Proteins Genetic Code 4b'
Summary
TLDRThis lecture delves into the translation process of the genetic code, detailing how scientists deciphered the genetic code table using various nucleotide combinations and the ribosome binding assay. It explains the universality and degeneracy of the genetic code, the role of start and stop codons, and the interaction between codons and anticodons on tRNAs. The lecture also covers the wobble hypothesis, isoaccepting tRNAs, and the process of amino acid activation and charging of tRNAs by aminoacyl-tRNA synthetases. Finally, it touches on the structure and function of ribosomes, setting the stage for a deeper exploration of translation's initiation, elongation, and termination stages in subsequent lectures.
Takeaways
- 🧬 The genetic code is deciphered through various experiments involving nucleotide combinations and ratios.
- 🔬 Scientists used different ratios of nucleotides like two U for every C to calculate the ratios and measure amino acid formation.
- 🧪 Ribosome binding assays developed by Nierenberg and Leder helped to definitively solve the genetic code for every single codon.
- 📚 The genetic code is a triplet code, with each codon made up of three nucleotides, and it is continuous and non-overlapping.
- 🌐 The genetic code is nearly universal across organisms, with a few exceptions in some organisms like mitochondria and certain protozoa.
- 🔄 The genetic code is degenerate, meaning multiple codons can code for the same amino acid, providing redundancy.
- ⏹ There are three stop codons that signal the end of protein translation and one start codon (AUG) that initiates translation.
- 🔀 The interaction between codons and anticodons on tRNAs is crucial for protein synthesis, with wobble allowing for flexibility in base pairing.
- 🔋 Amino acids are attached to their specific tRNAs through a two-step process involving aminoacyl-tRNA synthetases and ATP hydrolysis.
- 📉 Translation occurs in three stages: initiation, elongation, and termination, with ribosomes playing a central role in protein synthesis.
Q & A
What is the significance of dinucleotides, trinucleotides, and tetranucleotides in the genetic code?
-Dinucleotides, trinucleotides, and tetranucleotides are significant in the genetic code because they were used in experiments to understand how different nucleotides could combine. By varying the ratios of these nucleotides, scientists could deduce the ratios of amino acids formed and thus determine which codons coded for which amino acids.
How did the ratio of U to C nucleotides affect the formation of nucleotide combinations?
-By altering the ratio of U to C nucleotides, scientists could influence the likelihood of forming certain dinucleotides like UCU or CUU over others, such as all Cs. This manipulation helped them calculate the ratios and measure the amino acids that formed, aiding in decoding the genetic code.
What was the role of polynucleotides in decoding the genetic code?
-Polynucleotides, such as polyU, polyG, polyC, and polyA, were used in assays to change the ratio of different nucleotides. This allowed scientists to experiment with different combinations and ratios, which was crucial in decoding the genetic code.
What technique helped scientists to definitively solve every codon in the genetic code?
-The ribosome binding assay, developed by Nirenberg and Leder, was the technique that helped definitively solve every codon in the genetic code. This assay allowed for the binding of tRNA molecules to a ribosome mRNA complex in the absence of protein synthesis, enabling the synthesis of known three-nucleotide codons.
How did the ribosome binding assay work in the context of decoding the genetic code?
-In the ribosome binding assay, tRNAs bound to different amino acids were introduced to a specific codon. The tRNA that matched the codon would bind to the ribosome. Unbound tRNAs would pass through a filter, while ribosomes with bound tRNAs would be caught. This allowed scientists to detect which amino acid was bound to the tRNA inside the ribosome.
What is the significance of the genetic code being nearly universal?
-The near universality of the genetic code means that almost all organisms share the same code. This allows for the transfer of genes between different organisms in research and genetic engineering, as the genetic code functions similarly across species.
What does it mean for the genetic code to be degenerate?
-The genetic code is degenerate because multiple codons can code for the same amino acid. This means that there is more than one way to code for each amino acid, providing redundancy in the genetic code.
What are start and stop codons, and how do they function in protein synthesis?
-Start codons, such as AUG, signal the beginning of protein synthesis. Stop codons, on the other hand, signal the cell to stop translating. There are three specific codons for stop and one for start, which are crucial for controlling the process of translation.
How do codons interact with anti-codons during protein synthesis?
-Codons on mRNA interact with anti-codons on tRNA through complementary base pairing. The anti-codon of the tRNA is in a three-to-five prime order and matches the five-to-three prime oriented codon on mRNA. This interaction is essential for the correct amino acid to be added to the growing polypeptide chain.
What is the role of aminoacyl-tRNA synthetases in protein synthesis?
-Aminoacyl-tRNA synthetases are enzymes that attach the correct amino acid to its corresponding tRNA, ensuring that the tRNA is 'charged' with the appropriate amino acid. They recognize the tRNA's anticodon and sometimes other regions of the tRNA to perform this task accurately.
How does the process of translation occur in three stages?
-Translation occurs in three stages: initiation, elongation, and termination. Initiation involves the binding of mRNA to the small ribosomal subunit, the initiation tRNA, and the large ribosomal subunit. Elongation is the process of adding amino acids to the growing polypeptide chain. Termination occurs when a stop codon is reached, signaling the end of translation.
Outlines
🧬 Lecture on Translation and the Genetic Code
The lecture delves into the exploration of the genetic code through various experiments involving nucleotides. The speaker discusses how different nucleotides were combined in various ways to form dinucleotides, trinucleotides, and tetranucleotides. They explain how altering the ratio of U and C nucleotides could impact the formation of amino acids and how these ratios were used to decipher the genetic code. The lecture also touches on the use of polynucleotides and random copolymers to further understand the genetic code. A significant breakthrough came with the development of a ribosome binding assay by Nirenberg and Leder, which allowed for the definitive mapping of codons to amino acids by synthesizing known three-nucleotide codons and observing which amino acids would bind. This method revolutionized the understanding of the genetic code.
🌐 Characteristics of the Genetic Code
This section highlights the universality and degeneracy of the genetic code. The genetic code is nearly universal across organisms, with few exceptions like mitochondria and some protozoa. The code is degenerate, meaning multiple codons can code for the same amino acid, providing redundancy. The lecture also discusses the concept of start and stop codons, which are crucial for initiating and terminating protein synthesis. The interaction between codons and anticodons on tRNAs is explained, emphasizing the importance of base pairing and the role of 'wobble' at the third position of the anticodon, which allows for flexibility in codon recognition. The existence of isoaccepting tRNAs, which can carry the same amino acid but have different anticodons, is also mentioned.
🔬 Details of Translation and the Role of tRNAs
The paragraph focuses on the process of translation, where the genetic message in mRNA is translated into a polypeptide chain. It describes the directionality of translation, with the mRNA being read from the 5' to 3' end, and the polypeptide being synthesized from the N-terminus to the C-terminus. The role of tRNAs in translation is emphasized, with each tRNA carrying a specific amino acid and having an anticodon that matches the mRNA codon. The concept of 'charging' tRNAs, where amino acids are attached to their corresponding tRNAs, is introduced. The process involves two steps: activation of the amino acid by ATP and transfer to the tRNA. The lecture also mentions the role of aminoacyl-tRNA synthetases, enzymes that ensure the correct amino acid is attached to the correct tRNA.
🧪 Experiments on tRNA and mRNA Recognition
This part of the lecture discusses experiments that were conducted to understand how mRNA recognizes tRNAs. The speaker describes an experiment where the amino acid carried by a tRNA was chemically altered to determine whether the mRNA recognizes the anticodon or the amino acid itself. The results showed that the mRNA only recognizes the anticodon, not the amino acid, as a tRNA with an altered amino acid was still recognized and incorporated into the growing polypeptide chain. This finding underscores the specificity of the genetic code and the importance of the anticodon in translation.
📚 Overview of the Translation Process
The final paragraph provides an overview of the translation process, which involves three main stages: initiation, elongation, and termination. The paragraph describes the initiation phase, where the mRNA binds to the small ribosomal subunit, the initiation tRNA binds to the small subunit, and the large ribosomal subunit joins the complex. The lecture concludes with a mention of the upcoming discussion on the elongation and termination steps in the next lecture. The paragraph also touches on the structure of ribosomes and their role in facilitating translation, with mRNA and tRNAs fitting into specific sites within the ribosome to carry out protein synthesis.
Mindmap
Keywords
💡Dinucleotides
💡Trinucleotides
💡Ribosome Binding Assay
💡Codon
💡Amino Acids
💡tRNA
💡Anticodon
💡Wobble Hypothesis
💡Isoaccepting tRNAs
💡Start and Stop Codons
💡Aminoacyl-tRNA Synthetase
Highlights
Lecture discusses the process of deciphering the genetic code and the methods used.
Different nucleotides were combined in various ways to understand their roles in the genetic code.
The ratio of nucleotides was altered to form different nucleotide combinations, influencing the amino acid formation ratios.
By measuring amino acid ratios, scientists could deduce the codon coding for specific amino acids.
Polynucleotides like polyU, polyG, polyC, and polyA were used to change the nucleotide ratios.
Ribosome binding assay developed by Nierenberg and Leder helped to definitively solve the genetic code.
The assay allowed for the synthesis of known three-nucleotide codons to match specific amino acids.
The genetic code is a triplet code, with each codon consisting of three nucleotides.
The genetic code is continuous and non-overlapping, reading every three nucleotides without skipping.
The genetic code is nearly universal, with minor exceptions in some organisms like mitochondria.
The code is degenerate, with multiple codons coding for the same amino acid.
Methionine has a special start codon, AUG, which initiates protein synthesis.
There are three stop codons that signal the end of translation.
Anticodons on tRNAs interact with mRNA codons through complementary base pairing.
Wobble base pairing allows for some flexibility in the third nucleotide of the codon.
Isoaccepting tRNAs can have different anticodons but carry the same amino acid.
Translation is the process of converting the genetic message from mRNA into a polypeptide chain.
The ribosome is the site of protein synthesis, where mRNA and tRNAs interact.
Translation is divided into three stages: initiation, elongation, and termination.
Transcripts
okay so this is the second lecture in
the lecture set on translation and the
genetic code all right so I apologize
bit of a rough stop at the end of that
first lecture in this set um so I was
talking about they would have these
different dinucleotides trinucleotides
tetranucleotides and they would randomly
go together um and the other thing they
could do is so they could put together U
and C but not as a d nucleotide but they
put U and C together and they'd alter
the ratio so they'd put two U for every
C so you'd more likely get a U or a ucu
or a cuu um than you were to get all C's
or a couple of C so they could calculate
the ratios and then they could then
measure the ratio of the amino acids
that formed and they could go back and
sort out um what codon coded for what
amino acids so by doing all these
different combinations of ratios of
different nucleotides making D TR
nucleotides all those different things
they able to solve actually most of the
genetic code table but do weren't quite
able um to get it all doing it that way
until they um worked on another
technique which we talk about in a
minute so as I mentioning there they
could use all of one polyu polyg polyc
polya random copolymers and there they
could again change the ratio of the
different nucleotides together known
copolymers these are the known
dinucleotides tri-nucleotides that we
just saw on um this slide here and the
final uh type of assay that was
developed really helped them to
definitively solve for every single
codon and this was a ribosome binding
assay so uh nerber and leader developed
a ribosome binding assay where the tRNA
molecules would bind to a ribosome mRNA
complex in the absence of protein
synthesis so they're able to come up
with a technique where they actually
didn't have to
translate all they needed was the TRNA
that was bound to an amino acid to bind
to a ribosome and mRNA and important for
this is they could then synthesize known
three nucleotide codons so they could
put together exactly in in the order in
which they wanted three nucleotides so
they knew exactly what the codon was and
that worked in this system so they
didn't have to do all this random type
of thing they could put together the
three nucleotides they wanted and then
they knew exactly which amino acid would
bind that so um what does that look like
so again a still cell free or sorry this
is the um first methodology I forgot
about having the slide so self-re
extract here so in Beal translation they
mix different polymers together mix
amino acids and then they would measure
um the different poly pepid that formed
and which amino acids were in them this
um ribosome binding assay then was an
assay where they had a
ribosome okay they had a bunch of trnas
so here's a bunch of trnas all of which
or bound to different amino acids and
then they would give the system or put
in the system a specific codon all right
and so what happened in the system then
is the TRNA that matched that codon
would bind in the ribosome so what they
do is then they'd run all of that
through a filter okay and all the
Unbound trnas the tras that weren't
inside of the ribosome would flow
through the filter the ribosomes would
get caught on the filter
they then could detect what amino acid
was bound to that TRNA so basically a
one at a time type experiment they did
one codon then the next and the next so
they had to do all 64 codons and again
they knew what a lot of those would be
and so this verified all that and then
it solved um the few that they didn't
quite know at that point here's another
figure showing essentially the same
thing so here's the ribosome there's the
codon or tri-nucleotide um there's the
TRNA with an amino acid on it and it
runs through a filter and again you can
see the trnas go through the holes or
the pores in the filter where the
ribosome gets caught and then they can
detect what um amino acids on the TRNA
inside of that ribosome all right so
when all of a sudden done they sorted
out what um every single nucleate or
every single codon coded for what amino
acid was coded for by it and then
there's you can find this you find lots
of these on Google you just type in a
Code table and you find lots here's just
one example of it um and showing you all
the different different codons and then
next to it all the different amino acids
that are in it all right so what are
some of the uh characteristics then of
this uh genetic code so some of this be
repeat what I said a little bit earlier
um it is a tripet code okay and we just
talked about that the codon is made up
of three nucleotides three for triplet
it is continuous um didn't talk about
this explicitly but there's no skips or
jumps there's no periods no commas um it
reads each
three nucleotides it doesn't skip over
any of them as it's going um it's non
overlapping we talked about this earlier
each successive groups of three it
doesn't um it does this on the bottom
here three and then three and then three
it doesn't overlap does not do that all
right um it's I have Universal here it's
almost Universal almost all organisms
share the same code um there's again
it's biology so there are a few
exceptions mitochondria has a few
exceptions there's some organisms like a
protozoan has some um exceptions but
it's nearly Universal and that actually
allows scientists and research labs when
you're using model organisms to put
genes from one organism into another
organism and it works the same way um
the gentic code is said to be degenerate
and what that means is that there are
multiple codons that code for the same
amino acid all right so every amino acid
is coded for by at least one codon and
most of the amino acids are coded for by
multiple codons so if you look at the
genetic code table we can see that here
so here's Lucine these four nucleotides
code for lucing and addition Lucine
actually has two more nucleotides so six
different nucleotides code for Lucine
veine has four lots of them have four as
you can see in his whole row here some
of them only have a couple you know
alanine there spine Hine um see cine S2
tyrosine S2 gline S4 and arine S4 so um
different ones methine
however only has one and it also is a
special um codon it's called the start
codon because when protein when
translation starts protein synthesis
starts it always starts at an Aug okay
so there can be augs later in the
protein and so it's just a regular
methine you know get put in but it
always starts at Aug there are also
three codons that are called stop codons
originally they're called nonsense
codons because they didn't make any
sense they didn't bind to any
amino acid in any of the assays and and
didn't produce any um or didn't put any
amino acids in when these particular
codon showed up and then they realized
that they're stop codons and this is
what tells the the cell then to stop
translating and we'll see how that works
and we talk about the details of
translation all right so you have uh
stop signals those are the stop codons
which I just pointed out and the start
signal which is at aug okay so there's
specific Cod a specific codon for start
there are three different specific
codons for
stop all right so how do codons which we
just talked about interact with
anti-codons which we talked about when
we talked about the structure of trnas
that those three nucleotides on the uh
anti-codon loop of the TRNA interact
with the codon so the anti-codon of the
TRNA in a thre pre to five Prime order
matches or is complimentary to the codon
on the MRNA a 5 to3 Prime orientation so
again and and this is a good time to
talk about when base pairing occurs
between
nucleotide chains two two different
nucleotide strands whether it be DNA and
DNA making the double helix whether it's
RNA binding to DNA say when SI RNA binds
to RNA um or sorry that's RNA RNA or
when RNA binds a DNA say when an SN RNA
a snurp the RNA and a snurp binds to to
um an hn RNA um or in this case where
the codon anti-codons base pair it is
always went the base with the strands of
DNA in opposite orientations or
anti-parallel to one another okay they
are always anti-parallel that's how um
hydrogen bonding between bases takes
place so here is the MRNA 5 Prime 3
Prime and this isn't label on this side
but that means five Prime is on the C
end here three prime is on the the G end
here and for this TRNA a five Prime is
on the G end three prime is on the a end
okay it has to read from over here five
Prime to 3 Prime over here because mRNA
is five Prime to 3 Prime they have to be
anti- parallel okay so codon is on the
MRNA anti-codon on the TRNA um there's a
little bit of an exception to this um
base pairing the base pairing doesn't
always have to be all three nucleotides
there's something called wobble and
wobble occurs at the anti-codon five
Prime nucleotide okay or other said
otherwise the three prime nucleotide in
the
codon so the five Prime nucleotide of
the anti-codon does is not constrained
to bind as tightly many of the codons
that code for the same amino acids
differ at the three prime end um this
doesn't mean that any wobble occurs all
the time they're actually wobble rules
not all five Prim nucleotides of the
anti-codon can base pair with any three
prime nucleotides in the codon okay so
there's specific rules to this so um the
reason this works is for example here
with Lucine right as long as you bind
the C and the U it doesn't matter what
nucleotide is in this third spot C and U
is enough to code for locing so Proline
if you have CC doesn't matter what the
third nucleotide is it's still going to
code for Proline and so built in there
is some wobble um and for some and again
this is for all of the amino acids but
for some of them um it doesn't matter
what that third nucleotide is in the
codon um isoaccepting
trnas isoaccepting trnas are trnas that
accept the same amino acid but are
transcribed from different genes and
therefore have different anti-codons
okay so it's just another way saying
there's multiple trnas for any given uh
amino acid and this is because there's
multiple codons for any given amino acid
um I think I have a picture here the
wobble rules yeah so here it shows um
this TRNA caring phenol alanine has an A
A but it's able to recognize uuu so the
a and u a and u are correct base pairs
but G and U normally do not base pair
and if we go and look at uuu in the um
genetic code table here so uu is phenol
alanine
and u u c oops wrong one U uu or u u c
is phen alanine so a a will bind because
the wobble rules to uuu but it also bind
to UU C which is also pheno alanine so
our cells then can get away with having
a single TRNA a okay that has a a that
can bind to UU and uu C because of
wobble at this third position and again
there are rules so if the third base MRA
is a the base and anti-codon can
actually be quite a few different things
let's go to the simplest one if Third
Base m c you combine the G which is
normal you combine the a which is not
normal and I which is in aene one of
those modified bases so there are
certain restrictions to what the third
base
uh in sorry Third Base either in the
MRNA which is the first on the five
Prime and in the
TRNA all right so let's start talking
about the details of translation we've
been talking about details but let's
talk about how translation occurs so the
genetic message encoded by the MRNA is
translated in a five to3 Prime Direction
okay and the polypeptide is made in the
N Terminus to C Terminus Direction all
right so that's the colinearity the n
termin lines up with the five Prim Prime
end of the MRNA the C Terminus lines up
with the three prime end of the MRNA
ribosomes bind the MRNA and TRN carrying
amino acids are brought in each amino
acid binds to its own specific TRNA and
remember we talked about their
isoaccepting trnas and the codon on the
MRNA binds with the anti-codon on the
TRNA trnas are similarly shaped to fit
inside the ribos
they have sort of this L shaped in three
dimensions and you can see here here's
two different um tras one for phenoline
one for aspartate and they have this
similar upside down L shape okay so
they're slightly different if you look
at them closely um just a little bit
different but um they have the similar
shape and that's because they need to
fit in a similar um place inside of the
ribosome so the MRNA
or sorry the MRNA codon recognizes the
TRNA anti-codon the MRNA does not
recognize the amino acid bound to it and
scientists didn't know this initially so
they didn't experiment to figure it out
they want to know what is recognized by
the MRNA and to do this um they did
experiments where the amino acid bound
to the TRNA was chemically
changed so they had a TRNA that normally
carried cysteine here and they could
treat it chemically with nickel hydride
and change it to a TRNA that had the
anti-codon for cine but now was carrying
alanine so a
slide to show this so here's TRNA
carrying cysteine here so normally what
happens is if they gave it a ugc poly
ugc mRNA it would always put in 16 cine
so again inv vital translation
experiment but if they treated this with
nickel um it would change a cysteine to
an alanine so the TRNA still has the
anti-codon for cysteine but it's
carrying an alanine so they want to know
well is is the MRNA going to recognize
um this anticodon and put an alanine or
is it going to recognize this one and
put in Cy so it turns out that when they
did this used the
ugu this TRNA would still be recognized
because it had the cysteine anticodon
but it would put in alanine because that
had been changed so in other words the
MRNA has no idea what amino acid is
being carried on the TRNA it only
recognizes those three um nucleotides in
the
anti-codon all right so the process of
attaching an amino acid to its TRNA is
called charging the TRNA or amino
isolation a class of enzymes called
aminal ACL TRNA synthetases help charge
trnas there's at least one Amino ACL
TRNA synthetase for each amino acid it
recognizes the trna's anticodon in
sometimes other regions of the TRNA in
order to attach correct amino acid so
this sort of then answers the question
well how does a cell put the correct
amino acid on the correct TRNA or the
TRNA that has the correct anti-codon
these Amino ACL TRNA synthesises are
responsible for that recognition they
recognize the correct TR RNA with the
correct anti-codon and put the correct
amino acid on it or attach to that TRNA
an activated TRNA with its proper amino
acid attach is called an amino Amino ACL
TRNA or sometimes called a Charged TRNA
so charging a TRNA is a two-step process
that requires the hydrolysis of ATP and
so we'll talk about this in two steps
one is joining the amino acid to ATP and
the other then is transferring the amino
acid onto the
TRNA all right so I have steps one in
steps two and I have a couple sides to
look at as we look at this so in Step
One the amino acid and
ATP bind to the amino ACL TRNA
synthetase okay so you have the amino
acid here and you have ATP here they
will bind then to the Mr or to the TRNA
synthetase two phosphates are hydrolized
right it's from ATP it loses two
phosphates the leaves so two phosphates
are hydrolized from the ATP forming
bound to the amino acid all right so
this will be bone to the amino acid and
the correct TRNA binds the enzyme so
then the TRNA comes in and binds the
enzyme this step is required to make
step two energy favorable and step two
is the amino acid now is transferred
from
the to the
TRNA okay so the amino acid is
transferred for to TRNA which is then
released this step from the enzyme the
amino acid through its carboxy group is
attached to the two prime or thre Prime
hydroxy of the ribos sugar adenosine and
so see what that looks like here you've
seen this so you have the acceptor stem
here so you have the free three prime
Denine here and it's either going to be
attached to the two prime or the three
prime hydroxy to the carboxy group so
that's the little blue squiggly here is
the calent bond between the TRNA and the
amino
acid all right um this next side
probably shows these steps um a little
bit better so again step one your
specific amino acid
here ATP here they're both going to bind
to the amino ACL TR synthetase then
what's going to happen is the TP is
going to be hydrolized all right so two
phosphates are leave or pyrro phosphat
is going to leave that provides the
energy then to Coal link a Denine
monophosphate to the specific amino acid
then the T specific TRNA is going to
bind to the amino ACL TR synthetase the
energy then this bond is going to be
broken between the phosphate and the
amino acid that energy is going to be
transferred to form the calent bond
between the TRNA and the amino acid and
then the amino Amino AAL TRNA synthetase
is going to release the charged or amino
isolated TRNA so this side if you're
going to study this is probably the
better side it shows the steps more
explicitly than that previous side that
I started with um this side here shows a
little bit better look at um how the
amino a acid is um cently linked so
here's anticodon it just flipped from
the direction the other ones we looked
at so
CC phosphate and a all right and five
Prime here so one prime two prime three
prime so you have the two prime hydroxy
and the three prime hydroxy here um and
the amino acid could be linked to either
this one shows the amino acid linked to
the three prime hydroxy group so that o
belong to ribos and see o carboxy group
here there is a calent bond and again a
molecule water is released and forming
that calent
Bond so that's how amino acids get on
trnas a cell actively carrying out
protein synthesis will have a large
number of charged or amino isolated
trnas floating around in the cytoplasm
waiting to participate in protein
synthesis all right ribosomes um another
major part that is involved in
translation ribosomes are the location
for protein synthesis the MRNA and trnas
must fit into the ribosome to carry out
this function the MRNA is completely in
the small subunit so where are these
things the MRNA is completely within the
small subunit the anti-codon of the TRNA
is also in the small subunit and the
TRNA Bridges the small and large subunit
with the amino acid in the large subunit
um when the ribosomes come together well
so anticodon also in the small subunit
TR um spans large and small and amino
acid then is in the large subunit so
when the ribosome comes together then it
creates different sites known as a p and
E site the a site or aminal ACL site is
the site that binds the incoming TRNA or
the amino isolated or charged TRNA as it
comes in that is the aite the pite or
pepal P stands for pepal contains a TRNA
that holds on to the growing peptide
chain so as peptide chain grows it's
always linked to a TRNA and that TRNA
resides in the psite the eite stands for
exit contains the deated TRNA as it is
released so as the TRNA
transfers the growing peptide that is on
it it will then release through the exit
site so we can take a look at some of
this here so um image up here just shows
an mRNA and a bunch of ribosomes on it
okay and the green is the growing amino
acid coming off it so um an mRNA can be
used multiple times by multiple
ribosomes to make multiple polypeptides
all right so it's not a one shot deal
just um uh an x-ray model of ribosome
and down here is ribosome and in the
next slide I've enlarged that a little
bit so you can see a little bit better
here um so here you go e p a sites so
here's the MRNA it's in the small
subunit site TRNA is going to interact
or the anti coding interact in small
subun TRNA going to span large and small
polypeptide and amino acid that's linked
to the TRNA is going to then in the
large subunit so a is where the TRNA is
going to come in that has a single amino
acid P then is the TRNA now as you can
see with the growing polypeptide once
this polypeptide is transferred over and
we'll go through these steps this deated
TRNA will go to the eite and exit out of
the
ribosome um it's another model showing
similar thing shows actually the a site
being occupied by a TRNA with an amino
acid on it and a P with a growing
polypeptide and the exit site where the
last de isolated TRNA had just
left all right so now we'll start
getting into the details of translation
translation can be divided into three
stages or steps we have initiation
elongation and termination very similar
to transcription having three steps
we'll do the same thing here for
translation um so initiation in involves
binding of the MRNA to the small rabal
subunit that's number one binding of the
initiation TRNA to the small ribosomal
subunit and binding of the large
ribosomal subunit to the small subunit
so there's a number of steps in
initiation and um so just a little bit
of introduction we'll end this portion
here and we'll start with the steps in
the next um lecture
5.0 / 5 (0 votes)