Nucleolus Flashcards
In most cells, what percentage of RNA is rRNA?
over 80%
What is rRNA?
ribosomal RNA
What does rRNA form?
the core of a ribosome
Why do cells require so much rRNA?
because most cells require ~10 million ribosomes in every generation
How many copies of each type of rRNA does each ribosome need?
one
T or F: nuclei only contain one nucleolus
False, some contain many nucleoli
T or F: the nucleolus is surrounded by a membrane and is considered a distinct organelle
false! it does not have a membrane and is not considered an organelle
What occurs in the nucleolus?
ribosome synthesis
Where is the nucleolus located?
it is the dark structure within the nucleus
Where are ribosomes synthesized?
in the nucleolus within the nucleus
What 3 things does the nucleolus contain in humans?
newly made ribosome subunits
newly made rRNA transcripts
DNA clusters from 5 different chromosomes where rRNA genes are present
How is so much rRNA produced?
the DNA sequences coding for rRNA are repeated hundreds of times throughout the genome in clusters
How many times are the DNA sequences for rRNA repeated?
hundreds of times throughout the genome in clusters
What are the 3 basic compartments of the nucleolus?
the granular component
the fibrillar center
the dense fibrillar components
Describe the granular component
a compartment of the nucleolus that holds ribosomes in the process of assembly
What structure makes up the bulk of the nucleolus?
the granular component
what 2 structures are within the granular component?
the fibrillar centre and the dense fibrillar component
Describe the fibrillar centre
a structure within the granular component of the nucleolus that holds the DNA with genes for rRNA
it is surrounded by the DFC
Describe dense fibrillar component
a structure within the granular component of the nucleolus that is the site of rDNA transcription to make rRNA
it surrounds the fibrillar centre
Where is the fibrillar centre located in relation to the dense fibrillar component?
the FC is surrounded by the DFC (hence the name: fibrillar CENTRE)
Where does transcription take place?
in the dense fibrillar component within the granular component of the nucleolus
What 2 components does a ribosome have?
a protein component made up of dozens of different proteins
an rRNA component
How many different proteins can make up the protein component of a ribosome?
dozens
Which of the 2 components of ribosomes is the predominate component? What is the ratio of the components?
the rRNA is predominant
65% rRNA to 35% protein
Which component of a ribosome is also the catalytic component?
rRNA
What is another word for a ribosome because of its catalytic activity?
the rRNA has catalytic activity which makes the ribosome a ribozyme
how many subunits make up a ribosome?
2 main ones
What are the 2 main subunits of a ribosome?
there is one large and one small
In eukaryotes, what are the 2 subunits?
60S large
40S small
In prokaryotes, what are the 2 subunits?
50S large
30S small
T or F: the composition of subunits in prokaryotic ribosomes is the same as eukaryotic ribosomes. Why/why not?
false
prokaryotes = 50S large and 30S small
eukaryotes = 60S large and 40S small
what does the S stand for in the ribosome subunit structure (50S, 30S, 60S, 40S)?
Sveldburg unit
What is the Sveldburg unit?
a measurement of the rate of sedimentation
What is the Sveldburg unit based on?
the sedimentation rate of a particle in an ultracentrifuge when placed in a tube with a density gradient of increasing sucrose content
What magnitude of an S coefficient will a larger particle have? explain
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)
T or F: S values are not additive (ie., 30S + 50S does not necessarily mean there’s 80S altogether)
true
If the S value is low, what does that say about the particle?
the particle is small/has low density and it did not sink deep into the sucrose density gradient
How many distinct rRNAs and proteins does the large eukaryotic subunit have?
the 60S subunit has 3 distinct rRNAs and 49 proteins
What are the 3 rRNAs in the large eukaryotic subunit (60S)?
5S
5.8S
28S
Which is the biggest rRNA that makes up the eukaryotic large subunit?
28S is 4700 nucleotides long
How many rRNAs and proteins make up the small eukaryotic ribosomal subunit?
1 rRNA and 33 proteins
What is the rRNA that along with proteins, makes up the small eukaryotic ribosomal subunit (40S)?
18S rRNA (1900 nucleotides)
What is the S value of the entire eukaryotic ribosome?
80S
Describe the general structure of rRNA molecules that make up a eukaryotic ribosome? what does this contribute to?
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
T or F: each rRNA has the same shape
False, each rRNA has a distinct shape
T or F: the S value of the large eukaryotic subunit is 100S
FALSE! it’s 80S! remember it’s not additive (ie., not 40S + 60S)
How many locations are there in the genome that encode rRNA?
2
How many genes are there that encode rRNA?
2
Where are the 2 genes that code for rRNA located?
1 is located within the nucleolus
1 is located outside the nucleolus but inside the nucleus
Which rRNAs does the gene in the nucleolus code for?
it codes for 5.8S, 18S, and 28S to make a large pre-rRNA transcript
Which rRNA does the gene outside the nucleolus code for?
the 5S rRNA
How many times are the genes that code for rRNA repeated?
These 2 genes are repeated hundreds of times throughout the genome
How does the large rRNA gene distribute itself within the nucleolus?
it has multiple copies in clusters at one or more locations and is separated by an intergenic spacer
What is in between copies of the large rRNA genes?
intergenic spacers
How are coding segments separated within a copy of the large rRNA gene?
by intragenic spacers
Describe intergenic spacer
A space that separates copies of the large rRNA genes within the nucleolus
Describe intragenic spacer
the space that separates the coding segments (with rRNA sequences) within a copy of a large rRNA gene
What transcribes the large rRNA gene?
RNA Polymerase I
Describe the process of rRNA transcription. What does this produce?
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
What does transcription of the large rRNA gene produce?
the 45S pre-rRNA molecule
T or F: intragenic spacers are removed when the large rRNA gene is being transcribed
False
they are transcribed and then removed during processing
What is the purpose of removing the intragenic spacers of the large rRNA gene?
to release the separate rRNA segments (5.8S, 18S, 28S)
What processes the pre-rRNA transcript?
snoRNPs
small nucleolar ribonucleoproteins
Describe snoRNPs (including their full name)
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
How do snoRNPs help in processing?
they cleave the transcribed intragenic spacers by complimentary base pairing and do a process similar to splicing in mRNA
When are ribosomal proteins added to the RNA? What is the purpose of this?
when processing is still happening
to produce the large ribosomal subunit
How many nucleotides is the rRNA precursor?
13,000
T or F: the pre-rRNA is extensively modified before the 3 rRNA molecules are separated out
true
give one example of how the rRNA precursor can be modified
methylation
When looking at a TEM image of the nucleolus, how can you tell which end transcription is beginning/ending?
beginning: where the fibrils are the shortest
ending: where the fibrils are the longest
Looks like a Christmas tree
What 6 things should you be able to see on a TEM image of a nucleolus?
rDNA
rRNA fibrils
transcription start/end
intergenic spacers
RNA pol I
snoRNPs
What transcribes the 5S rRNA gene?
RNA polymerase III
How does the 5S gene distribute itself within the nucleus?
it is in clusters of DNA outside the nucleolus
What is a unique feature of the 5S rRNA?
its promoter region is completely within the coding segment
Describe transcription of the 5S rRNA gene
initiation involves an interaction between the internal promoter, transcription factors, and RNAPIII
intergenic spacers are transcribed and have to be destroyed during processing
T or F: snoRNPs are required for processing the 5S rRNA gene
False
T or F: during transcription of the 5S rRNA gene, intragenic spacers are transcribed
False, only intergenic spacers are transcribed
Why would there not be any intragenic spacers in the 5S rRNA gene?
because there’s only 1 rRNA molecule - the 5S
How is the small ribosomal subunit assembled?
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
Where does the small ribosomal subunit assemble and where is it transported to?
assembled in the nucleolus and transported to the cytoplasm
Where is the 5S rRNA transported to?
it is transported from the nucleus into the nucleolus
What does the 5S rRNA bind to when it reaches the nucleolus? What does this make?
the 28S, 5.8S, and proteins to make the large ribosomal subunit
Where is the large ribosomal subunit transported from and to?
from the nucleolus
to the cytoplasm
Where are ribosomal proteins made? Where are they then imported to? What do they contribute to?
made in the cytosol
imported to the nucleus
contribute to either the small or large ribosomal subunit
What allows ribosomal proteins synthesized in the cytosol to move to the nucleus?
an NLS
T or F: the small and large ribosomal subunits remain separate in the cytosol until they associate during translation initiation
true
