Ch 26 the Transcription and Translation Process for the MCAT

8/16/2019 Ch 26 the Transcription and Translation Process for the MCAT

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Ch 26 The Transcription and Translation Process for the MCAT: Help and Review

Lesson 2 - Transcription of Messenger RNA (mRNA) from DNA

In this lesson, you ill gain a thorough un!erstan!ing of ho transcription or"s# $e ill

in%estigate ho DNA is transcri&e! into RNA ith the help of a promoter an! RNA polymerase#

Learn the purpose of messenger RNA an! e'plore the three phases of transcription#

Intro!uction to Transcription

%eryone "nos that DNA contains the instructions for li%ing things# ut if you*%e e%er &een

confuse! a&out ho DNA actually turns into a li%ing creature, then you*re not alone# Most

people, e%en scientists, get o%erhelme! at the !etails in%ol%e! in all the steps of the central

!ogma# The central !ogma !escri&es the entire +o of genetic information from DNA to RNA to

the nal pro!uct, a protein#

To simplify it all in my hea!, I thin" of it li"e I*m folloing a recipe# DNA is li"e the mastercoo"&oo" an! RNA is li"e the car! that I copy a recipe onto# I imagine that I ha%e to ma"e a

recipe car! &ecause my mom ons the coo"&oo", an! I can*t ta"e it ith me# The coo"&oo" is

similar to DNA, hich li%es insi!e the cell*s nucleus an! can*t &e ta"en out# o DNA has to &e

copie! into RNA, hich can &e ta"en outsi!e the nucleus#

$hy !o e nee! to ta"e genetic information out of the nucleus. It*s &ecause e*re going to use

it to ma"e a protein, hich e can only !o in the cytoplasm# The cytoplasm contains all the

machinery, or the e/uipment, that is use! to ma"e the proteins# It*s 0ust li"e ho my "itchen

contains all the e/uipment for coo"ing my recipe# If I*m going to ma"e my foo! insi!e my

"itchen, then I nee! my recipe car!# If e*re going to ma"e proteins in the cytoplasm, then e

nee! the genetic recipe# That recipe comes in the form of RNA#

In this lesson, e*re only going to &e tal"ing a&out transcription# That means e on*t &e

getting through the entire central !ogma# $e*ll only get to the point here e en! up ith RNA#

In or!er to in%estigate the steps in%ol%e!, e*ll ha%e to ta"e a loo" at some close-up images that

sho ho all of the molecules are arrange!#

ometimes in &iology, hen you*re learning the tiny !etails, it*s easy to lose sight of the &ig

picture# 1ou may &egin to on!er e'actly here e are in the conte't of a cell# ut remem&er

that transcription starts ith DNA, an! DNA li%es in the nucleus# o throughout this entire

lesson, "eep in min! that all of it happens insi!e the nucleus of a cell#

ense tran! an! Antisense tran!

Transcription is the copying of genetic information from the form of DNA to the form of RNA#

Remem&er that RNA is a single-stran!e! molecule it !oesn*t ha%e a complementary stran! li"e

DNA !oes# It*s only half a la!!er, or a single stran! of nucleoti!es# In or!er to ma"e the RNA

stran!, e only nee! one of the original DNA stran!s# $e*ll tal" later a&out hich DNA stran! to

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promoter itself is not actually part of the "eratin gene, an! it !oesn*t co!e for any amino aci!s#

It*s simply a signal for transcription to &egin#

Another chun" of DNA sits at the en! of the "eratin gene# It*s sort of li"e the nish line, an! it*s

calle! the terminator# No, !on*t &e afrai! of the terminator8 It*s only a stop signal#

*Termination* means to stop, or en!, a process# o the promoter is the nucleoti!e se/uence infront of the gene that signals the &eginning of transcription, an! the terminator is the

nucleoti!e se/uence at the en! of the gene that signals the en! of transcription#

3romoters signal transcription to &egin hile terminators signal to en! it

3romoter Terminator Diagram

Initiation, longation an! Termination

RNA polymerase recogni4es the promoter an! &in!s to it on the DNA molecule# This is the

o9cial &eginning of transcription, an! e call this phase initiation# That*s not har! to

remem&er *initiation* 0ust means *the &eginning of something#* If I as copying a recipe from

my mom*s giant coo"&oo", then initiation oul! &e the moment here I sit !on at the ta&le

ith my pen an! recipe car!#

:ere, initiation is the rst phase of transcription, !uring hich RNA polymerase attaches to the

promoter an! &egins to &uil! mRNA# The promoter for our "eratin gene only e'ists on one

stran! of the DNA, hich is ho e "no hich stran! is the antisense stran!# ;or the ne't

gene, the promoter might &e on the same stran!, or it coul! &e on the opposite stran!# It

!oesn*t really matter# %ery gene has its promoter on one or the other stran!, an! it %aries

from gene to gene#


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Transcription Termination 3hase

It*s a pretty intricate !ance here, &ecause the DNA molecule has to split open to allo

transcription to occur, an! it has to come &ac" together as soon as that part is transcri&e!# o,

DNA splits open, gets transcri&e! an! 4ips &ac" together all ithin a %ery short !istance#There*s no room for e'tra molecules in there# o hat happens to the mRNA that*s still groing

longer. $ell, it 0ust peels aay from the DNA template an! hangs o5 to the si!e#

1ou can thin" of it li"e a person "nitting a scarf# As the "nitter creates more an! more length of

fa&ric, the front of the scarf trails o5 to the si!e of the person*s lap# Anyay, this hole phase

of transcription is calle! elongation, &ecause the mRNA stran! elongates as transcription

continues !on the length of the gene# o, elongation is the mi!!le phase of transcription,

!uring hich mRNA gros longer ith each a!!itional nucleoti!e#

No e*re at the en! of the gene, an! it*s time to nish up# 7an you guess the name of the last

phase here. I*ll gi%e you a hint= it has something to !o ith the RNA polymerase nally reachingthe terminator at the en! of the gene# That*s right, it*s calle! termination#

RNA polymerase sees the signal that it*s time to stop transcription# It stops a!!ing RNA

nucleoti!es an! comes o5 from the DNA gene# It also !rops the mRNA that it as &uil!ing

&ecause it*s nally !one# Then it +oats o5 to !o more transcription somehere else# o

termination is the nal phase of transcription, !uring hich the RNA polymerase reaches the

terminator an! !etaches from the gene an! the mRNA#

Lesson ummary

All three phases of transcription occur insi!e a cell*s nucleus# Remem&er, e%erything e*%e

!iscusse! in this lesson is relate! to transcription, hich is only the rst part of the central

!ogma# The purpose of transcription is to pro%i!e a copy of genetic information that can tra%el

outsi!e the nucleus in preparation for translation# $e start ith the genes in the original DNA,

an! e en! ith the genes in mRNA# >ust to &e sure that e*%e got all the steps here, let*s al"

through transcription from the top#

Transcription &egins hen RNA polymerase attaches to a promoter ithin DNA# This is calle!

initiation# The DNA molecule splits open an! allos RNA polymerase to a!! RNA nucleoti!es

onto the DNA template# The DNA stran! that ser%es as the template is calle! the antisense

stran!#

longation is the phase in hich the RNA molecule gros longer as transcription continues

!on the length of the gene# Termination occurs hen RNA polymerase reaches the terminator

an! !etaches from &oth stran!s# The RNA that*s pro!uce! at the en! of transcription can &e

one of three !i5erent types# The type that e*re concerne! ith here, hich enco!es the

se/uence for amino aci!s, is messenger, or mRNA#


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Lesson ? - Regulation of 6ene 'pression= Transcriptional Repression an! In!uction

Do our genes or" the same ay all the time. :o !o e regulate the e'pression of our

genes. 'plore the %arious ays organisms control gene transcription through repression an!

in!uction of operons#

6enes an! 6ene 'pression

:a%e you e%er ishe! that you ha! an i!entical tin. ometimes I*m 0ealous of people ho

ha%e tins# I thin" it oul! &e interesting to see ho another me oul! turn out# I mean,

i!entical tins !on*t turn out completely i!entical# @sually, they en! up slightly !i5erent

heights, ith slightly !i5erent facial features an! !i5erent personalities# ome i!entical tins

are easy to tell apart e%en though their genes are e'actly the same# ut that seems o!!, !oesn*t

it. If i!entical tins ha%e the same e'act genes, then hy are there noticea&le !i5erences at all.

Are genes really the only things that !ene our physical features.


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coul! en! up ith ay too much hair, really long nails or really thic", tough s"in# Life oul! &e a

pain if this happene! to us8 o it*s important that e regulate the e'pression of the "eratin

gene# Regulation of gene e'pression !escri&es a %ariety of mechanisms &y hich our cells

control the amount of protein that*s pro!uce! &y our genes#

3ro"aryotic %s# u"aryotic Transcription

6ene regulation happens !i5erently !epen!ing on hether the organism is a pro"aryote or a

eu"aryote# Do you remem&er the !i5erence &eteen these to. Let*s /uic"ly remem&er#

u"aryotes are organisms, li"e plants, animals, fungi an! protists, that all ha%e cells ith nuclei

an! organelles insi!e# Most eu"aryotes are multicellular# A pro"aryote is a single-celle!

organism, li"e &acteria, that !oesn*t ha%e a nucleus or organelles insi!e# ince a eu"aryotic cell

has a nucleus, an! a pro"aryotic cell !oesn*t, the regulation of transcription is !i5erent

&eteen the to#

In a eu"aryote, the mRNA that is transcri&e! in the nucleus must pass through the nuclear

en%elope to &e translate! in the cytoplasm# efore it can lea%e, it has to &e processe!# omeparts are a!!e! to the stran!, an! some are ta"en out# There*s more to it than that, &ut e*ll

sa%e eu"aryotic RNA processing for another time#

@nli"e in eu"aryotes, multiple genes get translate! together in pro"aryotes

3ro"aryotic Transcription

In a pro"aryote, there*s no nuclear en%elope, so the mRNA can &egin translation right there in

the cytoplasm# Transcription an! translation o%erlap ith each other# o the pro!uction of

proteins actually &egins &efore the mRNA stran! is complete# @nli"e eu"aryotes, pro"aryotes

ha%e more than one gene on an mRNA stran!# o ith the o%erlap of processes, all the genes

on the mRNA en! up getting translate! together# @sually, an organism !oesn*t ant to

translate !i5erent proteins at the same time &ecause !i5erent proteins are in%ol%e! in

!i5erent cellular acti%ities# o, in a pro"aryote, genes that are relate! to each other are foun!

si!e-&y-si!e on the actual DNA# 7lusters of relate! genes are calle! operons# $hen an entire

operon is translate!, a hole team of proteins is pro!uce!# %ery protein on the team

contri&utes to the same cellular function#

The Lac


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en4ymes# The set of three genes is an e'ample of an operon# cientists call this one the lac

operon &ecause it controls the pro!uction lactose-!igesting en4ymes#

No, e*re going to loo" insi!e your intestine for a minute# Let*s say that e%ery morning, hen

you a"e up, you alays !rin" a full glass of mil"# ut for the rest of the !ay, you !on*t ha%e any

more !airy pro!ucts# Do you thin" that the lac operon ma"es the lactose-!igesting en4ymes ata constant rate, all !ay long.


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A segment of DNA that co!es for a specic protein is calle! a gene# 6enes are e'presse! hen

they are transcri&e! into mRNA an! translate! into protein# 6ene e'pression is carefully

regulate! &y all organisms so that the correct amount of each protein is ma!e# u"aryotic

organisms regulate their gene e'pression !i5erently than pro"aryotes# $hile eu"aryotic RNA is

processe! in the nucleus, pro"aryotic RNA is arrange! in clusters of relate! genes calle!operons#

y stu!ying the lac operon foun! in # coli &acteria, &iologists learne! a&out gene regulation

an! the processes of repression an! in!uction# Repression is a !ecrease in gene e'pression#

In!uction is an increase in gene e'pression !ue to the presence of an in!ucer# $hile our genes

pro%i!e all the instructions for the proteins e ma"e, our in!i%i!ual traits are in+uence! &y the

regulation of gene e'pression#

Lesson E - :o An


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This lesson ill al" you through the main parts of an operon, !escri&ing ho each of the

segments or"# $e*ll ta"e a closer loo" at the lac operon an! ho it respon!s to repressors

an! in!ucers# Then, e*ll re%isit repression an! in!uction for a thorough un!erstan!ing of ho

an operon controls transcription in the pro"aryotic cell#

Anatomy of an


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lactose is &oun! to it# o it +oats o5 an! frees up the operon to start transcription again# RNA

polymerase soops in to &in! to the promoter, transcription continues all !on the operon,

an! &efore you "no it, etty, 6ail, an! Theo are &ac" in &usiness8 $hich is great, &ecause e

ha%e plenty more lactose molecules that nee! to &e &ro"en !on#

This re%ersal of repression is hat e call in!uction# $e*%e mentione! &efore that in!uction isan increase in gene e'pression !ue to the presence of an in!ucer# $e no "no that lactose

itself is the in!ucer# An in!ucer &in!s to a repressor an! ma"es it fall o5 the operator# ut, if

you thin" a&out it, lactose !i!n*t really in!uce transcription# It only stoppe! transcription from

&eing &loc"e!# Lactose ma!e transcription possi&le &y ma"ing its &loc"age impossi&le# It

in!uce! transcription &y repressing its repression8 I "no, it seems o!!, &ut it ma"es a lot of

sense if you consi!er ho the &o!y or"s# $hene%er e consume something, e nee!

en4ymes to &rea" it !on# $hene%er e*re not consuming that thing, then e !on*t nee! the

en4ymes# Lactose ensures its on !igestion &y in!ucing the transcription of lac operon*s

en4ymes# $hen lactose isn*t there anymore, then the repressor &ecomes acti%e again# The

acti%e repressor stops transcription, so e !on*t aste energy ma"ing en4ymes e !on*t nee!#

Lesson ummary

It turns out that controlling transcription really isn*t as simple as +ipping a sitch# Transcription

in the pro"aryotic cell is regulate! &y the comple' interactions of DNA, en4ymes, an! regulatory

proteins# ut hen you step &ac" from the mechanics of it all, you can still thin" of repression

an! in!uction li"e turning an operon o5 an! on#

The operon is e5ecti%ely the center of transcriptional control# In a!!ition to its main structural

genes, the operon houses an operator an! a promoter# In front of the promoter lies a

regulatory gene that pro!uces repressor proteins# $hen a repressor is in its acti%e state, it

&in!s to the operator# This &loc"s the promoter lying 0ust upstream an! "eeps RNA polymerase

from attaching to it# The full e5ect is a &loc"age of transcription, or transcriptional repression#

ut once an in!ucer shos up, it ma"es the repressor fall o5 the operator, alloing RNA

polymerase to &in! to the promoter# Transcription &egins here an! continues !on the operon

until all of the structural genes are transcri&e!# In the case of the lac operon, the structural

genes co!e for three !i5erent en4ymes that are responsi&le for lactose &rea"!on#


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All li%ing things ha%e metho!s for regulating their genes &y controlling hen an! ho their

genes are e'presse!# $hen scientists rst learne! a&out gene regulation, they stu!ie! the

&acterium # coli an! one of its gene clusters= the lac operon# cientists use! the lac operon to

stu!y gene regulation# Their research re%eale! plenty of information a&out the regulation of

operons, &ut it !i!n*t tell them much a&out the regulation of human genes# After all, ehumans !on*t ha%e operons# :umans, remem&er, are eu"aryotes#

@nli"e the pro"aryotic &acteria, eu"aryotes !on*t ha%e clusters of relate! genes#


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together to ma"e a continuous, shorter stran!# This process is calle! RNA splicing# It pro!uces a

*nal !raft* of the mRNA &efore translation gets un!er ay# RNA splicing is the remo%al of

introns an! 0oining of e'ons in eu"aryotic mRNA# It also occurs in tRNA an! rRNA#

plicing is accomplishe! ith the help of spliceosomes, hich remo%e introns from the genes in

RNA# pliceosomes are compose! of a mi'ture of protein an! small RNA molecules# They locatethe en!s of the introns, cut them aay from the e'ons, an! 0oin the en!s of a!0acent e'ons

together#


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e'tra mo!ications to the mRNA stran!# %en though they*re ma!e of nucleoti!es, they !on*t

actually co!e for proteins#

They simply act li"e a uniform orn &y a referee in a foot&all game &y protecting the mRNA

from attac" &y cellular en4ymes an! i!entifying it as an important element in the game of gene

e'pression# 7apping an! tailing occurs insi!e the nucleus, right after transcription an! RNAsplicing#


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RNA splicing is the remo%al of non-co!ing introns an! the 0oining of e'ons &y spliceosomes# The

presence of introns is thought to assist gene regulation &y ay of comple' metho!s li"e

alternati%e splicing#

After splicing, a F* cap an! a poly(A) tail are a!!e!, an! this *nal !raft* is calle! mature mRNA#

plicing, capping, an! tailing comprise RNA processing in the nucleus#


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:ere*s our rst gene on the mRNA stran!# It rea!s, A @ 6 A A 6 @ 6 6 @ A 6# ut, I !i!n*t rea! it

the ay I shoul!# >ust li"e tRNA, e nee! to rea! it in the form of co!ons, three letters at a time#

o it really rea!s, A@6, AA6, @66, an! @A6# Let*s &egin ith A@6 an! n! it on our co!on

chart# $hich amino aci! !oes A@6 co!e for. ust "eep in min! that co!on

recognition is the a&ility of mRNA co!ons to &e matche! ith the correct amino aci!s#

The 6enetic 7o!e is Degenerate an! @ni%ersal


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The RNA co!on chart is a tool that e in%ente! to help us un!erstan! co!on recognition# y

loo"ing at the chart, e can see that each co!on species one particular amino aci!# ut

remem&er, each amino aci! may ha%e more than one co!on that it is specie! &y# ecause

there are more co!ons than amino aci!s, e say that the genetic co!e is re!un!ant# ;or

e'ample, &oth co!ons @@@ an! @@7 specify the amino aci! phenylalanine# cientists ha%e aspecial term for !escri&ing this i!ea# They say that co!ons are !egenerate# No, that !oesn*t

mean that co!ons are inferior or corrupte!# It 0ust means that elements ith slightly !i5erent

structures can perform the same e'act function# o co!ons are !egenerate &ecause !i5erent

co!ons can gi%e the same instructions a&out hich amino aci! is specie!#

o amino aci!s can &e in!icate! &y more than one co!on# ut each co!on only species one

amino aci!# ;or this reason, scientists say that the genetic co!e has no am&iguity# In other

or!s, e can &e sure that the co!on @@7 ill alays specify the amino aci! phenylalanine# $e

ne%er ha%e to on!er hether @@7 might in!icate some other amino aci!# In fact, this is true

for almost e%ery li%ing thing# If you loo" at the genetic co!e for an animal, hether it*s a

mammal, a &ir!, a reptile, or a sh, the co!on @@7 ill alays co!e for phenylalanine# This e%enhol!s true for plants, fungi, &acteria, an! %iruses# cientists say that the genetic co!e is

uni%ersal, hich means it is use! the same ay for all the organisms on arth#

The RNA co!e chart is helpful in un!erstan!ing co!on recognition

7o!on Recognition 7hart

Actually, there are some micro&es an! other li%ing structures that tea" the co!e a little# ut

for the most part, e can assume that all li%ing things use the same co!ons to specify the same

amino aci!s# In fact, that*s hy scientists ha%e &een a&le to pro!uce some ama4ing organisms

&y inserting genes from one creature into another# $e no ha%e &acteria that manufacture

human insulin, crops that are resistant to pests an! !isease, an! e%en +uorescent gloing sh8

All &ecause our genetic co!e is uni%ersal# $ithout the uni%ersality of the genetic co!e, none of

these achie%ements oul! &e possi&le#

Lesson ummary

The genetic co!e is use! in the process of translation to con%ert the mRNA co!ons into a chain

of amino aci!s# The RNA co!on chart helps us to i!entify hich co!ons specify hich aminoaci!s# 7o!on recognition !escri&es the a&ility of co!ons to match ith the appropriate amino

aci!s# 6oo! co!on recognition is essential to the correct assem&ly of polypepti!es, hich in

turn lea!s to the pro!uction of the right proteins# The genetic co!e is !egenerate, meaning that

!i5erent co!ons can specify the same amino aci!# Also, the genetic co!e is nearly uni%ersal,

hich means that across all organisms, the co!on specications for amino aci!s remain fairly

consistent#


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In the case of genetic translation, e ha%e a molecule that acts as an interpreter &eteen

co!ons an! amino aci!s# It*s a special type of RNA calle! transfer RNA, or tRNA for short#

Transfer RNA is the type of RNA that interprets the mRNA co!e !uring translation#

Transfer RNA acts as an interpreter &eteen co!ons an! amino aci!s

Transfer RNA Interpreter

$hen a cell is rea!y to ha%e its genetic co!e translate! into polypepti!es, it rst has to &e

prepare! ith all the essential amino aci!s# Remem&er, there are 2J !i5erent amino aci!s

in%ol%e! in ma"ing our proteins# 3roteins can only &e ma!e correctly hen the amino aci!s are

assem&le! in the right or!er# The 0o& of tRNA is to match up the amino aci!s ith the correct

co!ons in the mRNA stran!#

$e can loo" at our co!on chart to get an i!ea of hat tRNA*s 0o& is li"e# Let*s see here# If e

ere tRNA an! e rea! the co!on @@@, then e oul! "no to gra& a phenylalanine# If erea! the co!on A67, then e oul! "no to gra& a serine# Transfer RNA rea!s a gene*s co!ons

from start to nish an! matches the amino aci!s in the correct or!er#

The Antico!on

That soun!s simple enough, &ut ho !oes tRNA go a&out matching the co!ons to the correct

amino aci!s. :o !oes it "no hen it*s foun! a co!on an! an amino aci! that are suppose!

to go together. Li"e all RNAs, tRNA is a single stran! of nucleoti!e &ases# ut a tRNA is short -

only a&out J nucleoti!es long - an! it fol!s up on itself so that some parts are actually !ou&le-

stran!e!# It tists into a funny shape that sort of loo"s li"e a four-leaf clo%er, or a T-shape#

That*s con%enient, &ecause it helps to remem&er that tRNA is shape! li"e a T#

At one en! of the T is the attachment site for a certain amino aci!# At the other en! is a set of

three nucleoti!es that match the co!on that species the same amino aci!# Note that these

nucleoti!es on the tRNA !o not ma"e up a co!on# They are, in fact, the e'act opposite - an

antico!on# The antico!on is a perfect complementary match to a co!on# o, if the co!on for

serine is A67, then the antico!on for serine is @76# The tRNA that contains the antico!on @76

ill also &e the tRNA that attaches to a serine amino aci!#

ust remem&er that 6 an! 7 are &oth consonants, an! A an! @ are &oth %oels#

To get an antico!on, sitch the consonants 6 an! 7 or the %oels A an! @


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Antico!on

o, let*s loo" at the co!on A67 on the mRNA stran!# The complement for A is going to &e @ on

the antico!on# The complement for 6 is going to &e 7, an! the complement for 7 is going to &e

6# o, if e put it all together, e get @76 as the antico!on for the co!on A67#

7o!on Recognition y tRNA

$e alrea!y "ne a&out co!on recognition &efore e got into all this stu5 a&out tRNA an!

antico!ons# ut no e*%e &egun to see the mechanics that are in%ol%e! at the molecular le%el#

o, let*s go ahea! an! e'pan! our !enition of co!on recognition# It*s the a&ility of tRNA to

match a co!on to the appropriate antico!on an!, therefore, the correct amino aci!# tRNA

recogni4es the co!on &y using its on complementary antico!on# It*s 0ust li"e a loc" an! "ey

you "no you ha%e the right "ey hen it ts into the loc"#

The attachment site for the amino aci! is also sitting on the tRNA# These &in! together in asimilar ay# ince &oth the amino aci! an! the mRNA co!on &in! to the tRNA, then they can &e

&rought together as an appropriate match# >ust to &e sure e*%e got e%erything clear, let*s try

an e'ample of genetic translation# $e*%e !one it &efore, &ut this time e*ll &ring tRNA an! the

antico!ons into the picture# Then e*ll really get an un!erstan!ing of ho co!on recognition

or"s#

$e*ll &egin ith the folloing mRNA stran!= A @ 6 @ A @ 7 A 7 @ A A# $e*ll !ra a line in

&eteen each set of three &ases# That*s hat scientists !o# o, e get a total of E co!ons= A@6,

@A@, 7A7, an! @AA# The A@6 co!on e "no is the start co!on, an! it co!es for methionine#

o, rst, e nee! a tRNA that matches ith methionine on one en! an! contains the correct

antico!on at the other en!# The antico!on for A@6 is @A7# :ere*s a tRNA ith the antico!on

@A7, an! it*s &ringing in a methionine attache! to its other en!# 7o!on recognition happens

hen tRNA pairs ith the mRNA insi!e the ri&osome# An! no e*%e gotten starte! ith

methionine#

The ne't co!on is @A@, for hich the amino aci! is tyrosine# The antico!on that pairs ith @A@

is A@A# o here*s a tRNA ith the antico!on A@A, carrying in the amino aci! tyrosine# It pairs

ith mRNA at the ri&osome, an! no tyrosine is a!!e! ne't to methionine#

The ne't co!on is 7A7, for hich the amino aci! is histi!ine# The antico!on for 7A7 is 6@6#

:ere comes a tRNA ith the antico!on 6@6, attache! to the amino aci! histi!ine# The tRNApairs ith mRNA at the ri&osome, an! no histi!ine is a!!e! to the en!#


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important organelles that ma"e polypepti!e assem&ly possi&le# In this lesson, e*re going to

focus on the ri&osome an! the role it plays in helping to &egin genetic translation#

Ri&osomes an! 3epti!e on!s

Translation is easier to un!erstan! hen you thin" of it li"e folloing a recipe for your fa%orite!ish# The mRNA stran! is li"e the recipe &ecause it contains all the instructions for ma"ing a

pro!uct# mRNA is the type of RNA that enco!es the genetic information foun! in DNA# $hen

mRNA lea%es the nucleus, it rst goes loo"ing for a tiny structure calle! a ri&osome# This ill &e

the actual site of translation#

1ou might recall that a ri&osome is a cell organelle that helps in assem&ling chains of amino

aci!s# It*s ma!e of ri&osomal RNA, or rRNA, an! protein# Ri&osomes are locate! on the rough

en!oplasmic reticulum, or RR, that surroun!s a cell*s nucleus# In fact, ri&osomes are the

reason that the RR is calle! rough they gi%e the en!oplasmic reticulum a rough appearance

hen %iee! un!er a microscope#

Anyay, you can thin" of the ri&osome as sort of the *s"illet* of translation it*s the place here

all the action happens# The ri&osome ser%es as a central hu& here all of the ingre!ients are

com&ine! &y the cell*s machinery# o if the ri&osome is the s"illet, an! mRNA is the recipe, then

hat e'actly are the ingre!ients.

3roteins are ma!e up of many of the tenty a%aila&le amino aci!s

Tenty Di5erent Amino Aci!s

The ingre!ients for our protein pro!uct are going to &e the amino aci!s# 1ou may recall that

amino aci!s are the organic molecules that ser%e as the monomers for proteins# There are 2J

!i5erent amino aci!s to choose from, an! their e'act com&inations are uni/ue to e%ery protein,

so it*s crucial that e put the amino aci!s in the correct or!er#

$hen to amino aci!s are 0oine! together &y a chemical &on!, e call it a pepti!e &on!# A

pepti!e &on! is a co%alent &on! &eteen to amino aci!s# $e "no that proteins are ma!e

from long chains of amino aci!s# o if you ha%e an amino aci! chain, then you also ha%e lots of

pepti!e &on!s# ;or this reason, e often use the or! polypepti!e to !escri&e an amino aci!

chain# A polypepti!e is a chain of amino aci!s lin"e! together &y pepti!e &on!s# *3oly* means*many,* 0ust li"e in the or! *polymer,* an! *pepti!e* refers to a pepti!e &on!, so a molecule ith

many pepti!e &on!s is calle! a polypepti!e# If a polypepti!e is the nal pro!uct that e get

from translation, then the pepti!e &on!s are li"e the mi'ing of the ingre!ients in our s"illet#

Ri&osome tructure


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$e*%e got a lot of ne terms an! molecules +oating aroun! here# o let*s loo" at ho

e%erything ts together# $e*ll start ith the cell# Insi!e the cell is the nucleus, an! surroun!ing

it is the en!oplasmic reticulum# $e can see !ots on the rough R, hich are the ri&osomes#

They li"e to sit close to the nucleus here &ecause they*re aiting for the mRNAs to come out#

The mRNA stran!s result from transcription, hich happens insi!e the nucleus# o once

transcription is !one, mRNA comes out, an! sitting there aiting are all the ri&osomes# ThemRNAs lin" up ith the ri&osomes in or!er to start translation#

ust imagine that, generally, the RNA stran!s can mo%e in an! out of the

ri&osome hene%er they ant to#

3epti!e on! ;ormation


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aci!s in the process of translation, &ut ho !oes polypepti!e assem&ly actually occur. There

are three important steps to the process of translation#

There*s a &eginning step, calle! initiation, a mi!!le step, calle! elongation, an! a nal step,

calle! termination# These three or!s may soun! familiar to you# The same terms are use! in

transcription to !escri&e the steps in%ol%e! in ma"ing the mRNA stran!# ut, here intranslation, e*re ma"ing a polypepti!e stran!# In either case, e*re ma"ing a long molecule out

of a chain of smaller su&units# o, hether e*re referring to transcription or translation, the

three terms accurately !escri&e the mechanics of the process# Let*s al" through each step,

one at a time#

Initiation

In initiation mRNA is attache! to tRNA, hich is attache! to the specie! amino aci!

mRNA Translation Initiation tep

$e*ll start ith initiation# During initiation, the mRNA, the tRNA, an! the rst amino aci! all

come together ithin the ri&osome# The mRNA stran! remains continuous, &ut the true

initiation point is the start co!on, A@6# Remem&er that the start co!on is the set of three

nucleoti!es that &egins the co!e! se/uence of a gene# Remem&er also that the start co!on

species the amino aci! methionine# o, methionine is the name of the amino aci! that is

&rought into the ri&osome rst#

An! ho !i! methionine get itself to the ri&osome. y attaching to the tRNA that contains the

right antico!on# The antico!on for A@6 is @A7# $e "no that &ecause of the rules of

complementary &ase pairing# The tRNA ith the antico!on @A7 ill automatically match to the

co!on A@6, &ringing the methionine along for the ri!e# o, there you ha%e it - mRNA is attache!

to tRNA, an! tRNA is attache! to methionine# That*s initiation#

longation

The ne't step ma"es up the &ul" of translation# It*s calle! elongation, an! it*s the a!!ition of

amino aci!s &y the formation of pepti!e &on!s# longation is 0ust hat it soun!s li"e= a chain of

amino aci!s gros longer an! longer as more amino aci!s are a!!e! on# This ill e%entually

create the polypepti!e#

No that e*%e &egun ith the start co!on, the mRNA shifts a little through the ri&osome so

that the ne't co!on is up for gra&s# Let*s say the ne't co!on is @A@# o, no e nee! a tRNA

that has the matching antico!on, A@A#


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Then the rst tRNA lea%es e%eryone else &ehin! an! +oats o5 to n! more or" to !o# 3oor

methionine8 No it*s 0ust !rifting aroun! li"e a lonely "ite in the in!8 That tRNA left methionine

hanging &y only one anchor= its pepti!e &on! ith tyrosine# The tyrosine is still attache! to its

on tRNA, hich, in turn, is clinging to the mRNA insi!e the ri&osome# Alrea!y e can see the

&eginnings of a polypepti!e elongating outar!#

3olypepti!es form as amino aci!s are a!!e! !uring the elongation step

mRNA Translation longation tep

houl! e al" through that process one more time. Let*s "eep e%erything 0ust as e ha%e it

here an! mo%e on to a!! our thir! amino aci!# mRNA shifts o%er again, an! no the thir!

co!on is rea!y for a match# $hat*s that co!on. 7A7# :ere comes a tRNA ith the matching

antico!on, 6@6# It*s also &rought us a histi!ine, since 7A7 co!es for histi!ine# The tRNA*santico!on matches up ith the mRNA*s co!on, putting the histi!ine in perfect position for

ma"ing a pepti!e &on! ith tyrosine#

o no e ha%e methionine, tyrosine, an! histi!ine all connecte!# $e on*t &e nee!ing

tyrosine*s tRNA anymore, so that tRNA !etaches an! +oats aay, 0ust li"e the rst one !i! in the

&eginning# No e ha%e an e%en longer "ite methionine an! tyrosine are !rifting aroun! ith

only their pepti!e &on!s to hol! them !on to the ri&osome#

ut the histi!ine is still connecte! to its on tRNA, an! it*ll stay that ay until it has the ne't

amino aci! to latch onto# 1ou can see ho this chain of amino aci!s oul! gro longer as each

ne co!on is translate!# The a!!ition process an! pepti!e &on! formation continues o%er an!

o%er again until the chain is a&out one hun!re! amino aci!s long#

Termination

The chain nally en!s hen a stop co!on mo%es into the ri&osome# This is the nal step of

translation, calle! termination# Termination &egins ith the arri%al of one of the three stop

co!ons= @AA, @A6, or @6A# $hen any of these enters the ri&osome, the last amino aci! cuts o5

its anchor to the last tRNA# The tRNA an! ri&osome are no longer nee!e!# The gene has &een

successfully translate!, an! no e ha%e a complete! polypepti!e#

In termination, a stop co!on signals the en! of polypepti!e synthesis

mRNA Translation Termination tep


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The polypepti!e is nally free - free to !rift happily all o%er the cytoplasm, untethere! &y

co%alent &on!s8 This nal step, termination, is the en! of polypepti!e synthesis, signale! &y a

stop co!on entering the ri&osome#

o, !oes that mean e*%e nally &uilt a protein. $ell, no, &ut e ha%e &uilt a polypepti!e# It

often ta"es more than one polypepti!e to &uil! a protein# Remem&er that protein synthesis is aseparate process from translation# o, 0ust &ecause you*%e ma!e a polypepti!e !oesn*t mean

you*%e ma!e a protein#

Lesson ummary

No that e*%e or"e! through all the !etails of translation, e can gi%e it a more specic

!enition# $e can say that= translation is the synthesis of a polypepti!e using the genetic co!e

foun! in mRNA# It still ts into our earliest outline of the central !ogma# Translation is the

mechanism &y hich the information in RNA is transforme! into a protein# In this lesson, e

learne! the three steps of translation# Let*s go &ac" an! ma"e sure e*%e got all of them nice

an! clear#

Initiation of translation occurs hen mRNA, tRNA, an! an amino aci! meet up insi!e the

ri&osome#


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How -o Transcription &actor 'inding (ites Wor1?

Transcription factor binding site are a part of either the pro"oter or enhancer regionof a gene% A gene pro"oter sits pstrea" to a gene and contains three i"portantregions% These are the reglator! protein binding site) the transcription factor

binding site) and the R.A pol!"erase binding site% &igre 3 shows a st!li4ed pictreof a pro"oter and enhancer% An enhancer is sall! far pstrea" of a gene% 'indingof transcription factors to an enhancer will sti"late transcription at a higher ratethan if onl! the pro"oter was bond% (o) binding of a T& to an enhancer "eans gofaster5

Transcription &actor 'inding (ites

0"age of a Transcription &actor 'inding (ite

A single T& cant start transcription% 0n fact) "an! proteins are needed% /nce the*rst T& is bond) secondar! factors will bind in a predeter"ined order% This reslts inthe for"ation of a transcription co"ple#% /nce this co"ple# is created) R.Apol!"erase can bind to the -.A and start transcription% Transcription is the wa!-.A beco"es a -.A prodct) either protein or R.A% 0t is li1e "a1ing a photo cop! of a boo1 page% Trning on or o transcription is the wa! we reglate gene e#pression%

How -o We &ind Transcription &actor 'inding (ites?

There are "an! wa!s to *gre ot if a particlar piece of -.A is or has atranscription factor binding site within it% 7o can actall! do a browser search forseveral dierent progra"s which will anal!4e a stretch of -.A se$ence% These

progra"s ta1e the se$ence !o sb"it and predict whether or not there is atranscription factor binding site based on several dierent algorith"s% 8oo1ing fortranscription factor binding site in this wa! is considered an in silico approach% 0nsilico is an e#pression sed to "ean perfor"ed on co"pter or via co"ptersi"lation%

/nce !o identif! a transcription factor binding site) !o need to prove that it isindeed a transcription factor binding site% Again) there are several "ethods fordoing this% /ne of the easiest and best wa!s is to "a1e a "tation in thetranscription factor binding site which will "a1e the transcription factor that bind toit nable to do so% This will test whether !o have reall! identi*ed a transcriptionfactor binding site or not%

Creating a test is done in several steps% &irst) !o isolate !or transcription factorbinding site -.A and attach it to a reporter gene% A reporter gene is an! genewhose protein prodct has an observable eect sch as glowing% (econd) !o "a1ea s"all change in the se$ence +a "tation,% .e#t) !o "a1e "ltiple copies of!or "tated transcription factor binding site and pt the" into a circlar piece of-.A called a vector% Then) !o pt the vector into a "odel cell sch as !east orbacteria% &inall!) !o wait to see if !or gene is e#pressed) that is) do !or bacteria


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Documents

Ch 26 the Transcription and Translation Process for the MCAT