Gene Expression: From Gene to Protein

CAMPBELL BIOLOGY IN FOCUSUrry • Cain • Wasserman • Minorsky • Jackson • Reece

14Gene Expression: From Gene to Protein

鄭先祐 (Ayo) 教授國立臺南大學生態科學與技術學系

Ayo website: http://myweb.nutn.edu.tw/~hycheng/

Overview: The Flow of Genetic Information

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

The DNA inherited by an organism leads to specific 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

Concept 14.1: Genes specify proteins via transcription and translation

How was the fundamental relationship between genes and proteins discovered?

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.

Nutritional Mutants in Neurospora: Scientific Inquiry

George Beadle and Edward Tatum disabled genes in bread mold one by one and looked for phenotypic changes.

They studied the haploid bread mold because it would be easier to detect recessive mutations.

They studied mutations that altered the ability of the fungus to grow on minimal medium.

Figure 14.2a

Neurosporacells

Each survivingcell forms a colony ofgeneticallyidentical cells.

Individual Neurosporacells placed on completegrowth medium.

Cells subjectedto X-rays.

2

1 3

Figure 14.2b

No growth

Growth

Mutant cells placedin a series of vials,each containingminimal mediumplus one additionalnutrient.

Surviving cellstested for inabilityto grow onminimal medium.

Growth

Control: Wild-typecells in minimalmedium

4

5

The researchers amassed a valuable collection of Neurospora mutant strains, catalogued by their defects.

For example, one set of mutants all required arginine for growth

It was determined that different classes of these mutants were blocked at a different step in the biochemical pathway for arginine biosynthesis

The Products of Gene Expression: A Developing Story

Some proteins are not enzymes, so researchers later revised the one gene–one enzyme 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 DNA and protein synthesis.

RNA is chemically similar to DNA, but RNA has a ribose sugar and the base uracil (U) rather than thymine (T).

RNA is usually single-stranded.

Getting from DNA to protein requires two stages: transcription and translation.

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 eukaryotes, the nuclear envelope separates transcription from translation.

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

Eukaryotic mRNA must be transported out of the nucleus to be translated.

Figure 14.4

Nuclearenvelope

Pre-mRNA

mRNA

DNA

RNA PROCESSING

TRANSCRIPTION

TRANSLATION

Polypeptide

Ribosome

mRNA

DNATRANSCRIPTION

TRANSLATION

Polypeptide

Ribosome

(a) Bacterial cell (b) Eukaryotic cell

Figure 14.4a-2

mRNA

DNA

TRANSCRIPTION

TRANSLATION

Polypeptide

Ribosome

(a) Bacterial cell

Figure 14.4b-1

Nuclearenvelope

Pre-mRNA

DNATRANSCRIPTION

(b) Eukaryotic cell

Figure 14.4b-2

Nuclearenvelope

Pre-mRNA

mRNA

DNA

RNA PROCESSING

TRANSCRIPTION

(b) Eukaryotic cell

Figure 14.4b-3

Nuclearenvelope

Pre-mRNA

mRNA

DNA

RNA PROCESSING

TRANSCRIPTION

TRANSLATION

Polypeptide

Ribosome

(b) Eukaryotic cell

Figure 14.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 an amino 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 14.5

DNAtemplatestrand

Protein

mRNA

3

Trp

TRANSCRIPTION

TRANSLATION

Amino acid

Codon

5

35

3

5

Phe Gly Ser

GU G U UU G G UC C A

CA C A AA C C AG G T

GT G T TT G G TC C A

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 strand for any given gene.

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

Each codon specifies the amino acid (one of 20) to be placed at the corresponding position along a 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.

Codons must be read in the correct reading frame (correct groupings) in order for the specified polypeptide to be produced.

Codons are read one at a time in a nonoverlapping fashion.

Figure 14.6

UUU

Second mRNA base

UUC

UUA

UUG

UCU

UCC

UCA

UCG

UAU

UAC

UAA

UAG

UGU

UGC

UGA

UGG

CUU

CUC

CUA

CUG

CCU

CCC

CCA

CCG

CAU

CAC

CAA

CAG

CGU

CGC

CGA

CGG

AUU

AUC

AUA

AUG

ACU

ACC

ACA

ACG

AAU

AAC

AAA

AAG

AGU

AGC

AGA

AGG

GUU

GUC

GUA

GUG

GCU

GCC

GCA

GCG

GAU

GAC

GAA

GAG

GGU

GGC

GGA

GGG

Fir

st m

RN

A b

ase

(5

en

d o

f co

do

n)

U

C

A

G

U

C

A

G

U

C

A

G

U

C

A

G

U

C

A

G

U C A G

Phe

Leu

Ser

Tyr Cys

Trp

Met orstart

Stop

Stop Stop

Arg

Gln

His

ProLeu

Val Ala

Asp

Glu

Gly

IIeThr

Lys

Asn

Arg

Ser

Th

ird

mR

NA

bas

e (

3 e

nd

of

cod

on

)

Evolution of the Genetic Code

The genetic code is nearly universal, shared by the simplest bacteria and the most complex animals.

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

Figure 14.7

(a) Tobacco plant expressinga firefly gene

(b) Pig expressing a jellyfishgene

Concept 14.2: Transcription is the DNA-directed synthesis of RNA: a closer look

Transcription is the first stage of gene expression.

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

RNA polymerases assemble polynucleotides in the 5 to 3 direction.

However, RNA polymerases can start a chain without a primer.

Figure 14.8-1Transcription unit

RNA polymerase

Promoter

Template strand of DNA

Start point

RNA transcript

UnwoundDNA

Initiation

35

35 3

5

35

1


RNA polymerase

Promoter


Start point

RNA transcript

UnwoundDNA

RewoundDNA

RNA transcript

Direction oftranscription(“downstream”)

Initiation

Elongation

35

35

35 3

5

35

35

35

2

1


RNA polymerase

Promoter


Start point

Termination

Completed RNA transcript

RNA transcript

UnwoundDNA

RewoundDNA

RNA transcript

Direction oftranscription(“downstream”)

Initiation

Elongation

35

35

35 3

5

35

35

35

35

35

35

3

2

1

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 14.UN02

DNA

Pre-mRNA

mRNA

Ribosome

Polypeptide

TRANSLATION

TRANSCRIPTION

RNA PROCESSING

Figure 14.9

Transcription factors

TATA box

Promoter Nontemplate strand

Start point

Transcriptioninitiationcomplex forms.

Transcription initiation complex

DNA

RNA transcript

A eukaryoticpromoter

Several transcriptionfactors bind to DNA.

35

5 3 35

35

35

3

2

1

Templatestrand

Transcription factors

RNA polymerase II

35

35 TA T A A A A

A T 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.

Figure 14.10

Nontemplate strand of DNA

Direction of transcription

RNA polymerase

3

53

5

RNA nucleotides


Newly made RNA

3 end

5

UC

U

G

A

A

A

A

AA

A

AA

A

T T T

TT

T

T

CC

C

CCC C

G

GG

U

Concept 14.3: 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 altered.

Also, usually some interior parts of the molecule are cut out and the other 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 G nucleotide 5 cap.

The 3 end gets a poly-A tail.

These modifications share several functions.

Facilitating the export of mRNA to the cytoplasm.

Protecting mRNA from hydrolytic enzymes.

Helping ribosomes attach to the 5 end.

Figure 14.UN03

DNA

Pre-mRNA

mRNA

Ribosome

Polypeptide

TRANSLATION

TRANSCRIPTION

RNA PROCESSING

Figure 14.11

Protein-coding segmentPolyadenylation

signal

G P

A modified guaninenucleotide added tothe 5 end

50–250 adeninenucleotides added to the 3 end

35

5 Cap 5 UTR 3 UTR Poly-A tailStartcodon

Stopcodon

P P AAUAAA …AAA AAA

Split Genes and RNA Splicing

Most eukaryotic mRNAs have long noncoding stretches of nucleotides that lie between coding regions.

The noncoding regions are called intervening ( 介於其間的 ) sequences, or introns.

The other regions are called exons and are usually translated into amino acid sequences.

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

Figure 14.12

Introns cut out andexons spliced together

31–104

5 Cap

5 UTR 3 UTR

Poly-A tail

Codingsegment

1–146

AAUAAA

105– 146

5 Cap Poly-A tail

1–30

mRNA

Pre-mRNA

Intron Intron

Many genes can give rise to two or more different polypeptides, depending on which segments are used as exons.

This process is called alternative RNA splicing.

RNA splicing is carried out by spliceosomes.

Spliceosomes consist of proteins and small RNAs

Figure 14.13

Spliceosomecomponents

5

Pre-mRNA

5

mRNA

Intron

Spliceosome

Exon 1

Small RNAs

Exon 2

Exon 2Exon 1

Cut-outintron

Ribozymes

Ribozymes are RNA molecules that function as enzymes

RNA splicing can occur without proteins, or even additional RNA molecules

The introns can catalyze their own splicing

Concept 14.4: Translation is the RNA-directed synthesis of a polypeptide: a closer look

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

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 14.UN04

DNA

mRNA

Ribosome

Polypeptide

TRANSLATION

TRANSCRIPTION

Figure 14.14

5

tRNA

Polypeptide

Ribosome

Anticodon

mRNA

Codons 3

tRNA withamino acidattached

Amino acids

Gly

Trp

Phe

A A A

A C C

CC

G

U U U G G CU G G

The Structure and Function of Transfer RNA

Each tRNA can translate a particular mRNA codon into a given amino acid.

A tRNA molecule consists of a single RNA strand that is only about 80 nucleotides long.

Flattened into one plane, a tRNA molecule looks like a cloverleaf.

In three dimensions, tRNA is roughly L-shaped, where one end of the L contains the anticodon that base-pairs with an mRNA codon.

Figure 14.15a

5

3

Anticodon

Amino acidattachment site

Hydrogenbonds

A

A

G

A

CG

CC

C

C

UA

C G

UU

A

A A

A

C

G U

A

C G U

A

C

GU

AC

G

U

*

CG

*

G

GU

A

AA

A

C C

C

CC

GG

GG

G

UUU

U

GG

G

G

A

A

**

*

*

*

*

**

*

(a) Two-dimensional structure

*

Figure 14.15b

Anticodon

Amino acidattachment site

Anticodon

A A G53

Hydrogenbonds

53

(b) Three-dimensional structure

(c) Symbol used in this book

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(擺動 ) and allows some tRNAs to bind to more than one codon.

Figure 14.16-1Tyrosyl-tRNAsynthetase

Tyrosine (Tyr)(amino acid)

Amino acidand tRNAenter activesite.

Tyr-tRNA

ComplementarytRNA anticodon

1

UA A




Tyr-tRNA


Using ATP,synthetasecatalyzescovalentbonding.

AMP 2

ATP

2

1

UA A

P i




Tyr-tRNA


AminoacyltRNAreleased.

Using ATP,synthetasecatalyzescovalentbonding.

AMP 2

ATP

2

3

1

UA A

P i

Ribosomes

Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons during protein synthesis.

The large and small ribosomal are made of proteins and ribosomal RNAs (rRNAs).

In bacterial and eukaryotic ribosomes the large and small subunits join to form a ribosome only when attached to an mRNA molecule.

Figure 14.17a

tRNAmolecules

A

Largesubunit

Smallsubunit

Growing polypeptide Exit tunnel

E P

mRNA5

3

(a) Computer model of functioning ribosome

Figure 14.17b

PE A Largesubunit

Smallsubunit

P site (Peptidyl-tRNA binding site)

Exit tunnel

E site (Exit site)

mRNA binding site

(b) Schematic model showing binding sites

A site (Aminoacyl-tRNA binding site)

Figure 14.17c

Growing polypeptide

tRNA

5

3

EmRNA

(c) Schematic model with mRNA and tRNA

Codons

Amino end Next amino acidto be added to

polypeptidechain

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

Ribosome Association and Initiation of Translation

The initiation stage of translation brings together mRNA, a tRNA with the first amino acid, and the two ribosomal subunits

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)

Figure 14.18

A

Small ribosomal subunit bindsto mRNA.

GTP

P siteU A

mRNA

53

Met

P i

mRNA binding site

Start codonSmallribosomalsubunit

InitiatortRNA

53

53

Large ribosomal subunit completes the initiation complex.

U G

C53

Translation initiation complex

Largeribosomalsubunit

AE

Met

GDP

1 2

The start codon is important because it establishes the reading frame for the mRNA

The addition of the large ribosomal subunit is last and completes the formation of the translation initiation complex

Proteins called initiation factors bring all these components together

Elongation of the Polypeptide Chain

During elongation, amino acids are added one by one to the previous amino acid at 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

Translation proceeds along the mRNA in a 5 to 3 direction

Figure 14.19-1Amino endof polypeptide

mRNAP

site

P i

5

3E

GTP

Asite

GDP

Codon recognition

E

P A

1


mRNAP

site

P i

5

3E

GTP

Asite

GDP

Peptide bondformation

Codon recognition

E

P A

E

P A

2

1


mRNARibosome ready fornext aminoacyl tRNA P

site

P i

5

3E

GTP

Asite

GDP

Peptide bondformation

Codon recognition

Translocation

E

P A

E

P A

P i

GTP

GDP

E

P A

3

2

1

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 then comes apart

Figure 14.20-1

Ribosome reaches astop codon on mRNA.

5

3

Releasefactor

Stop codon(UAG, UAA, or UGA)

1

Figure 14.20-2

Freepolypeptide


5

3

Releasefactor


5

3

Release factorpromoteshydrolysis.

21

Figure 14.20-3

Freepolypeptide


5

3

2 GTP

2 GDP Pi

Releasefactor


5

35

3

Ribosomalsubunits and othercomponentsdissociate.

Release factorpromoteshydrolysis.

2 31

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.

During synthesis, a polypeptide chain spontaneously coils and folds into its three-dimensional shape.

Proteins may also require post-translational modifications before doing their jobs.

Targeting Polypeptides to Specific Locations

Two populations of ribosomes are evident in cells: free ribosomes (in the cytosol ( 細胞質液 )) 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.

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 14.21

Polypeptidesynthesisbegins.

SRPbinds tosignalpeptide.

SRPbinds toreceptorprotein.

SRPdetachesandpolypeptidesynthesisresumes.

Signal-cleavingenzyme cutsoff signalpeptide.

Completedpolypeptidefolds intofinalconformation.

Signalpeptideremoved

ERmembrane

Signalpeptide

Protein

SRP receptorprotein

Ribosome

mRNA

CYTOSOL

ER LUMEM

SRP

Translocation complex

1 2 3 4 5 6

Making Multiple Polypeptides in Bacteria and Eukaryotes

In bacteria and eukaryotes multiple ribosomes translate an mRNA at the same time

Once a ribosome is far enough past the start codon, another ribosome can attach to the mRNA

Strings of ribosomes called polyribosomes (or polysomes) can be seen with an electron microscope

Figure 14.22a

Incomingribosomalsubunits

(a) Several ribosomes simultaneously translating onemRNA molecule

Start of mRNA(5 end)

End of mRNA(3 end)

Growingpolypeptides

Completedpolypeptide

Polyribosome

Figure 14.22b

(b) A large polyribosome in a bacterialcell (TEM)

Ribosomes

mRNA

0.1 m

Bacteria and eukaryotes can also transcribe multiple mRNAs form the same gene

In bacteria, the transcription and translation can take place simultaneously

In eukaryotes, the nuclear envelope separates transcription and translation

Figure 14.23

RNA polymerase

mRNA

0.25 m

DNA

Polyribosome

RNA polymerase DNA

Polyribosome

Direction oftranscription

Ribosome

mRNA (5 end)

Polypeptide(amino end)

Figure 14.24

A

U A

tRNA

5 Cap

3

mRNA

Ribosomalsubunits

Aminoacyl(charged)tRNA

PE

G

5

3

Ribosome

Codon

Anticodon

TRANSLATION

Aminoacid

Aminoacyl-tRNAsynthetase

CYTOPLASM

NUCLEUS

AMINO ACIDACTIVATION

Intron

5 C

ap

Poly-A

TRANSCRIPTION

RNAPROCESSING

RNAtranscript

RNA transcript(pre-mRNA)

RNApolymerase

Exon

DNA

Poly-A

Poly-A

U GUG U U

A A A

A C C UA

CAE

Figure 14.24a

tRNA

mRNA

5

3

Aminoacid

Aminoacyl-tRNAsynthetase

CYTOPLASM

NUCLEUS

AMINO ACIDACTIVATION

Intron

5 C

ap

TRANSCRIPTION

RNAPROCESSING

RNAtranscript

RNA transcript(pre-mRNA)

RNApolymerase

Exon

DNA

Poly-A

Poly-A


Figure 14.24b

A

U A

5 Cap

3

mRNA

Ribosomalsubunits


PE

G

Ribosome

Codon

Anticodon

TRANSLATION

5 C

ap

Poly-A

U GUG U U

A A A

A C C UA

CAE

Growing polypeptide

Concept 14.5: 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 or a few nucleotide pairs of a gene

The change of a single nucleotide in a DNA template strand can lead to the production of an abnormal protein

Figure 14.25

AG G

Wild-type hemoglobin

mRNA

53

mRNA

Wild-type hemoglobin DNA

535

3TC C

TG GAC C 5

3

AG G5 53 UG G 3

Normal hemoglobin

Sickle-cell hemoglobin

Mutant hemoglobin DNA

Sickle-cell hemoglobin

ValGlu

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

Figure 14.26

mRNA

DNA template strand

StopCarboxyl end

ProteinAmino end

Phe GlyMet Lys

A G3 T CC 5T T T TA A A AC C

5 3TA A A A AT T T TG G G G C

U5 3G CA U U G GGA A A AU U

Wild type

Phe GlyMet Lys

A A3 T CC 5T T T TA A A AC C

5 3TA A A A AT T T TG G G G T

U5 3G UA U U G GGA A A AU U

Phe SerMet Lys

A G3 T CC 5T T T TA A A AC T

5 3TA A A A AT T T TG G A G C

U5 3G CA U U A GGA A A AU U

Met

A C3 T CT 5A T A TC A A GC A

5 3TA T A T AG T T CG A T G G

U5 3G GA G U U GAU A U AU U

Leu AlaMet Lys

A A3 T GC 5T T T

A

A A C TC C

5 3TA A A AGT T AG G G C T

U5 3G UA U G G GGA A A

U

U A

GlyMet Phe

A T3 T TA 5A A

T T

C C G

C

C A

5 3TA T T

A

A

G G C

T

G T T A A

U5 3G AA G C U AUU U

A G

G

A

Met

A G3 T CC 5A T T TA A A AC C

5 3TA T A A AT T T TG G G G C

U5 3G UA U U G GGU A A AU U

Stop Stop

Stop

Stop

Stop

A instead of G

Silent (no effect on amino acid sequence)

(a) Nucleotide-pair substitution

U instead of C

T instead of C

Missense

A instead of G

A instead of T

Nonsense

U instead of A

Extra A

Frameshift causing immediate nonsense(1 nucleotide-pair insertion)

(b) Nucleotide-pair insertion or deletion

Extra U

Frameshift causing extensive missense(1 nucleotide-pair deletion)

missing

missing

missing

missing

No frameshift, but one amino acid missing(3 nucleotide-pair deletion)

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

Figure 14.26a

DNA template strand

mRNA

StopCarboxyl end

Protein

Amino endPhe GlyMet Lys

3 55 35 3

Wild type

Phe GlyMet Lys

3 55 3

5 3

Stop

A instead of G

Nucleotide-pair substitution: silent

U instead of C

T A T T A A A A T TC C C C GTA T TA A AAT TG G G CG

UA U UA A AAU UG G G CG

T A T T A A A A T TC C C C ATA T TA A AAT TG G G TG

UA U UA A AAU UG G G UG

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

Substitution mutations are usually missense mutations

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

Figure 14.26b

DNA template strand

mRNA

StopCarboxyl end

Protein


3 55 35 3

Wild type

Phe SerMet Lys

3 55 3

5 3

Stop

T instead of C

A instead of G

Nucleotide-pair substitution: missense

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

TA T TA A AAT TG G G CG


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

TA T TA A AAT TG A G CG

UA U UA A AAU UG A G CG

Figure 14.26c

DNA template strand

mRNA

StopCarboxyl end

Protein


3 55 35 3

Wild type

Nucleotide-pair substitution: nonsense

3 55 3

5 3Met

Stop

A instead of T

U instead of A




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

TA T TT A AAT TG G G CG

UA U UU A AAU UG G G CG

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 of the genetic message, producing a frameshift mutation

Figure 14.26d

DNA template strand

mRNA

StopCarboxyl end

Protein


3 55 35 3

Wild type

Nucleotide-pair insertion: frameshift causing immediate nonsense

Met

3 55 3

5 3

Stop

Extra A

Extra U







A

T

U

Figure 14.26e

DNA template strand

mRNA

StopCarboxyl end

Protein


3 55 35 3

Wild type

Nucleotide-pair deletion: frameshift causing extensive missense

Leu AlaMet Lys

3 5A

5 3

5 3U

missing

missing




T A T T A AG

A T TC C C C G

TA T TA A AATG G CG

UA U UA A AAG UG G CG

Figure 14.26f

DNA template strand

mRNA

StopCarboxyl end

Protein


3 55 35 3

Wild type

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

GlyMet Phe

3 5T T C

5 3

5 3A GA

Stop

missing

missing




T A A A C C GC A A T T

TA G GT T CT T A AG

UA U U UG G G C U A A

Mutagens

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

Mutagens are physical or chemical agents that can cause mutations

Researchers have developed methods to test the mutagenic activity of chemicals

Most cancer-causing chemicals (carcinogens) are mutagenic, and the converse is also true

What Is a Gene? Revisiting the Question

The definition of a gene has evolved through the history of genetics

We have considered a gene as

A discrete unit of inheritance

A region of specific nucleotide sequence in a chromosome

A DNA sequence that codes for a specific polypeptide chain

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Gene Expression: From Gene to Protein

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