Proteins Translation 4c'

Summary

This lecture delves into the intricacies of translation and the genetic code, focusing on the three stages of translation: initiation, elongation, and termination. It distinguishes between prokaryotic and eukaryotic initiation, highlighting the role of the Shine-Dalgarno sequence in prokaryotes and the scanning model in eukaryotes. The lecture also covers the essential components of the translation machinery, such as ribosomes, tRNAs, and various initiation and elongation factors. It further explains the process of peptide bond formation and the role of release factors in terminating translation. Additionally, it touches on the impact of antibiotics that target bacterial translation without affecting eukaryotic cells.

Takeaways

• 🔬 Translation is a three-step process: initiation, elongation, and termination, which is crucial for protein synthesis.
• 🌐 In prokaryotes, translation initiation involves the Shine-Dalgarno sequence, which helps the small ribosomal subunit bind to the mRNA at the correct site.
• 🔑 The initiator tRNA carries a modified methionine (N-formyl-methionine) in prokaryotes, which is the first amino acid added during translation.
• 🧬 Eukaryotic initiation differs as it lacks the Shine-Dalgarno sequence and instead uses a scanning mechanism to find the start codon, often the first AUG.
• 🌟 Both prokaryotic and eukaryotic translation initiations require initiation factors and GTP for energy.
• 🔄 Elongation is a cyclic process involving tRNA binding, peptide bond formation, and translocation, which is facilitated by elongation factors and GTP.
• 🏁 Termination occurs when a ribosome encounters a stop codon, and release factors help in detaching the completed polypeptide chain from the tRNA.
• 🌱 Eukaryotes have a single release factor, eRF, compared to multiple release factors in prokaryotes, highlighting a key difference in the termination process.
• 📚 Translation can occur in the cytoplasm or on the endoplasmic reticulum, influenced by signal peptides that direct the ribosome and mRNA to the ER for proteins destined for secretion or organelles.
• 💊 Antibiotics often target the differences in translation mechanisms between prokaryotes and eukaryotes, inhibiting bacterial protein synthesis without affecting human cells.

Q & A

What are the three steps of translation?

The three steps of translation are initiation, elongation, and termination.

What is the role of the Shine-Dalgarno sequence in translation initiation?

The Shine-Dalgarno sequence is a purine-rich sequence upstream from the AUG initiation codon in procaryotes that helps align the mRNA and small ribosomal subunit at the correct location for translation initiation.

How does the initiation of translation differ between procaryotes and eukaryotes?

In procaryotes, translation initiation involves the Shine-Dalgarno sequence and binding directly to the ribosomal binding site. In eukaryotes, initiation involves binding of the 40S subunit to the 5' cap of the mRNA with the help of eukaryotic initiation factors, followed by scanning for the first AUG codon.

What is the function of the initiator tRNA?

The initiator tRNA carries the first amino acid, methionine (or its modified form, formylmethionine in procaryotes), to the ribosome during translation initiation.

What is the role of GTP in translation initiation?

GTP provides the energy required for the binding of the small ribosomal subunit to the mRNA and for the association and dissociation of initiation factors during the initiation process.

How does elongation factor Tu (EF-Tu) contribute to translation elongation?

Elongation factor Tu (EF-Tu) in procaryotes helps deliver aminoacyl-tRNAs to the ribosome by facilitating their binding to the A site of the ribosome.

What is the significance of the peptidyl transferase center in translation?

The peptidyl transferase center, which is part of the 23S rRNA in the large ribosomal subunit, catalyzes the formation of peptide bonds between amino acids during translation elongation.

How does translocation occur during translation elongation?

Translocation is facilitated by elongation factors (EF-G in procaryotes and EF2 in eukaryotes) and GTP, which move the ribosome one codon along the mRNA, shifting the peptidyl-tRNA from the A site to the P site and the empty tRNA from the P site to the E site.

What are the termination codons, and how does translation termination occur?

The termination codons are UAA, UAG, and UGA, which do not code for any amino acids. During translation termination, release factors bind to these codons in the A site, stimulating peptidyl transferase to cleave the bond between the polypeptide chain and the tRNA, releasing the completed protein.

How can multiple ribosomes translate a single mRNA molecule simultaneously?

Multiple ribosomes can bind to a single mRNA molecule and translate it simultaneously, forming a polyribosome (or polysome) complex, where each ribosome is translating a different section of the mRNA into a polypeptide chain.

How do antibiotics target bacterial translation without affecting eukaryotic translation?

Antibiotics target specific components or processes of bacterial translation that differ from those in eukaryotes, such as binding to the 30S ribosomal subunit in bacteria, which has a different structure than the 40S subunit in eukaryotes, thus inhibiting bacterial protein synthesis without affecting eukaryotic cells.

Outlines

00:00

🔬 Translation Initiation in Prokaryotes

This paragraph delves into the process of translation initiation in prokaryotes, highlighting the role of the Shine-Dalgarno sequence in mRNA. This purine-rich sequence, located upstream of the AUG initiation codon, is crucial for the correct positioning of the mRNA and small ribosomal subunit. The paragraph explains how the complementary nature of the Shine-Dalgarno sequence to the 16S rRNA in the small subunit facilitates the binding and alignment of the ribosome at the ribosome binding site. The importance of this sequence is underscored by research that involved sequencing and mutating this region to observe effects on translation initiation. The paragraph also introduces the concept of initiation factors and GTP's role in this process.

05:03

🌟 Initiation and Methionine's Role in Translation

The paragraph explains that all protein sequences begin with methionine, which is often removed later during protein processing. In prokaryotes, the initiator tRNA carries a modified methionine called N-formylmethionine, which is distinct due to a formaldehyde group added by a transformylase. This modification protects the growing polypeptide chain within the cell. The paragraph describes the formation of the 30S complex, which includes the mRNA, small ribosomal subunit, initiator tRNA, and initiation factors. It also details the subsequent binding of the 50S subunit to form the 70S initiation complex, setting up the ribosome with two binding sites: the A site for incoming aminoacyl tRNAs and the P site for the growing polypeptide chain. The paragraph transitions into discussing the differences in translation initiation between prokaryotes and eukaryotes, noting the absence of a Shine-Dalgarno sequence in eukaryotes and the use of a scanning mechanism instead.

10:06

🧬 Eukaryotic Translation Initiation and Elongation

This section contrasts eukaryotic translation initiation with that of prokaryotes, emphasizing the absence of a Shine-Dalgarno sequence and the reliance on a 5' cap and eukaryotic initiation factors, particularly eIF4E, which includes a cap-binding protein. The 40S subunit slides down the mRNA until it encounters the first AUG codon, often initiating translation there. The paragraph also covers the scanning model of translation initiation, introduced by Marilyn Kozak, and the loose consensus sequence surrounding the initiation codon. It transitions into the elongation phase of translation, detailing the three steps: aminoacyl tRNA binding to the A site, peptide bond formation catalyzed by peptidyl transferase (a ribozyme activity of the 23S rRNA), and translocation facilitated by elongation factors and GTP, which move the ribosome and tRNAs to the next codon.

15:08

🔋 Elongation and Termination of Translation

The paragraph focuses on the elongation phase of translation, illustrating how the ribosome cycles through binding, peptide bond formation, and translocation until a termination codon is reached. It describes the process of peptide bond formation between the amino acid in the A site and the growing polypeptide chain on the tRNA in the P site, facilitated by peptidyl transferase. The subsequent translocation phase, involving elongation factors and GTP, moves the ribosome and tRNAs to prepare for the next aminoacyl tRNA. The paragraph concludes with a discussion of translation termination, where release factors bind to the stop codon in the A site, stimulating peptidyl transferase to cleave the bond between the tRNA and the polypeptide chain, releasing the completed protein. It also mentions the concept of polysomes, where multiple ribosomes translate a single mRNA molecule simultaneously.

20:10

💊 Antibiotics and Translation Inhibition

The final paragraph discusses the spatial aspects of translation, noting that it can occur in the cytoplasm or be attached to the endoplasmic reticulum, with proteins being translated and translocated into the ER lumen. It touches on post-translational modifications such as phosphorylation and the role of chaperone proteins in protein folding. The paragraph emphasizes how the information encoded in the mRNA sequence not only determines the amino acid sequence of a protein but also contains signals for protein sorting and localization. It concludes by highlighting the medical significance of the differences between prokaryotic and eukaryotic translation, explaining how antibiotics can target bacterial translation without affecting eukaryotic cells, thus serving as a therapeutic strategy.

Keywords

💡Translation

Translation in the context of the video refers to the process by which the genetic information encoded in mRNA is used to synthesize proteins. It is central to understanding how genetic information is expressed in living organisms. The video discusses the three stages of translation: initiation, elongation, and termination, which are essential for the accurate synthesis of proteins.

💡Genetic Code

The genetic code is the set of rules by which information encoded in genetic material (DNA or mRNA sequences) is translated into proteins by living cells. The video script explains how the genetic code is read during translation, particularly focusing on the role of codons and anticodons in this process.

💡Initiation

Initiation is the first step in the translation process where the ribosome assembles and begins to read the mRNA sequence. The video describes how initiation occurs in both prokaryotes and eukaryotes, involving the binding of the small ribosomal subunit to the mRNA and the initiator tRNA.

💡Shine-Dalgarno Sequence

The Shine-Dalgarno sequence is a specific nucleotide sequence found upstream of the start codon in prokaryotic mRNA. It plays a crucial role in the initiation of translation by helping to align the ribosome with the correct start site on the mRNA. The video explains how this sequence is complementary to a sequence in the 16S rRNA of the small ribosomal subunit.

💡Elongation

Elongation is the stage of translation where the ribosome reads the mRNA codons and adds amino acids to the growing polypeptide chain. The video details the steps involved in elongation, including the binding of aminoacyl-tRNA to the A site, peptide bond formation, and translocation of the ribosome.

💡Termination

Termination is the final step in translation where the synthesis of the protein is completed. The video describes how this occurs when the ribosome encounters a stop codon on the mRNA, leading to the release of the completed polypeptide chain.

💡Ribosome

A ribosome is a complex molecular machine found within cells that facilitates the synthesis of proteins from mRNA. The video script discusses the structure and function of ribosomes, particularly how they change conformation during the elongation phase of translation.

💡Anticodon

An anticodon is a sequence of three nucleotides found on a tRNA molecule that is complementary to a specific mRNA codon. The video explains how anticodons play a critical role in ensuring the correct amino acid is added to the growing polypeptide chain during translation.

💡Methionine

Methionine is an amino acid that is always the first in the sequence of amino acids in a newly synthesized protein. The video describes how initiator tRNA carries methionine to the ribosome during the initiation phase of translation, with a special emphasis on the modified form, N-formylmethionine, in prokaryotes.

💡Eukaryotes

Eukaryotes are organisms whose cells have a nucleus enclosed within membranes. The video contrasts the translation process in eukaryotes with that in prokaryotes, highlighting differences such as the absence of a Shine-Dalgarno sequence and the use of a 5' cap on mRNA for initiation.

💡Polysome

A polysome, also known as a polyribosome, is a structure formed when multiple ribosomes are translating a single mRNA molecule simultaneously. The video mentions polysomes as an example of how multiple proteins can be synthesized from the same mRNA at the same time.

Highlights

Translation involves three steps: initiation, elongation, and termination.

In prokaryotes, translation initiation involves the Shine-Dalgarno sequence upstream from the AUG start codon.

The Shine-Dalgarno sequence helps the small ribosomal subunit bind to the mRNA at the correct site.

Initiation factors and GTP are crucial for the initiation process in both prokaryotes and eukaryotes.

Eukaryotic translation initiation differs by lacking a Shine-Dalgarno sequence and instead uses a 5' cap and scanning mechanism.

The initiator tRNA carries a modified methionine called N-formylmethionine in prokaryotes.

In eukaryotes, the initiator methionine tRNA is not modified with a formyl group.

Translation elongation consists of three steps: tRNA binding, peptide bond formation, and translocation.

Elongation factors Tu and Ts facilitate aminoacyl-tRNA binding in prokaryotes.

Peptide bond formation is catalyzed by peptidyl transferase, an activity of the 23S rRNA.

Elongation factor G (EF-G) in prokaryotes and EF2 in eukaryotes assist in translocation.

Translation termination occurs when a ribosome encounters a stop codon and release factors are involved.

In prokaryotes, release factors RF1 and RF2 bind to stop codons, while eukaryotes use a single release factor.

Multiple ribosomes can translate a single mRNA molecule simultaneously, forming a polyribosome or polysome.

Translation can occur in the cytoplasm or be attached to the endoplasmic reticulum for proteins destined for secretion.

Antibiotics often target bacterial translation machinery, exploiting differences between prokaryotic and eukaryotic systems.

Transcripts

all right welcome back again this is the

third lecture in the lecture sets on

translation and the genetic code all

right so we ended talking about the

three different steps of translation

initiation elongation and termination

and we talked very briefly about the um

what initiation involves getting that

mRNA and small ribosomal subunit

together um bringing in the initiation

TRNA and then finally bringing in that

large subunit um of the ribosome

together so we'll start by talking about

U translation initiation in procaryotes

and then we'll talk about um initiation

in ukar before moving on to elongation

so procaryotes contain a puring rich

sequence that is about four to seven

nucleotides long Upstream from the AUG

initiation codon and this sequence is

called the shine Del garno sequence okay

so this is going to answer the question

how do the MRNA and small ribosomal

subunit come together at the right place

all right so the shine Del garal

sequence then is in the MRNA it's

Upstream of where translation is going

to start and again translation is going

to start at that Aug codon okay the

start codon so what this looks like is

here so here is the MRNA five Prime to

three prime there's the star codon so

that's going to be the first codon used

to insert the first meth um amino acid

so Upstream to the left here there's

this sequence that's complimentary to

the sequence in the 16srrna so remember

the 16srna is in the small ribosomal

subunit in Pro carots so I mentioned

when you're talking about ribosomes and

the spbg units 16s and stuff I S of

mentioned that it's involved in this

before realize we hadn't quite got there

yet well here we are um and this

sequence then is complimentary so that

helps line up so you remember the 16s

RNA is within the small subunit it helps

line up the small subunit in the right

place along the MRNA so the sequence is

complimentary to the three or sorry the

shin Del sequence is complimentary to

the thre Prime end of this 16sr RNA in

the small ribosomal subunit this

complimentarity then allows a small

ribosomal subunit to bind the MRNA and

what is referred to as a ribosome

binding site so you have the shog garal

sequence and this whole thing where it

sits down and binds is called the

ribosome binding site and this is a site

where the ribosome becomes oriented in

the reading frame for the initiation of

protein synthesis okay so it just gets

it to the right spot so it can start at

the right spot so identifying the

ribosomal binding site and identifying

its necessary role and initiation for

translation was determined in two ways

so how do they know this was important

well they did a couple different things

first they took a bunch of mrnas and

they sequenced this region Upstream of

the star codon okay and what essentially

they're doing they're looking for a

consensus sequence and so they did this

did this they noticed that this sequence

okay or sequence very similar to it was

found in lots of mRNA so it looked

important how do you know for sure it's

important well what they did is they

mutated these sequences to see if

initiation of translation could still

proceed all right so they started making

mutations in here and notice that then

you couldn't start translation properly

then they made complimentary mutation so

they made a mutation here so let's say

they changes G these two G's to use and

you can't get binding and you don't get

translation but if you these are two U's

and then you change these two C's to A's

suddenly you return the complimentarity

and you return the function of being

able to start in the right place so just

a few lines of how research questions

are asked um and really asking whether

nucleotide sequences are important or

not a good way to do that is to mutate

them and see if they can't do what they

normally do and the other way is to try

and return the function by um if you

think complimentarity is important by

mutating um the other strand of RNA or

DNA that's interacting with

it all right so in addition to this

small ribosomal subunit binding

initiation also involves an initiator

TRNA okay three different initiation

factors all right so I don't have that

on this slide initiation Factor 1 2 and

three and GTP for energy so I'm going to

explain as we go through where the

initiation factors sort of come in and

leave and or GP GTP does but I'm not

going to ask you to remember

that specifically know they're involved

okay but I want you to know the other

details and not worry so much but just

know there are initiation factors in GTP

um

involved so initiation Factor one and

initiation factor three and the GTP

molecule are bound to the small

ribosomal subun subunit as it finds and

binds its ribosomal binding site so we

just explain how the small subunit with

the shin doal sequence binds to the

ribosomal binding site and you have an

ation factors in GTP involved Aug okay

again that codon encodes methionine thus

all protein sequences originally begin

with methionine which in many cases is

later removed during protein processing

so even though methionine is always the

first one during translation we'll learn

later and you might have learned in

other classes that proteins get

processed and lots time that involved

cleaving off part of the polypeptide

chain often times and Terminus um but

we're just talking about translation now

and methines always first the first

methionine that is inserted is inserted

by a special TRNA called the initiator

TRNA initiator TRNA carries

methine in procaryotes the initiator

methionine is modified by the addition

of a formal group added by a transformal

a and it's called formal methionine okay

so we have formal methionine so not only

does it have methionine but it has this

formal group covalently attached to the

methionine which makes it slightly

different than any other methionine that

would get um inserted during po during

synthesis of the protein this formal

group on the methionine then blocks the

aminal group of methine from reacting so

it protects the protein while it's being

made um within the cell so at this point

the MRNA 30s subunit formal methionine

TRNA initiation Factor 1 two is known as

the 30s complex okay so this 30s complex

so when all this stuff right above where

it says 30X complex is put together it's

called the 30X complex the 50s subunit

then binds to the 30s complex GTP is

hydrolized and released with initiation

Factor 1 and two and this is then the

70s initiation complex or the entire

initiation complex this complex now has

two binding sites the a site or amino

ail site which will bind any incoming

Amino ACL trnas and the peptidal or P

site which binds the TRNA possess

possessing the growing polypeptide chain

what's unique is at this point

protein synthesis starts with the

initiator methine TRNA in the P site and

so I'll take a look at that here all

right so here's the small ribosomal

subunit you get that they'll show you

just this small portion in pink here of

the 16s RNA that contains the sequence

that binds the shinal sequence so it'll

come in and there's initiation factors

um bind to the shinal sequence that will

Orient the smaller R subunit in the

right spot then the initiator TRNA

methionine will come in here and then

the large ribosomal binding sorry the

large ribosomal subunit will come in and

you can see here that when it's all put

together this initiator TRNA is in the

psite so this is the only point at which

you have a single amino acid on a TRNA

in the psite every time after this

you'll have a growing polypeptide chain

in this P site but this is how

initiation begins

all right so translation initiation in

UK carots is similar to procaryotes with

some differences of course one there is

no shog Garo sequence or equivalent

sequence the 40s subunit when it binds

to the MRNA it binds to the five Prime

cap with the help of a eukariotic

initiation Factor this eukariotic

initiation factor is e

if4 e so not F but 4E so eukariotic

initiation factor for E okay and this

initiation Factor actually consists of

multiple proteins including something

called the CBP that stands for cap

binding protein okay so the 40s subunit

is able to bind to the MRNA with the

help of initiation factor that contains

a cap binding protein this 4ds subunit

then migrates or slides down the five

Prime untranslated region within the

MRNA until it meets the first

Aug and then stops I should say most

often the first Aug is where it stops

this is actually or what this is called

is called the scanning model of

translation initiation all right and

this this model was first put forth by

Marilyn kak the initiation codon is

found within a loose consensus sequence

called the KAC KAC sequence after

Marilyn kak the first Aug is usually

always the initiation Aug okay okay

again biology so they're going to be

exceptions but most of the time you have

this loose consensus sequence within

that is the um a gene so I think I have

here yep so just comparing the two

different ways for RI ribosome RBS

ribosome binding sites shine Del garno

is for procaryotes UK carots you have

this KAC sequence and so here's the

loose consensus for the KAC sequence and

here then is the

initiator um codon within that sequence

all right so the difference is is um in

procaryotes it binds to the site and in

the right orientation here the small

subunit will bind upstream and sort of

scan until it binds more tightly in the

correct orientation um here all right so

here's a pretty complicated slide but um

just look at this part here here's the

MRNA so here's a complex so purple is

the small rabal subunit so you're going

to get initiation factors that are going

to help it again the cap binding protein

bind to the cap and it's going to scan

right until it gets to the right spot so

this is as far as we are right now right

in the middle here um and so the next

step then is the um initiator methionine

on the TRNA entering into the scene so

they show it very early here um so

eukariotic UK carots also have an

initiator methionine but is not modified

does does have a formal group added to

it like a procaryotic one does so in uh

ukar it's also believed that polya

binding protein pabp here also plays a

role in binding one of the initiation

factors at the five Fram cap so here's

initiation Factor 4E with cap binding

protein here it's thought that this poly

binding protein so you can see the MRA

sort of bends around here also helps

with this initiation

event so here's um just slide sort of

comparing bacterial factors initiation

factors elongation factors and release

factors so you got bacterial ones and

their counterparts in your carots and

again I'm not expect you to know all the

names of these things or exactly where

they're acting but but I do want you

know that that there are initiation

factors helping with that event there

are elongation factors helping with the

elongation and you'll see very

specifically how release factors are

involved in termination of translation

all right so to finish up um initiation

and UK carots again binding here getting

to the correct site this initiator TRNA

is in its location here bound to the 40s

subunit and then you get the 60s subunit

coming in and joining so you have the

initiation complex here all right the

complete initiation complex and again

just like in procaryotes in the P site

so you see p site here and the P site

will be where the initiator TRNA with

methine on it is

all right so that was initiation now

we'll move on to translation elongation

so elongation takes place in three steps

or three steps that we can talk about um

first is aminal ACL TRNA binds to the

asite so remember at initiation the

initiator TRNA is in the psite so the

first step is an amino ACL TNA entering

the asite um a peptide bond then forms

and finally the third step is

translocation transfer from the a to the

P site or moving from one site to the

next so you open up a new asite for the

next Amo isolated TRNA to enter so Amo

ACL TRNA binding to the asite is through

the anti-codon codon binding Okay so

we've talked about that so in

procaryotes this binding is facilitated

by GTP and elongation factors so there's

two different elongation factors in

procaryotes you have uh Tu which stands

for temperature unstable and there's

also elongation Factor TS um for temp

temperature stable so just historical

names and how they were named so when

youl TNA elongation Factor Tu complex

comes into the asite and then the eftu

can be uh released um the amino ACL TRNA

associate with elongation or yes

elongation Factor Tu to form a tary

complex which moves into the as site the

ribosome changes shape and then allows

eftu to be released the amuno ACL ends

of the 2 TRNA is those within the A and

P sites are now right next to each other

so the next step then is peptide bond

formation peptidal transferase catalyzes

the peptide bond formation between the

amino acid and the asite and the pite

immediately after the amino ACL TR

binding is complete this pepal

transferase activity is due in part to

the

23srrna

this means that this RNA is acting as an

enzy so it is known as a ribozyme now

both amino acids are linked to the TRNA

in the a

site so this TRNA in the a site is

therefore converted from an amino ACL

TRNA to a peptidal TRNA so last step

then is translocation after the

formation of the peptide bond a peptidal

TRNA is in the a site and an uncharged

TRNA is in the P site elongation factors

EF elongation factors EFG in procaryotes

ef2 in ukar and GTP play roles in this

translocation step EFG appears to fit

into the asite moving the new peptidal

TRNA into the psite and then releasing

the uncharged TRNA is released through

the eite and the ribosome advances to

the next codon placing the pepal TRNA in

the P site this process is repeated

until a termination codon is

reached all right so that was a bunch of

words U makes a little bit easier to

look at it so here's the initiation

complex um actually this shows it

already um three amino acids in but so

here's a peptidal TRNA sitting in the

pite so the next TRNA is going to come

into the a site and again elongation

Factor Tu help it comes in the next step

is peptidal transferase okay and what's

going to happen is

the peptide or growing polypeptide is

going to transfer from the TRNA and the

pite over to the TRNA and the asite it's

going to move from left to right here

which means that that first that green

that first amino acid methine will

always be on the end which it should be

so once that happens then you're going

to get

translocation okay so ribosome's going

to move which is going to push this

deated TRNA into the E site so it can

leave and it's going to move the TRNA

with the growing polypeptide chain into

the P site um the uncharged TRNA will

leave and now you're all set up again

just like you were up here except you

have one more long amino acid opened up

the amino ACL site for the next TRNA to

come in uh similar thing here so TRNA

growing polypeptide chain next one comes

into the a site pepal transferase

growing polypeptide moves from P to a

here get

translocation deated moves out you have

the new TRNA with the growing

polypeptide chain in the correct site so

this shows a little bit more detail

about that peptide bond we've already

looked at what the peptide bond forms

between but so here you have um just a

single

um amino acid here and the amino acid

and the asite peptide bond formation uh

catalyzed by pepal transferase and so it

shows here how this one moves so these

two groups here carboxy group and amino

group are going to join and so this

group here is still on that

end all right protein synthesis

terminates when a ribosome encounters

one of the termination codons also known

as nonsense codons at the close of one

of their elongation Cycles the

Terminator codon lies in the empty a

site after

translocation and so as you know there's

no

TRNA that um recognizes the codon for

the Stop codons so any of those

termination codons there's not a TRNA

that recognizes that all right so in

procaryotes a release Factor RF Factor

binds to the nonsense codon in the as

site there's a couple of different

release factors bind the two of the

different stop

codons release factor three is then a

termination stimulatory Factor so the

release factors stimulate pepal

transferase to cleave the coent bond

between the TRNA and the polypeptide

chain so let me take a look at what that

looks like so here we

have P site with the TR with growing

polypeptide chain stop codon is in the a

site you're going to get a release

factor is going to come in release

Factor 3 is going to help and it's going

to stimulate the TRNA to release the

growing polypeptide chain or you

simulate pepal transferase so basically

it's going to make pepal transferase

cleave this like it's going to join to

another amino acid except of course

there's no another amino acid here and

so it releases the polypeptide once it

release polypeptide that's released from

the ribosome the deated TRNA is release

and then what ends up happening is all

the parts um come apart at that stage so

in ukar termination occurs much the same

way UK carots just have a single release

Factor called e for ukar RF release

Factor one of the few cases I know of

where UK carots have less of something

than procaryotes too so it turns out

then that multiple ribosomes can bind a

single mRNA and translate all at the

same time and there's a term given for

this it's called a poly ribosome or

actually for short it's often called a

polysome so polysome is the complex of

mRNA and the ribosomes that are

simultaneously translating it and again

multiple ribosomes can bind and

translate protein on the same

mRNA and so here's a cting drawing of it

mRNA get multiple ribosomes on here

creating ever longer polypeptides here's

an electron micrograph of that actual

thing so the middle here this line is

the MRNA all these globular structures

are the ribosomes and all these things

little chain like things coming off are

growing polypeptides on that U growing

polypeptides that are being translated

so that's the sort of simplest

explanations of uh translation of course

translation can take place in a couple

different places when the in cell um you

can have it out in cytoplasm or attached

to the um er membrane and I have a

figure here showing that so you can have

out here in the cytoplasm where it just

happens or um sometimes when the

ribosome is undergoing translation of an

mRNA there'll be a particular signal

here called the signal peptide which is

recognized and that then can bring this

uh translating ribosome and mRNA to the

endoplasmic reticulum which then sort of

cycles that growing polypeptide or

translocates it through the membrane

into the Lumen where it can continue to

be made and then it can end up inside

the Lumen of the endoplasmic reticulum

rather than outside in the cytoplasm um

there of course other post- transational

modifications you take something like

cell biology you talk more extensively

about these sorts of things U there's

protein sorting during translation

there's signal sequences that send

protein certain places here's just one

example of a signal sequence sending the

protein to the Lumen of the er um

proteins get help with their folding

with other proteins called chaperonins

that help them do this lots of proteins

get phosphorilated there's a whole group

of proteins called kinases um that add

phosphate groups to different proteins

which either activate or deactivate

those proteins it's a way to regulate it

phosphatases do the exact opposite they

um remove phosphate groups from proteins

um and again the proteins get move from

one place to the other based on a

sequence of amino acids so you it's

interesting you have right a information

within the DNA that codes for the MRNA

that codes for the amino acid and

there's information within that

information um the order of the amino

acids that are made send its own

information as to where the protein

should go whether it should be sorted in

the ER or there's something called the

nuclear localization signal telling it

okay you made this protein here it's got

this signal on it we're supposed to ship

you back in the nucleus because that's

where your where your job

is um so just comparison bacterial nucar

translation so a good summary um of

what's uh what between bacterial and um

eukaryotic so you sort of compare some

of the differences that we've talked

about

here and um oh the other thing last

thing I to mention here is that um a lot

of antibiotics work by actually

specifically targeting or inhibiting

translation in bacterial cells so that

it inhibits bacterial cell translation

but doesn't inhibit translation in

eukariotic cells so those subtle tiny

differences between procaryotes and UK

carots can actually be taken advantage

of in a medical way okay there's

antibiotics that will stop bacterial

cells from being able to translate and

therefore kill the cells but will not

have any uh impact on our cells on

eukariotic cells uh so with that we'll

end this um series of lectures on

translation and genetic code