gene expression: from gene to protein

CAMPBELL

BIOLOGYReece • Urry • Cain • Wasserman • Minorsky • Jackson

© 2014 Pearson Education, Inc.

TENTHEDITION

CAMPBELL

BIOLOGYReece • Urry • Cain • Wasserman • Minorsky • Jackson

TENTHEDITION

17Gene Expression: From Gene to Protein

Lecture Presentation by Nicole Tunbridge andKathleen Fitzpatrick

http://www.mediaresource.org/instruct.htm


The Flow of Genetic Information

The information content of genes is in the specific sequences of nucleotides

The DNA inherited by an organism leads tospecific traits by dictating the synthesis of proteins

Proteins are the links between genotype and phenotype

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


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

Cells synthesize and degrade molecules in a series of steps, a metabolic pathway


Nutritional Mutants in Neurospora: Scientific Inquiry

George Beadle and Edward Tatum exposed bread mold to X-rays, creating mutants that were unable to survive on minimal media

Using crosses, they and their coworkers 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


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 the one gene–one polypeptide hypothesis

It is common to refer to gene products as proteins rather than polypeptides


Basic Principles of Transcription and Translation

RNA is the bridge between genes and the proteins for which they code

Transcription is the synthesis of RNA using information in DNA

Transcription produces messenger RNA (mRNA)

Translation is the synthesis of a polypeptide, using information in the mRNA

Ribosomes are the sites of translation


In prokaryotes, translation of mRNA can begin before transcription has finished

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

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


Figure 17.3

Nuclear envelope

CYTOPLASM

DNA

Pre-mRNA

mRNA

RibosomeTRANSLATION

(b) Eukaryotic cell

NUCLEUS

RNA PROCESSING

TRANSCRIPTION

(a) Bacterial cell

Polypeptide

DNA

mRNARibosome

CYTOPLASM

TRANSCRIPTION

TRANSLATION

Polypeptide


A primary transcript is the initial RNA transcript from any gene prior to processing

The central dogma is the concept that cells are governed by a cellular chain of command: DNA → RNA → protein


Figure 17.UN01

DNA RNA Protein


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

How many nucleotides correspond to anamino acid?


Codons: Triplets of Nucleotides

The flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, three-nucleotide words

The words of a gene are transcribed into complementary nonoverlapping three-nucleotide words of mRNA

These words are then translated into a chain of amino acids, forming 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′


During transcription, one of the two DNA strands, called the template strand, provides a template for ordering the sequence of complementary nucleotides in an RNA transcript

The template strand is always the same strandfor a given gene


During translation, the mRNA base triplets, called codons, are read in the 5′ → 3′ direction

Each codon specifies the amino acid (one of 20)to be placed at the corresponding position alonga polypeptide


Cracking the Code

All 64 codons were deciphered by the mid-1960s

Of the 64 triplets, 61 code for amino acids; 3 triplets are “stop” signals to end translation

The genetic code is redundant (more than one codon may specify a particular amino acid) butnot ambiguous; no codon specifies more thanone 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

Th

ird

mR

NA

ba

se (

3′ e

nd

of

cod

on

)

Fir

st

mR

NA

ba

se (

5′ e

nd

of

cod

on

)

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

Ala

Glu

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


Figure 17.6

Pig expressing a jellyfishgene

(b)Tobacco plant expressinga firefly gene

(a)


Concept 17.2: Transcription is the DNA-directed synthesis of RNA: A closer look

Transcription is the first stage of gene expression


Molecular Components of Transcription

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

The RNA is complementary to the DNA template strand

RNA polymerase does not need any primer

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


Figure 17.7-3Promoter 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′


The DNA sequence where RNA polymerase attaches is called the promoter; in bacteria, 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


RNA Polymerase Binding and Initiation of Transcription

Promoters signal the transcriptional start point and usually extend several dozen nucleotide pairs upstream of the start point

Transcription factors mediate the binding of RNA polymerase and the initiation of transcription

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


Figure 17.8Promoter 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 AAAA

TA 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

Nucleotides are added to the 3′ end of thegrowing RNA molecule


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

CC CU

U

C CT

TC

A

TT

GG

U


Termination of Transcription

The mechanisms of termination are different in bacteria and eukaryotes

In bacteria, the polymerase stops transcription at the end of the terminator and the mRNA can be translated without further modification

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 genetic messages are dispatched to the cytoplasm

During RNA processing, both ends of the primary transcript are usually altered

Also, usually certain interior sections of the molecule are cut out, and the remaining parts spliced together


Alteration of mRNA Ends

Each end of a pre-mRNA molecule is modified in a particular way

The 5′ end receives a modified nucleotide 5′ cap

The 3′ end gets a poly-A tail

These modifications share several functions

They seem to facilitate the export of mRNA to the cytoplasm

They protect mRNA from hydrolytic enzymes

They help ribosomes attach to the 5′ end


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

…


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

The RNAs of the spliceosome also catalyze the splicing reaction


Figure 17.12

Spliceosome 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 that function as enzymes and 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 asan enzyme

It can form a three-dimensional structure because of its ability to base-pair with itself

Some bases in RNA contain functional groupsthat may participate in catalysis

RNA may hydrogen-bond with other nucleicacid 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 proteinsan 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

Exon shuffling may result in the evolution of new proteins


Figure 17.13

GeneDNA

Transcription

RNA processing

Translation

Domain 3

Exon 1 Exon 3Exon 2 IntronIntron

Domain 2

Domain 1

Polypeptide


Concept 17.4: Translation is the RNA-directed synthesis of a polypeptide: A closer look

Genetic information flows from mRNA to protein through the process of translation


Molecular Components of Translation

A cell translates an mRNA message into protein with the help of transfer RNA (tRNA)

tRNAs transfer amino acids to the growing polypeptide in a ribosome

Translation is a complex process in terms of its biochemistry and mechanics


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

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


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


Because of hydrogen bonds, tRNA actually twists and folds into a three-dimensional molecule

tRNA is roughly L-shaped


Figure 17.15

Amino acidattachmentsite

Amino acidattachment site

5′

3′ACCACGCUUA

G

GC

GAUUU

A

AGAA CC

CU*

**C

GG U UG

C*

**

*C CUA G

GG

GAGAGC

CC

*U* G A

GGU

**

*A

A

A

G

C

UG

AA

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: a correct match between a tRNA and an amino acid, done by the enzyme aminoacyl-tRNA synthetase

Second: 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)

Bacterial and eukaryotic ribosomes are somewhat similar but have significant differences: some antibiotic drugs specifically target bacterial ribosomes without harming eukaryotic ribosomes


Figure 17.17a

Exit tunnel

Largesubunit

Smallsubunit

mRNA 3′5′

E P A

tRNAmolecules

Growingpolypeptide

(a) Computer model of functioning ribosome


Figure 17.17b

(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


Figure 17.17c

Growingpolypeptide

Next aminoacid to beadded topolypeptidechain

tRNA

3′

5′

mRNA

Amino end

Codons

E

(c) Schematic model with mRNA and tRNA


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 discharged tRNAs leave the ribosome


Building a Polypeptide

The three stages of translation

Initiation

Elongation

Termination

All three stages require protein “factors” that aid in the translation process

Energy is required for some steps also


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 C

GA UMetMet

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 andthird steps

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


Figure 17.19-1Amino endof polypeptide

Codonrecognition

1

3′

5′

E

P Asitesite

E

P A

mRNA

GTP

P iGDP +



Codonrecognition

1

3′

5′

E

P Asitesite

E

P A

mRNA

GTP

P i

2

GDP +

E

P A

Peptide bondformation



Codonrecognition

1

3′

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


Completing and Targeting the Functional Protein

Often translation is not sufficient to make a functional protein

Polypeptide chains are modified after translation or targeted to specific sites in the cell


Protein Folding and Post-Translational Modifications

During its synthesis, a polypeptide chain begins to coil and fold spontaneously to form a protein with a specific shape—a three-dimensional molecule with secondary and tertiary structure

A gene determines primary structure, and primary structure in turn determines shape

Post-translational modifications may be required before the protein can begin doing its particular job in the cell


Targeting Polypeptides to Specific Locations

Two populations of ribosomes are evident in cells: free ribosomes (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

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 tothe 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

mRNA

Ribosome

Polypeptidesynthesisbegins.

SRPbinds tosignalpeptide.

SRPbinds toreceptorprotein.

SRPdetachesandpolypeptidesynthesisresumes.

Signal-cleavingenzyme cutsoff signalpeptide.

Completedpolypeptidefolds intofinalconformation.

1 2 3 4 5 6


Making Multiple Polypeptides in Bacteria and Eukaryotes

Multiple ribosomes can translate a single mRNA simultaneously, forming a polyribosome (or polysome)

Polyribosomes enable a cell to make many copies of a polypeptide very quickly


Figure 17.22

0.1 µm

mRNA

Ribosomes

A large polyribosome in a bacterialcell (TEM)

(b)

Several ribosomes simultaneously translatingone mRNA molecule

(a)

Incomingribosomalsubunits

Completedpolypeptide

Growingpolypeptides

End ofmRNA(3′ end)

PolyribosomeStart ofmRNA(5′ end)


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


Figure 17.23

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


Figure 17.24DNA

RNApolymerase

RNA transcript(pre-mRNA)

Intron

Exon

Aminoacyl-tRNAsynthetase

Aminoacid

tRNA

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 GGG

A A AAnticodon

Codon

Ribosome

CA

U

3′

A


Concept 17.5: Mutations of one or a few nucleotides 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 can lead to the production of an abnormal protein


If a mutation has an adverse effect on the phenotype of the organism the condition is referred to as a genetic disorder or hereditary disease


Figure 17.25

Wild-type β-globin Sickle-cell β-globin

Mutant β-globin DNAWild-type β-globin DNA

mRNA mRNA

Normal hemoglobin Sickle-cell hemoglobin

Val

U GG

A CCG GT

Glu

G GA

G GAC CT3′

5′

5′ 5′

5′ 5′ 3′

3′

3′ 3′

3′ 5′


Types of Small-Scale Mutations

Point mutations within a gene can be divided into two general categories

Nucleotide-pair substitutions

One or more nucleotide-pair insertions or deletions


Substitutions

A nucleotide-pair substitution replaces one nucleotide and its partner with another pair of nucleotides

Silent mutations have no effect on the amino acid produced by a codon because of redundancy in the genetic code

Missense mutations still code for an amino acid, but not the correct amino acid

Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein


Figure 17.26a

Wild type

DNA template strand T T TA C C A A A C C G A T T

AAAAA T G G T T T G G C T3′ 5′ 3′

5′

AAUCGGUUUGAAGA U5′ 3′ mRNAProtein

Amino endMet Lys Phe Gly Stop

Carboxyl end

Nucleotide-pair substitution: silent

A instead of G

U instead of C3′

3′ 5′

StopMet Lys Phe Gly

GGUUUGAAGA U U A AU

T T TA C C A A A C C T TAA

AAA T G G T T T G G T AAT

5′

5′ 3′


Insertions and Deletions

Insertions and deletions are additions or losses of nucleotide pairs in a gene

These mutations have a disastrous effect on the resulting protein more often than substitutions do

Insertion or deletion of nucleotides may alter the reading frame, producing a frameshift mutation


Figure 17.26b

Wild type



5′



Carboxyl end

Nucleotide-pair substitution: missense

T instead of C

StopMet Lys Phe Ser

A instead of G5′ 3′

5′

5′ 3′

3′ A U G A A G U U U U A ACGA

A T G A A AATCGTTTG A

T A C C CA A A AT T T TT G


New Mutations and Mutagens

Spontaneous mutations can occur during DNA replication, recombination, or repair

Mutagens are physical or chemical agents that can cause mutations


Figure 17.26Wild 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 substitution

A 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 UUU

U

A A

AATCGGTTT GG A AA

T 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′

T

A A A A A

T T T TA A A A AACCCC

T T T T TTGGGG

G G G GUUU U A AA A AU

Met Lys Phe Gly Stop

U

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

A

T

A

A

T

A

A

T

A

T instead of C

A instead of G

Missense

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

A A A

T T TA A A A AGCCCC A

T T T T T TCGGGG3′ 5′

5′

5′ 3′

3′

T T

A A

AA

A A G

U U U U UCGGG

T T T T T

AGCC

CGGG

C

CTT

AAAA

T T T TT

A

AA AA C C C GA

A A A ATTTT GGGT C

A A AAUCGGGU U U U A


Figure 17.26c

Wild type



5′



Carboxyl end

Nucleotide-pair substitution: nonsense

A instead of T

U instead of A5′

5′

3′ 3′

3′

5′

Met Stop

A U G G U U U G G U U A A

AATCGGTTTGAGA T

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

T

AU


Figure 17.26d

Wild type



5′



Carboxyl end

Nucleotide-pair insertion: frameshift causing immediate nonsense

3′

3′

5′ 5′ 3′

5′

StopMet

A U G A A G U U U UG G C A A

Extra A

U

A

T

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

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

T


Figure 17.26e

Wild type



5′



Carboxyl end

Nucleotide-pair deletion: frameshift causing extensive missense

5′

5′ 3′ 5′

3′

3′

A missingA

AACGGUUGAAGUA

A

U

A A A A

AAA C C C C G T TTT

T G G T T G G TC

Met Lys Leu Ala

missingU

T


Figure 17.26f

Wild type

DNA template strand

3 nucleotide-pair deletion: no frameshift, but one amino acid missing

mRNAProtein


Carboxyl end

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

AAAAA T G G T T T G G C T

AAUCGGUUUGAAGA U

T

T T C

A C A A A C C G A T T

T T T T TA A ACGGG

missing

missingA A G

G G G C UUUUUA A A

GlyPheMet Stop

3′ 5′

5′

3′ 5′

3′

5′ 3′

3′

3′ 5′

5′


Figure 17.UN02

TRANSCRIPTION

RNA PROCESSING

TRANSLATION

DNA

Pre-mRNA

mRNA

Ribosome

Polypeptide


Figure 17.UN03

TRANSCRIPTION

RNA PROCESSING

TRANSLATION

DNA

Pre-mRNA

mRNA

Ribosome

Polypeptide


Figure 17.UN04

TRANSCRIPTION

TRANSLATION

DNA

mRNA

Ribosome

Polypeptide


Figure 17.UN06

Transcription unit

Template strandof DNARNA

polymerase

Promoter

RNA transcript

3′ 5′

5′ 3′ 3′

5′


Figure 17.UN07

Pre-mRNA

mRNA

IntronPoly-A tail

Exon 3′Intron ExonExon5′ Cap

5′

5′ UTR 3′ UTRCodingsegment


Figure 17.UN08

Polypeptide

Aminoacid

Anti-codon

mRNA

Codon

Ribosome

E A

tRNA

gene expression: from gene to protein

Documents

pearson education

synthesis of rna

gene products

dna rna protein

products of gene expression

translation of mrna

initial rna transcript

specific enzyme