Biology in Focus - Chapter 14

CAMPBELL BIOLOGY IN FOCUS

© 2014 Pearson Education, Inc.

Urry • Cain • Wasserman • Minorsky • Jackson • Reece

Lecture Presentations by Kathleen Fitzpatrick and Nicole Tunbridge

14Gene Expression: From Gene to Protein


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


Figure 14.1


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

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


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 14.2

Neurosporacells

Each survivingcell forms a colony ofgeneticallyidentical cells.

No growth

Growth

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

Surviving cellstested for inabilityto grow onminimal medium.

Individual Neurosporacells placed on completegrowth medium. Growth

Cells subjectedto X-rays.

Control: Wild-typecells in minimalmedium

2

1 3

4

5


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


Figure 14.3

Precursor

Gene A

Ornithine Citrulline Arginine

Gene B Gene C

EnzymeA

EnzymeB

EnzymeC


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-1

mRNA

DNATRANSCRIPTION

(a) Bacterial cell


Figure 14.4a-2

mRNA

DNATRANSCRIPTION

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


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


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′

3′5′

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

But it is 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

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


Figure 14.7a

(a) Tobacco plant expressinga firefly gene


Figure 14.7b

(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


Molecular Components of Transcription

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

Animation: Transcription Introduction


Figure 14.8-1Transcription unit

RNA polymerase

Promoter

Template strand of DNA

Start point

RNA transcript

UnwoundDNA

Initiation

3′5′

3′5′ 3′

5′

3′5′

1



RNA polymerase

Promoter


Start point

RNA transcript

UnwoundDNA

RewoundDNA

RNA transcript

Direction oftranscription(“downstream”)

Initiation

Elongation

3′5′

3′5′

3′5′ 3′

5′

3′5′

3′5′

3′5′

2

1



RNA polymerase

Promoter


Start point

Termination

Completed RNA transcript

RNA transcript

UnwoundDNA

RewoundDNA

RNA transcript

Direction oftranscription(“downstream”)

Initiation

Elongation

3′5′

3′5′

3′5′ 3′

5′

3′5′

3′5′

3′5′

3′5′

3′5′

3′5′

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.

3′5′

5′ 3′ 3′5′

3′5′

3′5′

3

2

1

Templatestrand

Transcription factors

RNA polymerase II

3′5′

3′5′ T A 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′

5′3′

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


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 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

3′5′

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 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


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 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

The tRNA contains an amino acid at one end and at the other end has a nucleotide triplet that can base-pair with the complementary codon on mRNA


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

tRNA molecules can base-pair with themselves

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

Video: tRNA Model


Figure 14.15

5′

Anticodon

3′

Amino acidattachment site

AnticodonAnticodon

A A G5′3′

Hydrogenbonds

5′3′


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

GGGG

G

UUU

U

GG

G

G

A

A

**

*

*

*

*

**

*

(b) Three-dimensional structure

(c) Symbol used in this book(a) Two-dimensional structure

*


Figure 14.15a

5′

3′

Anticodon


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


Anticodon

A A G5′3′

Hydrogenbonds

5′3′

(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 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 14.17

PE A

tRNAmolecules

A

Largesubunit

Smallsubunit

Growing polypeptide Exit tunnel

E P

mRNA5′

3′

Growing polypeptide

(a) Computer model of functioning ribosome

tRNA

5′

3′E

mRNA

(c) Schematic model with mRNA and tRNA

Codons

Amino end Next amino acidto be added to

polypeptidechain

Largesubunit

Smallsubunit

A site (Aminoacyl-tRNA binding site)

P site (Peptidyl-tRNA binding site)

Exit tunnel

E site (Exit site)

mRNA binding site

(b) Schematic model showing binding sites


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′E

mRNA

(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)

Animation: Translation Introduction


Figure 14.18

A

Small ribosomal subunit bindsto mRNA.

GTP

P siteU A

mRNA

5′3′

Met

P i

mRNA binding site

Start codonSmallribosomalsubunit

InitiatortRNA

5′3′

5′3′

Large ribosomal subunit completes the initiation complex.

U G

C5′3′

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′

3′E

GTP

+

Asite

GDP

Codon recognition

E

P A

1



mRNAP

site

P i

5′

3′E

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′

3′E

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′

3′5′

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


Protein Folding and Post-Translational Modifications

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) 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 14.22

Incomingribosomalsubunits

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

Ribosomes

mRNA

(a) Several ribosomes simultaneously translating onemRNA molecule

0.1 µm

Start of mRNA(5′ end)

End of mRNA(3′ end)

Growingpolypeptides

Completedpolypeptide

Polyribosome


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 µm


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 µm

DNA

Polyribosome

RNA polymerase DNA

Polyribosome

Direction oftranscription

Ribosome

mRNA (5′ end)

Polypeptide(amino end)


Figure 14.23a

RNA polymerase

mRNA

0.25 µm

DNA

Polyribosome


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′ Cap

Poly-A

TRANSCRIPTION

RNAPROCESSING

RNAtranscript

RNA transcript(pre-mRNA)

RNApolymerase

Exon

DNA

Poly-A

Poly-A

U GUG U U

A A A

A CC U

ACAE


Figure 14.24a

tRNA

mRNA

5′

3′

Aminoacid

Aminoacyl-tRNAsynthetase

CYTOPLASM

NUCLEUS

AMINO ACIDACTIVATION

Intron

5′ Cap

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′ Cap

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

5′3′

mRNA

Wild-type hemoglobin DNA

5′3′5′

3′TC C

TG GAC C 5′

3′

AG G5′ 5′3′ 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 5′T T T TA A A AC C

5′ 3′TA A A A AT T T TG G G G C

U5′ 3′G CA U U G GGA A A AU U

Wild type

Phe GlyMet Lys

A A3′ T CC 5′T T T TA A A AC C

5′ 3′TA A A A AT T T TG G G G T

U5′ 3′G UA U U G GGA A A AU U

Phe SerMet Lys

A G3′ T CC 5′T T T TA A A AC T

5′ 3′TA A A A AT T T TG G A G C

U5′ 3′G CA U U A GGA A A AU U

Met

A C3′ T CT 5′A T A TC A A GC A

5′ 3′TA T A T AG T T CG A T G G

U5′ 3′G GA G U U GAU A U AU U

Leu AlaMet Lys

A A3′ T GC 5′T T T

A

A A C TC C

5′ 3′TA A A AGT T AG G G C T

U5′ 3′G UA U G G GGA A A

U

U A

GlyMet Phe

A T3′ T TA 5′A A

T T

C C G

C

C A

5′ 3′TA T T

A

A

G G C

T

G T T A A

U5′ 3′G AA G C U AUU U

A G

G

A

Met

A G3′ T CC 5′A T T TA A A AC C

5′ 3′TA T A A AT T T TG G G G C

U5′ 3′G 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′ 5′5′ 3′5′ 3′

Wild type

Phe GlyMet Lys

3′ 5′5′ 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

Animation: Protein Synthesis


Figure 14.26b

DNA template strand

mRNA

StopCarboxyl end

Protein


3′ 5′5′ 3′5′ 3′

Wild type

Phe SerMet Lys

3′ 5′5′ 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′ 5′5′ 3′5′ 3′

Wild type

Nucleotide-pair substitution: nonsense

3′ 5′5′ 3′

5′ 3′Met

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′ 5′5′ 3′5′ 3′

Wild type

Nucleotide-pair insertion: frameshift causing immediate nonsense

Met

3′ 5′5′ 3′

5′ 3′

Stop

Extra A

Extra U







A

T

U


Figure 14.26e

DNA template strand

mRNA

StopCarboxyl end

Protein


3′ 5′5′ 3′5′ 3′

Wild type

Nucleotide-pair deletion: frameshift causing extensive missense

Leu AlaMet Lys

3′ 5′A

5′ 3′

5′ 3′U

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′ 5′5′ 3′5′ 3′

Wild type

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

GlyMet Phe

3′ 5′T T C

5′ 3′

5′ 3′A GA

Stop

missing

missing




T A A A C C GC A A T TTA 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


A gene can be defined as a region of DNA that can be expressed to produce a final functional product, either a polypeptide or an RNA molecule


Figure 14.UN05a

thrA−

18

lacA

lacY

lacZ

lacl

recA

lexA

galR

met J

trpR3′5′

−1

7−

16

−15

−1

4−

13−

12

−1

1−

10 0

−1

−2

−3

−4

−5

−6

−7

−8

−9 1 2 3 4 5 6 7 8


Figure 14.UN05b

3′5′

−18

−1

7−

16

−15

−14

−1

3−

12−

11

−10 0−

1−

2−

3−

4−

5−

6−

7−

8−

9 1 2 3 4 5 6 7 8


Figure 14.UN05c

−18

−17

−1

6−

15

−1

4−

13

−1

2−

11

−10

0−

1−

2−

3−

4−

5−

6−

7−

8−

9 1 2 3 4 5 6 7 8


Figure 14.UN06

3′

5′3′5′ 3′

5′

RNA transcriptRNA polymerase

Transcription unit

Promoter

Template strandof DNA


Figure 14.UN07

5′ Cap

Pre-mRNA

mRNA

Poly-A tail


Figure 14.UN08

tRNA

Polypeptide

Ribosome

Aminoacid

Anti-codon

Codon

mRNA

E A


Figure 14.UN09

Biology in Focus - Chapter 14

Science

Transcript of Biology in Focus - Chapter 14