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BIOL 2321 - Cell Biology > Nucleolus > Flashcards

Nucleolus Flashcards

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1
Q

In most cells, what percentage of RNA is rRNA?

A

over 80%

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2
Q

What is rRNA?

A

ribosomal RNA

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3
Q

What does rRNA form?

A

the core of a ribosome

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4
Q

Why do cells require so much rRNA?

A

because most cells require ~10 million ribosomes in every generation

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5
Q

How many copies of each type of rRNA does each ribosome need?

A

one

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6
Q

T or F: nuclei only contain one nucleolus

A

False, some contain many nucleoli

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7
Q

T or F: the nucleolus is surrounded by a membrane and is considered a distinct organelle

A

false! it does not have a membrane and is not considered an organelle

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8
Q

What occurs in the nucleolus?

A

ribosome synthesis

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9
Q

Where is the nucleolus located?

A

it is the dark structure within the nucleus

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10
Q

Where are ribosomes synthesized?

A

in the nucleolus within the nucleus

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11
Q

What 3 things does the nucleolus contain in humans?

A

newly made ribosome subunits

newly made rRNA transcripts

DNA clusters from 5 different chromosomes where rRNA genes are present

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12
Q

How is so much rRNA produced?

A

the DNA sequences coding for rRNA are repeated hundreds of times throughout the genome in clusters

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13
Q

How many times are the DNA sequences for rRNA repeated?

A

hundreds of times throughout the genome in clusters

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14
Q

What are the 3 basic compartments of the nucleolus?

A

the granular component

the fibrillar center

the dense fibrillar components

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15
Q

Describe the granular component

A

a compartment of the nucleolus that holds ribosomes in the process of assembly

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16
Q

What structure makes up the bulk of the nucleolus?

A

the granular component

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17
Q

what 2 structures are within the granular component?

A

the fibrillar centre and the dense fibrillar component

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18
Q

Describe the fibrillar centre

A

a structure within the granular component of the nucleolus that holds the DNA with genes for rRNA

it is surrounded by the DFC

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19
Q

Describe dense fibrillar component

A

a structure within the granular component of the nucleolus that is the site of rDNA transcription to make rRNA

it surrounds the fibrillar centre

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20
Q

Where is the fibrillar centre located in relation to the dense fibrillar component?

A

the FC is surrounded by the DFC (hence the name: fibrillar CENTRE)

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21
Q

Where does transcription take place?

A

in the dense fibrillar component within the granular component of the nucleolus

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22
Q

What 2 components does a ribosome have?

A

a protein component made up of dozens of different proteins

an rRNA component

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23
Q

How many different proteins can make up the protein component of a ribosome?

A

dozens

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24
Q

Which of the 2 components of ribosomes is the predominate component? What is the ratio of the components?

A

the rRNA is predominant

65% rRNA to 35% protein

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25
Q

Which component of a ribosome is also the catalytic component?

A

rRNA

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26
Q

What is another word for a ribosome because of its catalytic activity?

A

the rRNA has catalytic activity which makes the ribosome a ribozyme

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27
Q

how many subunits make up a ribosome?

A

2 main ones

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28
Q

What are the 2 main subunits of a ribosome?

A

there is one large and one small

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29
Q

In eukaryotes, what are the 2 subunits?

A

60S large

40S small

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30
Q

In prokaryotes, what are the 2 subunits?

A

50S large

30S small

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31
Q

T or F: the composition of subunits in prokaryotic ribosomes is the same as eukaryotic ribosomes. Why/why not?

A

false

prokaryotes = 50S large and 30S small

eukaryotes = 60S large and 40S small

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32
Q

what does the S stand for in the ribosome subunit structure (50S, 30S, 60S, 40S)?

A

Sveldburg unit

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33
Q

What is the Sveldburg unit?

A

a measurement of the rate of sedimentation

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34
Q

What is the Sveldburg unit based on?

A

the sedimentation rate of a particle in an ultracentrifuge when placed in a tube with a density gradient of increasing sucrose content

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35
Q

What magnitude of an S coefficient will a larger particle have? explain

A

a larger particle will have a larger density and sink further into the density gradient tube when in the centrifuge (to match a denser sucrose level) and will therefore have a higher S coefficient (higher sedimentation rate)

36
Q

T or F: S values are not additive (ie., 30S + 50S does not necessarily mean there’s 80S altogether)

A

true

37
Q

If the S value is low, what does that say about the particle?

A

the particle is small/has low density and it did not sink deep into the sucrose density gradient

38
Q

How many distinct rRNAs and proteins does the large eukaryotic subunit have?

A

the 60S subunit has 3 distinct rRNAs and 49 proteins

39
Q

What are the 3 rRNAs in the large eukaryotic subunit (60S)?

A

5S
5.8S
28S

40
Q

Which is the biggest rRNA that makes up the eukaryotic large subunit?

A

28S is 4700 nucleotides long

41
Q

How many rRNAs and proteins make up the small eukaryotic ribosomal subunit?

A

1 rRNA and 33 proteins

42
Q

What is the rRNA that along with proteins, makes up the small eukaryotic ribosomal subunit (40S)?

A

18S rRNA (1900 nucleotides)

43
Q

What is the S value of the entire eukaryotic ribosome?

A

80S

44
Q

Describe the general structure of rRNA molecules that make up a eukaryotic ribosome? what does this contribute to?

A

they have complimentary regions and intrachain binding to form secondary structures

the unique shapes of each rRNA contributes to the overall shape of the ribosome

45
Q

T or F: each rRNA has the same shape

A

False, each rRNA has a distinct shape

46
Q

T or F: the S value of the large eukaryotic subunit is 100S

A

FALSE! it’s 80S! remember it’s not additive (ie., not 40S + 60S)

47
Q

How many locations are there in the genome that encode rRNA?

A

2

48
Q

How many genes are there that encode rRNA?

A

2

49
Q

Where are the 2 genes that code for rRNA located?

A

1 is located within the nucleolus

1 is located outside the nucleolus but inside the nucleus

50
Q

Which rRNAs does the gene in the nucleolus code for?

A

it codes for 5.8S, 18S, and 28S to make a large pre-rRNA transcript

51
Q

Which rRNA does the gene outside the nucleolus code for?

A

the 5S rRNA

52
Q

How many times are the genes that code for rRNA repeated?

A

These 2 genes are repeated hundreds of times throughout the genome

53
Q

How does the large rRNA gene distribute itself within the nucleolus?

A

it has multiple copies in clusters at one or more locations and is separated by an intergenic spacer

54
Q

What is in between copies of the large rRNA genes?

A

intergenic spacers

55
Q

How are coding segments separated within a copy of the large rRNA gene?

A

by intragenic spacers

56
Q

Describe intergenic spacer

A

A space that separates copies of the large rRNA genes within the nucleolus

57
Q

Describe intragenic spacer

A

the space that separates the coding segments (with rRNA sequences) within a copy of a large rRNA gene

58
Q

What transcribes the large rRNA gene?

A

RNA Polymerase I

59
Q

Describe the process of rRNA transcription. What does this produce?

A

transcription factors recruit RNAPI and bind to the single promoter to produce the initiation complex

the entire (large rRNA) gene is transcribed in one go (including the intragenic spacers) to produce the 45S pre-rRNA molecule

60
Q

What does transcription of the large rRNA gene produce?

A

the 45S pre-rRNA molecule

61
Q

T or F: intragenic spacers are removed when the large rRNA gene is being transcribed

A

False

they are transcribed and then removed during processing

62
Q

What is the purpose of removing the intragenic spacers of the large rRNA gene?

A

to release the separate rRNA segments (5.8S, 18S, 28S)

63
Q

What processes the pre-rRNA transcript?

A

snoRNPs

small nucleolar ribonucleoproteins

64
Q

Describe snoRNPs (including their full name)

A

small nucleolar ribonucleoprotein process the pre-rRNA transcript

they have folded snoRNAs in their structure

they also help in modifying the pre-rRNA before the rRNA molecules are separated

65
Q

How do snoRNPs help in processing?

A

they cleave the transcribed intragenic spacers by complimentary base pairing and do a process similar to splicing in mRNA

66
Q

When are ribosomal proteins added to the RNA? What is the purpose of this?

A

when processing is still happening

to produce the large ribosomal subunit

67
Q

How many nucleotides is the rRNA precursor?

A

13,000

68
Q

T or F: the pre-rRNA is extensively modified before the 3 rRNA molecules are separated out

A

true

69
Q

give one example of how the rRNA precursor can be modified

A

methylation

70
Q

When looking at a TEM image of the nucleolus, how can you tell which end transcription is beginning/ending?

A

beginning: where the fibrils are the shortest
ending: where the fibrils are the longest

Looks like a Christmas tree

71
Q

What 6 things should you be able to see on a TEM image of a nucleolus?

A

rDNA

rRNA fibrils

transcription start/end

intergenic spacers

RNA pol I

snoRNPs

72
Q

What transcribes the 5S rRNA gene?

A

RNA polymerase III

73
Q

How does the 5S gene distribute itself within the nucleus?

A

it is in clusters of DNA outside the nucleolus

74
Q

What is a unique feature of the 5S rRNA?

A

its promoter region is completely within the coding segment

75
Q

Describe transcription of the 5S rRNA gene

A

initiation involves an interaction between the internal promoter, transcription factors, and RNAPIII

intergenic spacers are transcribed and have to be destroyed during processing

76
Q

T or F: snoRNPs are required for processing the 5S rRNA gene

A

False

77
Q

T or F: during transcription of the 5S rRNA gene, intragenic spacers are transcribed

A

False, only intergenic spacers are transcribed

78
Q

Why would there not be any intragenic spacers in the 5S rRNA gene?

A

because there’s only 1 rRNA molecule - the 5S

79
Q

How is the small ribosomal subunit assembled?

A

the 18S rRNA molecule is packaged with proteins into the small subunit which is then shipped out of the nucleolus –> out of the nucleus –> into the cytosol

80
Q

Where does the small ribosomal subunit assemble and where is it transported to?

A

assembled in the nucleolus and transported to the cytoplasm

81
Q

Where is the 5S rRNA transported to?

A

it is transported from the nucleus into the nucleolus

82
Q

What does the 5S rRNA bind to when it reaches the nucleolus? What does this make?

A

the 28S, 5.8S, and proteins to make the large ribosomal subunit

83
Q

Where is the large ribosomal subunit transported from and to?

A

from the nucleolus

to the cytoplasm

84
Q

Where are ribosomal proteins made? Where are they then imported to? What do they contribute to?

A

made in the cytosol
imported to the nucleus
contribute to either the small or large ribosomal subunit

85
Q

What allows ribosomal proteins synthesized in the cytosol to move to the nucleus?

A

an NLS

86
Q

T or F: the small and large ribosomal subunits remain separate in the cytosol until they associate during translation initiation

A

true

BIOL 2321 - Cell Biology (26 decks)

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