Chapter 11 - The Nucleus
In humans, pre-ribosomal RNA consists of a large 45S RNA that contains nucleotide sequences corresponding to _______ different individual rRNAs found within ribosomes.
3
How has chromosome conformation capture (3C) analysis contributed to the identification of interactions within chromosomes?
3C has revealed interacting regions of chromosomes in living cells. Most interactions identified occur between regions on the same chromosome.
What is the size (in Svedberg units) of the pre-rRNA primary transcript?
45S
In addition to the rRNAs derived from the pre-rRNA primary transcript, what other components make up the two ribosomal subunits?
5S rRNA and multiple ribosomal proteins
Entry into the nucleus is limited by the _______ and the _______ complexes.
: nuclear envelope (nuclear membranes); nuclear pore
By restricting the traffic of proteins and RNA across the nuclear envelope, eukaryotic cells can regulate _______ in unique ways. a. translation and transcription b. transcription only c. translation only d. endocytosis
A
Chromatin that contains sequences that are transcribed is called a. euchromatin. b. a centromere. c. heterochromatin. d. a telomere.
A
Initial steps in the assembly of functional snRNPs occur in the a. cytoplasm. b. nucleolus. c. nucleoplasm. d. PML body.
A
Small molecules (< 20 kd) enter nuclei by way of a. passive diffusion through nuclear pore complexes. b. selective transport through nuclear pore complexes. c. active transport across the nuclear envelope membranes. d. passive diffusion across the nuclear envelope membranes.
A
The directionality of nuclear protein import is determined by a. an unequal distribution of Ran/GTP. b. importin. c. an ion gradient. d. the nuclear lamina.
A
The filaments of the nuclear lamina are composed of a type of a. intermediate filament. b. microtubule. c. microfilament. d. thick filament.
A
The nuclear basket protein complex is a component of the nuclear pore complex found a. inside the nucleus. b. buried in the nuclear envelope. c. on the cytoplasmic face of the pore. d. lining the central channel of the pore.
A
The nuclear lamina binds to a. both the inner nuclear envelope membrane and the chromatin. b. the inner nuclear envelope membrane only. c. the chromatin only. d. both the inner and outer nuclear envelope membranes.
A
The nuclear lamina is built of proteins called a. lamins. b. laminins. c. nucleons. d. nucleoplasmins.
A
The nucleolus is the site where a. ribosomal RNA is transcribed and ribosomes are partially assembled. b. DNA replication occurs. c. proteins recently imported from the cytoplasm are deposited. d. translation occurs.
A
The nucleolus is the site where _______ is(are) assembled. a. ribosomal subunits b. nuclear pores c. chromatin d. the nuclear matrix
A
The outer nuclear membrane is contiguous with the a. endoplasmic reticulum. b. nuclear lamina. c. Golgi apparatus. d. plasma membrane.
A
What is the average number of replication forks per DNA replication cluster in a mammalian cell nucleus? a. Approximately 20 b. Approximately 200 c. Approximately 4,000 d. Approximately 8,000
A
Which of the following is an example of nucleolar involvement in processing of non-rRNAs? a. Base modification within tRNAs catalyzed by snoRNPs b. Storage of pre-mRNA splicing factors c. Gene silencing d. Proteasomal proteolysis
A
How many copies of the large ribosomal RNA gene does the human genome contain?
About 200
What modification of nucleotides occurs during processing of pre-rRNA?
Addition of methyl groups and pseudouridylation
Because of the presence of a nuclear envelope, eukaryotes can perform several functions that prokaryotes cannot. Identify one of these functions.
Answers will vary, including posttranslational processing (e.g., splicing) of transcripts and regulation of RNA synthesis by regulating transport of transcription factors into the nucleus.
If you stained mammalian cells with fluorescent antibodies against the three different RNA polymerases, antibodies directed against which RNA polymerase would be expected to stain the nucleoli?
Antibodies directed against RNA polymerase I
20. Which of the following is a possible intermediate during pre-rRNA processing? a. An RNA molecule containing 18S + 5.8S rRNAs b. An RNA molecule containing 5.8S + 28S rRNAs c. An RNA molecule containing 5S + 28S rRNAs d. An RNA molecule containing 18S + 28S rRNAs
B
An important step in the import of the transcription factor NF-κB into the nucleus is regulated by a a. nuclease. b. protease. c. phospholipase. d. plasma membrane receptor that directly phosphorylates IκB.
B
Cajal bodies are thought to represent sites of a. DNA replication. b. snRNA processing and assembly. c. large ribosomal subunit assembly. d. small ribosomal subunit assembly.
B
Heterochromatin is normally associated with a. ribosomes. b. the nuclear envelope. c. only a few specific human chromosomes. d. mutated chromosomes.
B
Highly condensed, transcriptionally inactive chromatin is called a. euchromatin. b. heterochromatin. c. a chromatin domain. d. histone-containing chromatin.
B
Most snoRNAs a. splice cleaved mRNAs. b. splice cleaved rRNAs. c. cleave the 45S rRNA. d. guide regions of rRNAs to modifying enzymes.
B
Nuclear pores are organized with _______ symmetry. a. sixfold b. eightfold c. ninefold d. tenfold
B
Nuclear proteins that are stripped of their nuclear localization signal and injected into the cytoplasm of cultured cells will localize a. to the nucleus. b. in the cytoplasm. c. in a ring around the outside of the nuclear envelope. d. extracellularly.
B
Polycomb proteins consist of _______ complex(es) and are concentrated in Polycomb bodies within the nucleus. a. 1 b. 2 c. 3 d. either 2 or 3, depending on cell type
B
Ran GAP association with cytoplasmic filaments of the nuclear pore results in the conversion of the nucleotide that is bound to nuclear Ran into a. ADP. b. GDP. c. ATP. d. GTP.
B
Ribosomes contain one copy each of 5.8S, 18S, and 28S rRNA. The major mechanism ensuring that each is produced in equal molar amounts is the a. selective degradation of excess transcripts of the respective rRNAs. b. grouping of the DNA sequences encoding each rRNA into a single rRNA gene. c. outcome of coevolution of the separate genes. d. existence of common sequence features for transcription initiation for the genes encoding each rRNA.
B
The 5.8S rRNA associates with a. Cajal bodies. b. the large subunit of the ribosome. c. the small subunit of the ribosome. d. the interface between the large and small subunits of a functional ribosome.
B
The genes encoding 5.8S, 18S, and 28S ribosome RNA are clustered together in a. tandem arrays located on one chromosome. b. tandem arrays interspersed with tandem arrays of genes encoding 5S ribosomal RNA. c. tandem arrays located on three chromosomes. d. tandem arrays located on five chromosomes.
B
The nuclear envelope is continuous with the a. mitochondrial outer membrane. b. rough endoplasmic reticulum. c. Golgi apparatus. d. plasma membrane.
B
The nuclear localization signal is recognized by and binds to which protein in the process of nuclear protein import? a. Ran b. Importin c. Exportin d. The outer fibril protein
B
The region of chromosomes that codes for most of the rRNAs is found in the a. nucleolus. b. nucleolar organizing region. c. ribosomal assembly region. d. ribosomal organizer region.
B
Which of the following are not compartments in the nucleus? a. Clustered DNA replication sites b. Lipid droplets c. PML bodies d. Speckles enriched in RNA splicing components
B
Which of the following processes does not take place in the nucleus? a. DNA replication b. Translation c. RNA processing d. Transcription
B
Chromatin domains are loops of at least _______ base pairs of DNA. a. 1000 b. 10,000 c. 100,000 d. 1,000,000
C
Eukaryotic ribosomes leave the nucleus as a. intact 80S ribosomes. b. intact 70S ribosomes. c. 40S and 60S ribosomal subunits. d. 18S and 28S rRNAs.
C
Export of RNAs from the nucleus occurs primarily by a. passive diffusion through nuclear pore complexes. b. cotranscriptional insertion through protein pores of the nuclear envelope membrane. c. selective transport through nuclear pore complexes. d. release from the nucleus when it breaks down at mitosis.
C
Most snoRNAs function as a. catalytic RNAs. b. self-replicating RNAs. c. guide RNAs. d. self-splicing RNAs.
C
Newly replicated DNA is located in a few hundred spots in the nucleus because a. there are a few hundred origins of replication per nucleus. b. there are a few thousand origins of replication, but only a few hundred are active at any one time. c. there are a few thousand origins of replication active at any one time, but they are located in a few hundred discrete clusters. d. there are so many charges drawing the DNA molecules together.
C
Polycomb bodies act on _______ regions of chromatin, silencing gene expression. a. adjacent b. distant c. both adjacent and distant d. very short
C
Polycomb proteins promote heterochromatin formation and gene silencing by a. acetylating lysine 27 of histone 3. b. deacetylating lysine 27 of histone 3. c. methylating lysine 27 of histone 3. d. methylating lysine 9 of histone 3.
C
Protein transport into the nucleus occurs by a. diffusion through the nuclear envelope membranes. b. passive diffusion through nuclear pore complexes. c. selective transport through nuclear pore complexes. d. selective transport through individual subsets of nuclear pore complexes that are specific for individual classes of proteins.
C
Sites of newly replicated DNA can be detected with antibodies if the DNA is labeled with a. tritiated thymidine. b. tritiated uridine. c. bromodeoxyuridine. d. bromouridine.
C
The directionality of nuclear transport is determined by _______ in the nucleus and _______ in the cytosol. a. high Ran; low Ran b. low Ran; high Ran c. high Ran/GTP; high Ran/GDP d. high Ran/GDP; high Ran/GTP
C
The eukaryotic 45S pre-rRNA transcript codes for the a. 28S rRNA. b. 28S and 18S rRNAs. c. 28S, 18S, and 5.8S rRNAs. d. 28S, 18S, 5.8S, and 5S rRNAs.
C
The major pathway for molecules to pass into and out of the nucleus is through a. gap junctions between the nuclear envelope membranes. b. porin channels in the nuclear envelope membranes. c. nuclear pore complexes. d. diffusion through the membrane bilayers of the nuclear envelope.
C
The nuclear localization signal is typically a(n) _______, rich in the amino acids _______. a. α helix; Pro, Lys, and Arg b. α helix; Leu, Phe, and Gly c. short sequence; Lys and Arg d. short sequence; Pro, Leu, and Gly
C
Typically, nuclear localization signals are composed primarily of a. hydrophobic amino acids. b. acidic amino acids. c. basic amino acids. d. sulfur-containing amino acids.
C
Which of the following is not involved in the transport of RNAs out of the nucleus? a. Ran b. Expenditure of chemical energy c. Small RNAs crossing by passive diffusion d. Exportins
C
Which statement about heterochromatin is false? a. It is highly condensed chromatin. b. There are two forms—constitutive and facultative heterochromatin. c. It is transcriptionally active. d. It is largely localized to the nuclear periphery.
C
Which statement describing the chromosome conformation capture (3C) technique for chromosome structural analyses is true? a. Sequences of DNA that are physically distant from each other are chemically linked together. b. Most DNA-DNA interactions are between sequences on different chromosomes. c. Sequences separated by a centromere rarely interact with each other. d. All of the above
C
Chromosomes are distributed in the nucleus a. as cleavage fragments. b. as condensed mitotic chromosomes. c. randomly. d. in distinct territories.
D
Cleaving of pre-rRNA produces a. the 28S rRNA. b. the 18S rRNA. c. both 28S and 18S rRNAs. d. the 28S, 18S, and 5.8S rRNAs.
D
Emery-Dreifuss muscular dystrophy and Hutchinson-Gilford progeria are caused by mutations in genes that code for a. importin or Ran. b. Polycomb proteins. c. NF-κB. d. A-type nuclear lamins or emerin.
D
Lamins are associated with the inner nuclear envelope membrane via a. GPI anchors on lamins. b. lipid tails on lamins only. c. lamin-binding proteins in the nuclear envelope membrane only. d. lipid tails on lamins and lamin-binding proteins in the nuclear envelope membrane.
D
The fibrous proteins underlying the inner nuclear membrane are a. band 3s. b. collagens. c. keratins. d. lamins.
D
The first nuclear localization signal to be mapped is the signal responsible for the transport of a. nucleoplasmin. b. histone H1. c. lamin B. d. SV40 T antigen.
D
The mass of the nuclear pore complex is estimated to be a. smaller than that of a ribosome. b. about the same as that of a ribosome. c. about five times that of a ribosome. d. about 30 times that of a ribosome.
D
The most prominent nuclear body is the a. mitochondrion. b. endoplasmic reticulum. c. histone locus body. d. nucleolus.
D
The principal difference between eukaryotic and prokaryotic cells is that eukaryotic cells have a. larger ribosomes. b. cell walls, whereas prokaryotic cells do not. c. DNA replication and transcription in the same compartment, whereas prokaryotic cells do not. d. a nucleus, whereas prokaryotic cells do not.
D
What is the function of karyopherins? a. They attach to chromosomes in order to activate heterochromatin. b. They coat chromosomes. c. They define the structure of Cajal bodies. d. They transport macromolecules into or out of the nucleus.
D
Which of the following factors contributes to the relative ease of determining how pre-rRNA is processed? a. The abundance of rRNA genes b. The abundance of ribosomes (5-10 million) that need to be synthesized per cell cycle c. The grouping of rRNA genes into discrete nuclear compartments in the nucleoli d. All of the above
D
Which statement about snRNAs is true? a. They are synthesized in the nucleus and function in the cytoplasm. b. They are synthesized in the cytoplasm and function in the nucleus. c. They are synthesized in the nucleus and function in the nucleus without passing into the cytoplasm. d. They are synthesized in the nucleus, move to the cytoplasm to form functional complexes with proteins, and then return to the nucleus.
D
mRNA molecules are exported from the nucleus to the cytoplasm via a. a consensus sequence located at the 3ʹ terminus. b. the 7-methylguanosine cap structure. c. importin. d. a recruited protein complex.
D
snRNPs that are responsible for pre-mRNA splicing are assembled and stored in a. Cajal bodies. b. histone locus bodies. c. speckles. d. both Cajal bodies and speckles.
D
The nuclear localization signal (NLS) is not removed (cleaved off) after entry of the protein into the nucleus. In contrast, the targeting sequences of proteins destined for organelles such as the mitochondria are located at the N-terminus of the protein and are cleaved off once the proteins reach the lumen of the organelle. What are two likely reasons that the nuclear localization signal (NLS) is not cleaved off for nuclear proteins?
During mitosis, the nuclear envelope disintegrates, and nuclear proteins are released into the cytoplasm. Once the envelope re-forms around the daughter nuclei, the nuclear proteins need to be re-transported into the nuclei, explaining a need for retention of NLS. In some organisms, such as yeast, the nuclear envelope remains intact during mitosis. Thus, there are likely other reasons for retention of NLS that are of equal or greater importance. A second possible reason is that nuclear localization signals are internal to the polypeptide. Cleaved targeting sequences are typically located at the N-terminus of the protein. Cleavage of an internal sequence has much more consequence for the overall structure of the protein than removal of an N-terminal sequence. In addition, many nuclear proteins shuttle in and out of the nucleus, which would be a third reason for retaining the NLS.
How does euchromatin differ from heterochromatin with respect to transcription activity?
Euchromatin is transcriptionally active, while heterochromatin is transcriptionally inactive.
All molecules found in the nucleus have been transported there via a process that requires ATP.
F
Eukaryotic ribosomal RNAs are synthesized as three transcripts, with sizes of 28S, 18S, and 5.8S.
F
Ribosomal proteins are synthesized in the nucleolus and then assembled onto rRNAs.
F
Small nuclear RNAs are synthesized in the nucleus and function there without ever leaving the nucleus.
F
The nuclear envelope is composed of one nuclear membrane and an underlying nuclear lamina.
F
How have FISH approaches been applied to studying the arrangement of distribution of gene-rich chromosomes relative the nuclear periphery versus the center of the nucleus?
FISH stands for fluorescence in situ hybridization and can using gene specific probes localization in situ gene-rich chromosomal regions. Hence relative distribution within the nucleus can be determined.
How can fluorescence in situ hybridization be used to show that chromosomes occupy distinct domains of space within the nucleus?
Fluorescently labeled probes that base pair with repetitive sequences unique to each individual chromosome are hybridized to chromosomes in permeabilized cells.
Unlike eukaryotic cells, prokaryotic cells do not have a nucleus. What are some possible benefits of the evolution of a nucleus in eukaryotes?
In contrast to prokaryotes, eukaryotic RNA is extensively modified by processes such as splicing and RNA editing, which are critical to the accurate transfer of genetic information from gene to protein. Sequestering the chromosomes in a separate compartment allows RNA-processing events to be more efficient, because the enzymes involved are sequestered in the nucleus and hence have less interference from unrelated enzymes. In addition, because RNA is retained in the nucleus until the processing events are complete, inappropriate translation of unprocessed RNAs does not occur. Furthermore, an additional level of control comes from the entry of regulated transcription factor into the nucleus. This would not be possible if the cytosol were not separated from the nucleus and chromosomes.
Where do the final stages of ribosome subunit formation occur?
In the cytoplasm
Where are snoRNAs located in the nucleus?
In the nucleolus
What are LADs and NADs?
LADs are lamina associated domains in chromosomes and NADs are nucleolus associated domains.
What attributes of the locations where replication and transcription occur within the nucleus leads to these sites being designated as "factories"?
Lots of DNA replication and transcription into RNA occur within clustered regions of the nucleus where large complexes of proteins involved in each particular process are concentrated. Because of the copious production in these clustered regions and the volume of protein complexes produced, these regions resemble factories.
Nuclear localization signals are usually short sequences rich in which amino acids?
Lysine and arginine
What are nuclear speckles?
Nuclear speckles are storage sites for pre-mRNA splicing factors within the nucleus. Textbook Reference:
Where is 5S rRNA synthesized?
On a chromosome outside the nucleolar organizer region, namely chromosome 1
How does the structure of the outer side of the nuclear pore complex differ from that of the inner side?
On the outer side, eight cytoplasmic filaments, or spokes, are attached to the cytoplasmic ring. On the inner side, eight filaments extend from the pore to a ring in the nucleus, forming a nuclear basket.
Why is most heterochromatin bound to the periphery of the nucleus?
Proteins associated with heterochromatin bind to the nuclear lamina.
A major regulatory protein for nuclear import and export of proteins is the small GTPase _______.
Ran
What are the expected phenotypic consequences of a mutation in Ran GAP that reduces its affinity for cytoplasmic filaments of the nuclear pore?
Ran is important to both protein import into and export out of the nucleus. As part of importin recycling to the cytoplasm, Ran in the GTP-bound form complexes with importin inside the nucleus. Formation of this complex results in the release of importin from cargo and the importin/Ran-GTP complex is then recycled back to the cytoplasm. During nuclear protein export in general, GTP-bound Ran binds to exportin inside the nucleus to give rise to a Ran-GFP/exportin/cargo complex that is exported as a unit through the nuclear pore into the cytoplasm. In either case, Ran functions as a GTP-dependent molecular switch. Ran GAP bound to the cytoplasmic filaments of the nuclear pore complex activates the GTPase activity of Ran in the cytoplasm and bound GTP is cleaved to GDP. If Ran GAP were not bound to the cytoplasmic filaments, its effective local concentration would be diluted, and the efficiency of Ran activation would be low. Low affinity would cause a distinct decrease in the Ran GTP gradient across the nuclear pore. This would reduce the efficiency of both import into the nucleus, which is dependent on importin recycling, and export from the nucleus. Phenotypically, cell growth rates would slow and the defect might be so severe as to be lethal.
What is the basis for referring to replication and transcription sites in the nucleus as "factories"?
Replication sites, and transcription sites are clustered in the nucleus and produce multiple DNA molecules and proteins, respectively, in these clusters in a manner reminiscent of the manufacturing that occurs in factories.
In many organisms, the nuclear envelope breaks down during mitosis. As part of this breakdown, the nuclear lamina that underlies the nuclear envelope breaks down. In fact, many investigators think that the breakdown of the lamina initiates envelope breakdown. Propose a reversible mechanism for controlling the polymerization state of the proteins that make up the nuclear lamina.
Reversible phosphorylation/dephosphorylation would provide a mechanism for regulating the polymerization of nuclear lamins. When phosphorylated, negative charge repulsion would lead to dissociation, and after dephosphorylation repolymerization would occur. Specific protein kinases would be involved in the phosphorylation reaction and specific phosphatases in the dephosphorylation reaction. These events could be regulated by various signaling pathways.
How does the clustering and transcriptional pattern for eukaryotic ribosomal RNA genes ensure that there will be equal numbers of copies of the major RNAs available for ribosome assembly?
Ribosomal genes are organized in tandem repeats that contain one copy each of the 18S, 5.8S, and 28S rRNA sequence as a single gene unit. The transcript contains all three sequences, and hence, each is produced after processing of the pre-rRNA in the same amount. All this is located in the nucleolus. Transcription of 5S rRNA occurs outside the nucleolus and may not be as tightly linked in terms of amount.
What takes place in the granular component of the nucleolus?
Ribosome assembly
Give one example of how regulation of nuclear protein import is involved in gene regulation.
Some transcription factors that act on nuclear genes are found in the cytoplasm associated with inhibitor proteins like IκB. When IκB is phosphorylated and releases NF-κB, NF-κB can enter the nucleus because its nuclear localization signals are exposed; it can then bind to and activate certain genes.
Chromatin domains appear to represent discrete functional units that independently regulate gene expression.
T
Inactive, condensed chromatin is called heterochromatin.
T
Individual chromosomes occupy discrete territories within the nucleus.
T
Messenger RNAs are transported through the nuclear pores as ribonucleoprotein particles.
T
The activity of some proteins is regulated by controlling their ability to be imported into the nucleus.
T
The eukaryotic large ribosomal subunit contains the 28S, 5.8S, and 5S rRNAs, plus many different proteins.
T
The nuclear pore complexes provide the only known routes through which molecules can travel between the nucleus and the cytoplasm of interphase cells.
T
The nucleolus is the site of rRNA synthesis and processing and of ribosomal subunit assembly.
T
Within the nucleolus, what functions are associated with the fibrillar center, dense fibrillar component, and granular component?
The fibrillar center is the site of rRNA transcription. The pre-rRNAs are processed in the dense fibrillar component, and ribosomal subunits are then assembled in the granular component.
The nuclear envelope consists of what four major components?
The inner nuclear membrane, the outer nuclear membrane, the nuclear lamina, and nuclear pore complexes
Protein transport into the endoplasmic reticulum (ER) or mitochondria requires unfolding of the protein and threading the amino acid chain through a channel or pore into the organelle. In contrast, protein transport into the nucleus does not require unfolding of the protein. Why is unfolding unnecessary for nuclear protein import?
The nuclear pore complex is a huge structure composed of about 30 different pore proteins, and its central channel has a diameter of approximately 10-40 nm, which is large enough for even very large protein complexes. For this reason, it is unnecessary to unfold the protein, which can pass through the nuclear pore in its native state.
How does the nuclear envelope differ from the membrane of the endoplasmic reticulum?
The outer membrane of the nuclear envelope is enriched in proteins that bind the cytoskeleton and lacks the proteins that give the ER its tubular structure. The inner nuclear membrane contains about 60 unique transmembrane proteins, including those that interact with the nuclear lamina. The endoplasmic reticulum does not have nuclear pore complexes.
What is the role of nucleotide sequence in mediating the export of ribosomal RNA from the nucleus?
The recognized export sequences reside on the proteins associated with the ribosomal RNA rather than on the ribosomal RNA itself. For example, a specific exportin, Crm1, recognizes protein sequence features (i.e., amino acids) of proteins bound to the ribosomal RNA rather than the nucleotide sequence of the ribosomal RNA itself.
What snRNP modifications take place in Cajal bodies?
The snRNA component of the snRNPs can undergo ribose methylation and pseudouridylation.
What is the function of karyopherins?
They are nuclear transport receptor proteins.
. By what mechanism are most nuclear proteins specifically targeted to the nucleus for import?
They have a nuclear import signal (a short basic region, such as Pro-Arg-Lys-Arg) in their sequence that is recognized by cytosolic transport proteins (e.g., importins); these proteins then bind to the surface of the nuclear pore complex and are transported through the nuclear pore.
The activity of transcription factors is frequently regulated by controlling their localization within eukaryotic cells, such that only in response to a given stimulus is the transcription factor transported into the nucleus, where it can activate its target genes. For a transcription factor that has a nuclear localization signal (NLS), describe two mechanisms by which regulated transport can occur.
Two possible mechanisms involve masking of the NLS such that it is not recognized by importin. This masking can be: (1) intermolecular, whereby another protein binds and obscures the NLS, or (2) intramolecular, whereby the protein itself folds in such a way that the signal is masked. Upon stimulation, the masking protein dissociates or the protein structure is altered such that the NLS is exposed, and transport to the nucleus can occur. Other mechanisms are also possible. For example, the transcription factor may be tethered to another protein that is attached to a cytoplasmic structure, such as a membrane protein or a cytoskeletal protein. In this case, the nuclear localization signal is exposed, but the protein is physically retained in the cytoplasm until the appropriate stimulus liberates it, and it is free to make its way into the nucleus.
To determine a chromosome's distribution within the mammalian nucleus, probes were hybridized to repeated sequences rather than to a single gene. Why?
When tandemly arrayed, the repeated sequence gives a locally amplified signal. Hybridizing a nucleotide probe to a single gene on a chromosome would give the equivalent of a point signal for the chromosome. Chromosomes have many genes. Moreover, repeated sequences are spread across a given chromosome. Therefore, hybridizing a probe to a repeated sequence lights up the whole chromosome.
Mutations in the fibrous protein underlying the nuclear envelope can cause premature _______, or progeria.
aging
Transcriptionally active chromatin, which is called _______, is decondensed, while transcriptionally inactive chromatin, which is called _______, is highly condensed.
euchromatin; heterochromatin
Proteins called _______ act as receptors for cargo protein import into the nucleus, and proteins called _______ play the corresponding role in export from the nucleus.
importins; exportins
In addition to replication factories, the nucleus contains a number of discrete structures, maintained by protein-protein and protein-RNA interactions, that are referred to as _______.
nuclear bodies
In eukaryotic cells, the contents of the nucleus are separated from the cytoplasm by the _______.
nuclear envelope (nuclear membranes)
Underlying the nuclear envelope is the _______, a fibrous meshwork composed of proteins called _______.
nuclear lamina; lamins
Receptor-dependent protein transport into the nucleus requires the recognition of a _______ on the cargo protein.
nuclear localization signal (NLS)
The most prominent nuclear body, the _______, is the site of ribosome production and rRNA processing and synthesis.
nucleolus
Depending on size, molecules can enter the nucleus by the process of _______ or _______.
passive diffusion; selective transport
In ribosome assembly, _______ that were synthesized in the cytoplasm are assembled into complexes on pre-ribosomal RNA molecules.
proteins (ribosomal proteins)
What guides the enzymes that modify rRNAs to their proper positions on the pre-rRNA?
snoRNAs
Nuclear speckles are nuclear bodies that contain _______ and snRNPs.
splicing factors
Individual chromosomes occupy distinct _______ within the nucleus.
territories (regions)