteins are composed of 2 complexes:

Answer 17

1. a complex that binds to methylated H3 lysine 27 residues
2. an enzyme that methylates adjacent H3 lysine 27 residues

Question 18

methylation of H3 lysine 27 residues can () to generate domains of repressed chromatin

Answer 18

spread to adjacent nucleosomes

Question 19

domains in which polycomb proteins in the nucleus are localized; they are frequently associated with heterochromatin, consistent with their role in gene repression

Answer 19

Polycomb bodies

Question 20

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

Answer 20

Cajal bodies

Question 21

Cajal bodies are where the final stages of snRNP maturation occur; this includes

Answer 21

modification of snRNAs (ribose methylation and pseudouridylation)

Question 22

the enzyme responsible for RNA methylation () is concentration in Cajal bodies as well as in nucleoli

Answer 22

fibrillarin

Question 23

(), like snoRNAs serve as guide RNAs to direct the ribose methylation and pseudouridylation of snRNAs in Cajal bodies

Answer 23

Cajal-specific RNAs (scaRNAs),

Question 24

how are Cajal bodies said to have a role in telomerase assembly

Answer 24

they may promote the assembly of the RNA-protein telomerase complex as well as facilitate its delivery to telomeres

Question 25

discrete nuclear bodies containing components of the mRNA splicing machinery; this is where matured snRNPs are transferred to

Answer 25

nuclear speckles