Chapter 17

From Gene to Protein

Transcription and Translation

Overview: The Flow of Genetic Information

• The information content of DNA is in the form of specific sequences of nucleotides

• The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins

• Gene expression, the process by which DNA directs protein synthesis, includes two stages: transcription and translation

• The ribosome is part of the cellular machinery for translation, polypeptide synthesis

Evidence from the Study of Metabolic Defects

• In 1902, British physician Archibald Garrod first suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions

• He thought symptoms of an inherited disease reflect an inability to synthesize a certain enzyme

• Linking genes to enzymes required understanding that cells synthesize and degrade molecules in a series of steps, a metabolic pathway

Nutritional Mutants in Neurospora: Scientific Inquiry

• Beadle and Tatum exposed bread mold to X-rays, creating mutants that were unable to survive on minimal medium as a result of inability to synthesize certain molecules

• Using crosses, they identified three classes of arginine-deficient mutants, each lacking a different enzyme necessary for synthesizing arginine

• They developed a “one gene–one enzyme” hypothesis, which states that each gene dictates production of a specific enzyme

Figure 17.2

Precursor

Enzyme A

Enzyme B

Enzyme C

Ornithine

Citrulline

Arginine

No growth:Mutant cellscannot growand divide

Growth:Wild-typecells growingand dividing

Control: Minimal medium

Results Table

Wild type

Minimalmedium(MM)(control)

MM +ornithine

MM +citrulline

MM +arginine(control)

Summaryof results

Can grow withor without anysupplements

Gene(codes forenzyme) Wild type

Precursor

Ornithine

Gene A

Gene B

Gene C

Enzyme A

Enzyme B

Enzyme C

Enzyme A

Enzyme B

Enzyme C

Citrulline

Arginine

Precursor

Ornithine

Citrulline

Arginine

Precursor

Ornithine

Citrulline

Arginine

Precursor

Ornithine

Citrulline

Arginine

Enzyme A

Enzyme B

Enzyme C

Enzyme A

Enzyme B

Enzyme C

Class I mutants(mutation in

gene A)

Class II mutants(mutation in

gene B)

Class III mutants(mutation in

gene C)

Can grow onornithine,citrulline,or arginine

Can grow onlyon citrulline orarginine

Require arginineto grow

Class I mutants Class II mutants Class III mutants

Classes of Neurospora crassa

Con

ditio

n

The Products of Gene Expression: A Developing Story

• Some proteins aren’t enzymes, so researchers later revised the hypothesis: one gene–one protein

• Many proteins are composed of several polypeptides, each of which has its own gene

• Therefore, Beadle and Tatum’s hypothesis is now restated as one gene–one polypeptide

Basic Principles of Transcription and Translation

• Transcription is the synthesis of RNA under the direction of DNA

• Transcription produces messenger RNA (mRNA)• Translation is the synthesis of a polypeptide,

which occurs under the direction of mRNA• Ribosomes are the sites of translation

• In prokaryotes, mRNA produced by transcription is immediately translated without more processing--coupled

• In a eukaryotic cell, the nuclear envelope separates transcription from translation

• Eukaryotic RNA transcripts are modified through RNA processing to yield finished mRNA

• Cells are governed by a cellular chain of command: DNA RNA protein

Figure 17.3Nuclear envelope

CYTOPLASM

DNA

Pre-mRNA

mRNA

RibosomeTRANSLATION

(b) Eukaryotic cell

NUCLEUS

RNA PROCESSING

TRANSCRIPTION

(a) Bacterial cell

Polypeptide

DNA

mRNARibosome

CYTOPLASM

TRANSCRIPTION

TRANSLATION

Polypeptide

The Genetic Code• How are the instructions for assembling amino

acids into proteins encoded into DNA? • There are 20 amino acids, but there are only four

nucleotide bases in DNA• So how many bases correspond to an amino acid?

Codons: Triplets of Bases• The flow of information from gene to protein is

based on a triplet code: a series of nonoverlapping, three-nucleotide words

• These triplets are the smallest units of uniform length that can code for all the amino acids

• Example: AGT at a particular position on a DNA strand results in the placement of the amino acid serine at the corresponding position of the polypeptide to be produced

• During transcription, a DNA strand called the template strand provides a template for ordering the sequence of nucleotides in an RNA transcript

• During translation, the mRNA base triplets, called codons, are read in the 5 to 3 direction

• Each codon specifies the amino acid to be placed at the corresponding position along a polypeptide

Figure 17.4

A C C A A A C C G A G T

ACTTTT CGGGGT

U G G U U U G G C CU A

SerGlyPheTrp

CodonTRANSLATION

TRANSCRIPTION

Protein

mRNA 5′

5′

3′

Amino acid

DNAtemplatestrand

5′

3′

3′

Cracking the Code

• All 64 codons were deciphered by the mid-1960s• The genetic code is redundant but not ambiguous;

no codon specifies more than one amino acid• Codons must be read in the correct reading frame

(correct groupings) in order for the specified polypeptide to be produced

Figure 17.5Second mRNA base

Third

mR

NA

bas

e (3

′ end

of c

odon

)

Firs

t mR

NA

bas

e (5

′ end

of c

odon

)

UUU

UUC

UUA

UUG

Phe

Leu

Leu

Ile

Val

CUU

CUC

CUA

CUG

AUU

AUC

AUA

AUG

GUU

GUC

GUA

GUG

UCU

UCC

UCA

UCG

CCU

CCC

CCA

CCG

ACU

ACC

ACA

ACG

GCU

GCC

GCA

GCG GAG

GAA

GAC

GAU

AAG

AAA

AAC

AAU

CAG

CAA

CAC

CAU

Ser

Pro

Thr

AlaGlu

Asp

Lys

Asn

Gln

His

Tyr Cys

Trp

Arg

Ser

Arg

Gly

GGG

GGA

GGC

GGU

AGG

AGA

AGC

AGU

CGG

CGA

CGC

CGU

UGG

UGA

UGC

UGUUAU

UAC

UAA

UAG Stop

Stop Stop

Met orstart

U

C

A

G

U

C

A

G

U

C

A

G

U

C

A

G

U C A G

G

A

C

U

Evolution of the Genetic Code• The genetic code is nearly universal, shared by

the simplest bacteria to the most complex animals

• Genes can be transcribed and translated after being transplanted from one species to another

Pig expressing a jellyfishgene

(b)Tobacco plant expressinga firefly gene

(a)

Molecular Components of Transcription

• RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides

• RNA synthesis follows the same base-pairing rules as DNA, except uracil substitutes for thymine

• The DNA sequence where RNA polymerase attaches is called the promoter; in prokaryotes, the sequence signaling the end of transcription is called the terminator

• The stretch of DNA that is transcribed is called a transcription unit

Synthesis of an RNA Transcript• The three stages of transcription:

– Initiation– Elongation– Termination

Figure 17.7-3 Promoter Transcription unit

RNA polymeraseStart point

1

Template strand of DNARNAtranscript

UnwoundDNA

RewoundDNA

RNAtranscript

Direction oftranscription(“downstream”)

Completed RNA transcript

Initiation

Elongation

Termination

2

3

5′3′

5′

5′3′

5′3′

5′3′

5′

5′3′ 3′

5′3′

3′

5′3′

5′3′

Figure 17.9 Nontemplatestrand of DNA

5′

3′3′ end

5′

3′

5′

Direction of transcription

RNApolymerase

Templatestrand of DNA

Newly madeRNA

RNA nucleotides

A

A

AA

A AA

AA

CC

G G T TT

C C CU

U

C CT

TC

A

TT

GG

U

RNA Polymerase Binding and Initiation of Transcription

• Promoters signal the initiation of RNA synthesis• Transcription factors mediate the binding of RNA

polymerase and initiation of transcription in eukaryotes

• The completed assembly of transcription factors and RNA polymerase II bound to a promoter is called a transcription initiation complex

• A promoter called a TATA box is crucial in forming the initiation complex in eukaryotes

Promoter Nontemplate strand

15′3′

5′3′

Start point

RNA polymerase II

Templatestrand

TATA box

Transcriptionfactors

DNA3′5′

3′5′

3′5′

2

3

Transcription factors

RNA transcript

Transcription initiation complex

3′5′5′3′

A eukaryoticpromoter

Severaltranscriptionfactors bindto DNA.

Transcriptioninitiationcomplexforms.

T A T AAAATA A T T T T

Elongation of the RNA Strand• As RNA polymerase moves along the DNA, it

untwists the double helix, 10 to 20 bases at a time• Transcription progresses at a rate of 40 nucleotides

per second in eukaryotes• A gene can be transcribed simultaneously by

several RNA polymerases

Termination of Transcription• The mechanisms of termination are different in

prokaryotes and eukaryotes• In prokaryotes, the polymerase stops

transcription at the end of the terminator• In eukaryotes, RNA polymerase II transcribes

the polyadenylation signal sequence; the RNA transcript is released 10–35 nucleotides past this polyadenylation sequence

Concept 17.3: Eukaryotic cells modify RNA after transcription

• Enzymes in the eukaryotic nucleus modify pre-mRNA (RNA processing) before the messages are dispatched to the cytoplasm

• During RNA processing, both ends of the primary transcript are usually altered (5’ cap and 3’ polyA tail addition)

• Also, usually some interior parts of the molecule are cut out, and the other parts spliced together(RNA splicing)

Figure 17.10

A modified guaninenucleotide added tothe 5′ end

Region that includesprotein-coding segments

5′

5′ Cap

5′ UTR Startcodon

Stopcodon

G P P P

3′ UTR

3′AAUAAA AAA AAA

Poly-A tail

Polyadenylationsignal

50–250 adeninenucleotides addedto the 3′ end

…

These modifications share several functions:• They seem to facilitate the export of mRNA• They protect mRNA from hydrolytic

enzymes• They help ribosomes attach to the 5’ end

Split Genes and RNA Splicing• Most eukaryotic genes and their RNA transcripts

have long noncoding stretches of nucleotides that lie between coding regions

• These noncoding regions are called intervening sequences, or introns

• The other regions are called exons because they are eventually expressed, usually translated into amino acid sequences

• RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence

Figure 17.11

Pre-mRNA Intron Intron

Introns cut outand exonsspliced together

Poly-A tail5′ Cap

5′ Cap Poly-A tail

1–30 31–104 105–146

1–1463′ UTR5′ UTR

Codingsegment

mRNA

• In some cases, RNA splicing is carried out by spliceosomes• Spliceosomes consist of a variety of proteins and several

small nuclear ribonucleoproteins (snRNPs) that recognize the splice sites

Figure 17.12Spliceosome Small RNAs

Exon 2

Cut-outintron

Spliceosomecomponents

mRNA

Exon 1 Exon 2

Pre-mRNA

Exon 1

Intron

5′

5′

Ribozymes• Ribozymes are catalytic RNA molecules • Here they function as enzymes that can splice RNA• The discovery of ribozymes rendered obsolete the

belief that all biological catalysts were proteins

• Three properties of RNA enable it to function as an enzyme– It can form a three-dimensional structure because

of its ability to base-pair with itself– Some bases in RNA contain functional groups

that may participate in catalysis– RNA may hydrogen-bond with other nucleic

acid molecules

The Functional and Evolutionary Importance of Introns

• Some introns contain sequences that may regulate gene expression

• Some genes can encode more than one kind of polypeptide, depending on which segments are treated as exons during splicing

• This is called alternative RNA splicing• Consequently, the number of different proteins

an organism can produce is much greater thanits number of genes

• Proteins often have a modular architecture consisting of discrete regions called domains

• In many cases, different exons code for the different domains in a protein

Figure 17.13

GeneDNA

Transcription

RNA processing

Translation

Domain 3

Exon 1 Exon 3Exon 2 IntronIntron

Domain 2

Domain 1

Polypeptide

Molecular Components of Translation• A cell translates an mRNA message into protein

with the help of transfer RNA (tRNA) & ribosomes• tRNAs transfer amino acids to the growing

polypeptide in a ribosome• Molecules of tRNA are not identical:

– Each carries a specific amino acid on one end– Each has an anticodon on the other end; the anticodon

base-pairs with a complementary codon on mRNA

Figure 17.14

PolypeptideAminoacids

tRNA withamino acidattached

Ribosome

tRNA

Anticodon

Codons

mRNA

5′

U U U UG G G G C

A C C

A A A

CC

G

Phe

Trp

3′

Gly

The Structure and Function of Transfer RNA• A tRNA molecule consists of a single RNA strand that is only about 80

nucleotides long• Flattened into one plane to reveal its base pairing, a tRNA molecule looks

like a cloverleaf• Amino acid at one end of sock and anticodon at the other• Because of hydrogen bonds, tRNA actually twists and folds into a three-

dimensional molecule• tRNA is roughly L-shaped

ACC

Figure 17.15Amino acidattachmentsite Amino acid

attachment site5′

3′ACCACGCUUA

G

GC

GAUUUA

AGAA CC

CU*

**CG

G U UGC*

**

*C CUA G

GGGA

GAGC

CC

*U* G A

GGU**

*A

A

AG

CU

GAA

Hydrogenbonds

Hydrogenbonds

Anticodon Anticodon

Symbol usedin this book

Three-dimensionalstructure

(a) Two-dimensional structure (b) (c)Anticodon

5′3′A A G

3′5′

• Accurate translation requires two steps:– First step: a correct match between a tRNA and an amino acid,

done by the enzyme aminoacyl-tRNA synthetase– Second step: a correct match between the tRNA anticodon and

an mRNA codon

Flexible pairing at the third base of a codon is called wobble and allows some tRNAs to bind to more than one codon

Figure 17.16-3

Aminoacyl-tRNAsynthetase

1 Amino acidand tRNAenter activesite.

Tyr-tRNA

Tyrosine (Tyr)(amino acid)

Tyrosyl-tRNAsynthetase

ComplementarytRNA anticodon

A AU

tRNA

Aminoacid

Using ATP,synthetasecatalyzescovalentbonding.

23

ATP

AMP + 2P i

AminoacyltRNAreleased.

Computer model

Ribosomes• Ribosomes facilitate specific coupling of tRNA

anticodons with mRNA codons in protein synthesis• The two ribosomal subunits (large and small) are

made of proteins and ribosomal RNA (rRNA)

Figure 17.17

Exit tunnel

Largesubunit

Smallsubunit

mRNA 3′5′

E P A

tRNAmolecules

Growingpolypeptide

(a) Computer model of functioning ribosome

Growingpolypeptide Next amino

acid to beadded topolypeptidechain

tRNA3′

5′

mRNA

Amino end

Codons

E

(c) Schematic model with mRNA and tRNA(b) Schematic model showing binding sites

Smallsubunit

Largesubunit

Exit tunnel

A site (Aminoacyl-tRNA binding site)

P site (Peptidyl-tRNA binding site)

E site(Exit site)

mRNAbinding site

E P A

• A ribosome has three binding sites for tRNA:– The P site holds the tRNA that carries the growing

polypeptide chain– The A site holds the tRNA that carries the next

amino acid to be added to the chain– The E site is the exit site, where uncharged tRNAs

leave the ribosome

Building a Polypeptide• The three stages of translation:

– Initiation– Elongation– Termination

– All 3 stages require protein factors and some require energy in the form of GTP.

Ribosome Association and Initiation of Translation

• Initiation brings together mRNA, a tRNA with the first amino acid, and the two ribosomal subunits

• First, a small ribosomal subunit binds with mRNA and a special initiator tRNA

• Then the small subunit moves along the mRNA until it reaches the start codon (AUG)

• Proteins called initiation factors bring in the large subunit that completes the translation initiation complex

Figure 17.18

1 2

P site3′

Largeribosomalsubunit

Translation initiation complex

Large ribosomal subunitcompletes the initiationcomplex.

Small ribosomal subunit binds to mRNA.

mRNA binding site

Smallribosomalsubunit

InitiatortRNA

Start codon3′5′

E A

3′5′

GTP GDP

P i+

3′ 5′5′

U A CGA UMet

Met

mRNA

Elongation of the Polypeptide Chain

• During elongation, amino acids are added oneby one to the C-terminus of the growing chain

• Each addition involves proteins called elongation factors and occurs in three steps: codon recognition, peptide bond formation, and translocation

• Energy expenditure occurs in the first (docking in A site) and third steps(translocation)

• Translation proceeds along the mRNA in a5 → 3 direction′ ′

Amino endof polypeptide

Codonrecognition

13′

5′

E

P Asitesite

E

P A

mRNA

GTP

P iGDP +

Amino endof polypeptide

Codonrecognition

13′

5′

E

P Asitesite

E

P A

mRNA

GTP

P i

2

GDP +

E

P A

Peptide bondformation

Amino endof polypeptide

Codonrecognition

13′

5′

E

P Asitesite

E

P A

mRNA

GTP

P i

23 GTP

P i

GDP +

GDP +

Translocation

E

P A

Peptide bondformation

E

P A

Ribosome ready fornext aminoacyl tRNA

Termination of Translation

• Termination occurs when a stop codon in the mRNA reaches the A site of the ribosome

• The A site accepts a protein called a release factor

• The release factor causes the addition of a water molecule instead of an amino acid

• This reaction releases the polypeptide, and the translation assembly comes apart

Figure 17.20-3

31 2

Releasefactor

3′5′5′

3′

Stop codon(UAG, UAA, or UGA)

Ribosome reaches astop codon on mRNA.

Release factorpromotes hydrolysis.

Ribosomal subunitsand other componentsdissociate.

Freepolypeptide

3′

5′

2 GTP

2 GDP +2 P i

Polyribosomes• A number of ribosomes can translate a single

mRNA simultaneously, forming a polyribosome• Polyribosomes enable a cell to make many

copies of a polypeptide very quickly

LE 17-20

Ribosomes

mRNA

0.1 mThis micrograph shows a large polyribosome in a prokaryotic cell (TEM).

An mRNA molecule is generally translated simultaneouslyby several ribosomes in clusters called polyribosomes.

Incomingribosomalsubunits

Growingpolypeptides

End ofmRNA(3 end)

Start ofmRNA(5 end)

Polyribosome

Completedpolypeptides

Completing and Targeting the Functional Protein

• Often translation is not sufficient to make a functional protein

• Polypeptide chains are modified after translation• Completed proteins are targeted to specific sites

in the cell

Targeting Polypeptides to Specific Locations

• Two populations of ribosomes are evident in cells: free ribsomes (in the cytosol) and bound ribosomes (attached to the ER)

• Free ribosomes mostly synthesize proteins that function in the cytosol

• Bound ribosomes make proteins of the endomembrane system and proteins that are secreted from the cell (rough ER)

• Ribosomes are identical and can switch from free to bound

• Polypeptide synthesis always begins in the cytosol• Synthesis finishes in the cytosol unless the

polypeptide signals the ribosome to attach to the ER

• Polypeptides destined for the ER or for secretion are marked by a signal peptide

• A signal-recognition particle (SRP) binds to the signal peptide

• The SRP brings the signal peptide and its ribosome to the ER

Figure 17.21

Protein

ERmembrane

Signalpeptideremoved

SRP receptorprotein

Translocation complex

ER LUMEN

CYTOSOL

SRP

Signalpeptide

mRNARibosome

Polypeptidesynthesisbegins.

SRPbinds tosignalpeptide.

SRPbinds toreceptorprotein.

SRPdetachesandpolypeptidesynthesisresumes.

Signal-cleavingenzyme cutsoff signalpeptide.

Completedpolypeptidefolds intofinalconformation.

1 2 3 4 5 6

• A bacterial cell ensures a streamlined process by coupling transcription and translation

• In this case the newly made protein can quickly diffuse to its site of function

RNA polymerase

Polyribosome

DNAmRNA

0.25 µm

Ribosome

DNA

mRNA (5′ end)

Polyribosome

Polypeptide(amino end)

Direction oftranscriptionRNA

polymerase

• In eukaryotes, the nuclear envelop separates the processes of transcription and translation

• RNA undergoes processes before leavingthe nucleus

DNA

RNApolymerase

RNA transcript(pre-mRNA)Intron

Exon

Aminoacyl-tRNAsynthetase

AminoacidtRNA

Aminoacyl(charged)tRNA

mRNA

CYTOPLASM

NUCLEUS

RNAtranscript

3′

5′ Poly-

A

5′ Cap

5′ Cap

TRANSCRIPTION

RNAPROCESSING

Poly-A

Poly-A

AMINO ACIDACTIVATION

TRANSLATION

Ribosomalsubunits

E PA

E AA C C

U U U U U GGGA A A Anticodon

CodonRibosome

CAU

3′

A

Concept 17.5: RNA plays multiple roles in the cell: a review

Type of RNA FunctionsMessenger RNA (mRNA)

Carries information specifying amino acid sequences of proteins from DNA to ribosomes

Transfer RNA (tRNA)

Serves as adapter molecule in protein synthesis; translates mRNA codons into amino acids

Ribosomal RNA (rRNA)

Plays catalytic (ribozyme) roles and structural roles in ribosomes

Type of RNA FunctionsPrimary transcript

Serves as a precursor to mRNA, rRNA, or tRNA, before being processed by splicing or cleavage

Small nuclear RNA (snRNA)

Plays structural and catalytic roles in spliceosomes

SRP RNA Is a component of the the signal-recognition particle (SRP)

Video: protein synthesis

Concept 17.7: Point mutations can affect protein structure and function• Mutations are changes in the genetic material

of a cell or virus• Point mutations are chemical changes in just

one base pair of a gene• The change of a single nucleotide in a DNA

template strand leads to production of an abnormal protein

Figure 17.25Wild-type β-globin Sickle-cell β-globin

Mutant β-globin DNAWild-type β-globin DNA

mRNA mRNA

Normal hemoglobin Sickle-cell hemoglobinVal

U GG

A CCG GT

Glu

G GA

G GAC CT3′

5′

5′ 5′

5′ 5′ 3′

3′

3′ 3′

3′ 5′

Types of Point Mutations• Point mutations within a gene can be divided

into two general categories– Base-pair substitutions– Base-pair insertions or deletions

Substitutions• A base-pair substitution replaces one nucleotide

and its partner with another pair of nucleotides• Base-pair substitution can cause missense or

nonsense mutations• Missense mutations still code for an amino acid,

but not necessarily the right amino acid• Nonsense mutations change an amino acid

codon into a stop codon, nearly always leading to a nonfunctional protein

• Missense mutations are more common

Wild type

Stop

5′ 3′ 3′

Stop

3′ 5′

5′ Met

(b) Nucleotide-pair insertion or deletion

Carboxyl endAmino end

mRNAProtein

3′ 5′ 5′

DNA template strand

(a) Nucleotide-pair substitutionA instead of G

U instead of C

5′ 3′

3′ Stop

Extra A

Extra U

Frameshift (1 nucleotide-pair insertion)

missing

missing

Frameshift (1 nucleotide-pair deletion)

missing

3′ 5′

5′

missing

3′ 5′

3′ Met Lys Leu Ala

3′ 5′

3′

T A C T T C A A A C C G A T TATA G T A A A AC TTTTG G G

G G C U A AAAA U U UUUGG

A U G A A G G G C UUUU

A A

AATCGGTTT GG A AAT A C T T C A A C C G A T T

A

Met Lys Phe Gly

T A C C C C GA A A AA AAAA

A U G

G G G GT T T T

TTTTT C

G G G C UUUUAA A A

3′ 5′

5′

TA A A A A

T T T TA A A A AACCCCT T T T TTGGGG

G G G GUUU U A AA A AU

Met Lys Phe Gly StopU

Silent

3′ 5′

5′

5′ 3′

3′ Stop

Stop Stop3 nucleotide-pair deletionNonsense

A instead of T

U instead of A5′

5′

5′ 3′

3′

3′

Met Lys Phe Ser

Met Met Phe Gly

AT

A

AT

A

AT

A

T instead of C

A instead of G

Missense

U G G G G A AU U U U UU A

A A AT T TA A A A AGCCCC A

T T T T T TCGGGG3′ 5′

5′

5′ 3′

3′

T TA A

AAA A G

U U U U UCGGG

T T T T TAGCC

CGGGC

CTTAAAA

T T T TTA

AA AA C C C GAA A A ATTTT GGGT C

A A AAUCGGGU U U U A

Mutagens• Spontaneous mutations can occur during DNA

replication, recombination, or repair• Mutagens are physical or chemical agents that

can cause mutations

What is a gene? revisiting the question

• A gene is a region of DNA whose final product is either a polypeptide or an RNA molecule

LE 17-26TRANSCRIPTION

RNA PROCESSING

RNAtranscript5

Exon

NUCLEUS

FORMATION OFINITIATION COMPLEX

CYTOPLASM

3

DNA

RNApolymerase

RNA transcript(pre-mRNA)

Intron

Aminoacyl-tRNAsynthetase

AminoacidtRNA

AMINO ACID ACTIVATION

3

mRNA

A

P

E Ribosomalsubunits

5

Growingpolypeptide

E A

Activatedamino acid

Anticodon

TRANSLATION

Codon

Ribosome

Review compartments for eukaryotes