Transcript Ribosome

Ribosome
a presentation by Erin Husson
Just a quick overview of what
we’re going to cover…
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What ribosome is and what its subunits are
The purpose of ribosome
The process of protein synthesis, including:
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DNA to mRNA (transcription)
mRNA to protein (translation)
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Initiation
Elongation
End of translation
Just a quick overview of what
we’re going to cover…
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Structures of the two ribosome subunits
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The larger subunit
The smaller subunit
RNA’s relation to their structure
What is ribosome?
Ribosome - protein
synthesizer consisting
of two subunits
 Larger one, “50S”, is
upper picture. Smaller
is “30S”
(They look the same size
here because of space
restrictions.)
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50S and 30S???
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Related to their respective sizes. Numbers
actually measures of how quickly each
subunit sinks to the bottom of a container of
liquid when spun in a centrifuge
One subunit smaller than other, but both are
larger than average protein
A couple more nifty pictures…
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50S (left) and 30S. This time you can see them
from different angles, through different style of
picture
So what’s the purpose of
ribosome?
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Ribosome basically a protein factory.
Subunits each have role in making of proteins
To understand exactly what each subunit
does, it’s necessary to walk through protein
synthesis step by step
Protein synthesis
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Process starts from
DNA through
“transcription”
“Translation” is where
ribosome comes in.
Translation occurs
when protein formed
from code on mRNA
Ribosome carries out
the translation of the
nucleotide triplets
Protein synthesis
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Chart - visual image of
transcription and
translation in protein
synthesizing
DNA and RNA have
nucleotides that
determine kind of
protein
3 nucleotides = 1 amino
acid of a protein
Ribosome and RNA
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mRNA with code for proteins located at 30S
subunit
tRNAs responsible for carrying amino acids
to mRNA. Each tRNA has own nucleotide
triplet which binds to matching triplet on
mRNA, ex., tRNA with code AAA (triple
adenine) would match up with mRNA that has
code UUU (triple uracil)
Initiation:
The first phase of translation
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Translation begins
when mRNA attaches
to the 30S
tRNA comes and binds
to mRNA where
nucleotide code
matches
This triggers 50S
binding to 30S. 50S is
where all tRNAs will
bind. Now we move on
to elongation
Elongation:
The second phase
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Two binding sites on
50S: A site and P site,
which aid in continuing
translation
First tRNA connected at
A site. Now moves to P
site as another tRNA
approaches
Second tRNA binds to
A site
Elongation (continued)
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Peptide bond forms
between amino acids of
tRNAs (methionine and
proline)
First tRNA now
detached from its
amino acid, and it
leaves ribosome.
Second tRNA still has
proline and methionine
attached
Elongation (continued)
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The tRNA left now
moves to P site.
Ribosome ready to
accept another tRNA
and continue process
Each tRNA adds
another amino acid to
growing peptide chain
(thus “elongation”)
Eventually process has
to finish, however…
End of translation
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Ribosome was moving
along nucleotide triplets
one by one
Ribosome reaches
“stop codon,” peptide
chain finished. Last
tRNA leaves ribosome,
leaving behind
completed peptide
chain
End of translation (continued)
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Ribosome separates
from mRNA
Ribosome subunits also
separate, and will
remain this way until
another mRNA comes
along to restart the
process
Still awake?
It’s Pop Quiz Time!!!
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Here’s a little quiz on what we’ve covered so
far, just to keep your brain alert.
First question…
What are the two subunits of ribosome known
as, and which is which?
Answer: 50S (the larger one) and 30S (the
smaller one)
Another question…
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(This one’s multiple choice!)
The process of converting code on the mRNA
into a protein is called…?
A. Transition
B. Transduction
C. Translation
D. Transcription
And now…
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That’s the end of the quiz (whew)!
Any questions so far?
Okay, that’s taken care of. And now, without
further ado, we move on to…
Structures of the subunits
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50S and 30S besides difference in size, also
have somewhat different structures
Both are rather complicated, since they’re
much larger than average protein
We’ll cover structure of 50S first
The 50S subunit
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This part of ribosome contains
site where new peptide bonds
are formed when proteins are
synthesized
mRNA would be located
horizontally in groove across
middle
To help with protein synthesis,
subunit uses adenine RNA
nucleotide, shown by green dot
in center
The 50S subunit
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This part of ribosome contains
site where new peptide bonds
are formed when proteins are
synthesized
mRNA would be located
horizontally in groove across
middle
To help with protein synthesis,
subunit uses adenine RNA
nucleotide, shown by green dot
in center
The 50S subunit
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50S made of two RNA strands: a
long one, shown in an orangey
color, and shorter one, shown in
yellow
Blue things in the picture are a
few of many proteins that have
bound to ribosome’s surface
Many of proteins have long tails
that go inside ribosome and keep
the RNA strands in place
A bit more on 50S…
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50S is quite rigid,
folded and packed so
well that it’s virtually
immobile within its
structure
Contrasts with structure
of 30S, which we’ll now
move to
The 30S subunit
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Unlike 50S, 30S is fairly flexible
It needs to have movable regions because,
when it shifts from one mRNA nucleotide
triplet to another, movement is necessary to
aid this process
30S controls flow of information during
protein synthesis
The 30S subunit
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30S finds an mRNA
and then makes sure
that each tRNA is
matched up correctly
on mRNA
It’s been suggested that
mRNA enters through
small hole in 30S
(shown here in center
of left side)
The 30S subunit
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30S finds an mRNA
and then makes sure
that each tRNA is
matched up correctly
on mRNA
It’s been suggested that
mRNA enters through
small hole in 30S
(shown here in center
of left side)
The 30S subunit
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mRNA then extends up
into the place where
translation occurs,
located in cleft between
top part, “head”, and
bottom part, “body”
Structure and RNA
Recently discovered - about two-thirds of
ribosome’s mass made up of RNA
 Most important functions of ribosome
performed by RNA. This has been found
because of atomic knowledge of structures of
50S and 30S and their assemblage into 70S
(One might logically think it to be 80S, but it
really isn’t)
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Concluding…
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Although a good amount of progress has
been made in figuring out structures of 50S
and 30S, there is probably still a lot that
hasn’t been deciphered yet
It’s somewhat difficult to find out structures
because, as mentioned earlier, they’re
relatively large. Hopefully more will be
discovered about them in the future
Guess what…
It’s Quiz Time again!
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This is the last one, I promise. It’s a piece of
cake anyway…well, if you were paying
attention, that is.
Ribosome is primarily made up of…?
A. DNA
B. RNA
C. Proteins
Glad that’s over, huh? Now to summarize
what we covered…
To summarize…
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Ribosome: protein synthesizer consisting of
two subunits, 50S and 30S
Protein synthesis starts from DNA, which
carries the code for making the proteins. The
DNA is converted to mRNA by “transcription”,
and mRNA, in turn, is converted to protein by
“translation”
Ribosome translates nucleotide triplets on
mRNA into proteins
To summarize…
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tRNAs carry amino acids to ribosome to be
linked together by the ribosome. One by one,
they bind to the ribosome, add their amino
acid to the chain, and then depart
50S and 30S made primarily of RNA and are
both larger than the average protein. 50S
has rather rigid structure, while 30S has fairly
flexible one. Not all about them has been
discovered yet, but significant progress has
been made in that respect
The End
That’s a wrap!
Any questions?