CH 14: Gene to Protein Concept 14.1: Genes specify proteins via …bstephen.weebly.com/uploads/7/8/8/1/7881286/ch14_gento... · 2020. 1. 26. · UGG CUU CUC CUA CUG CCU CCC CCA CCG

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© 2014 Pearson Education, Inc.

CH 14: Gene to ProteinThe Flow of Genetic Information

The information of genes is in the sequences of

nucleotides in DNA

DNA 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


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

Archibald Garrod (1902)suggested that genes dictate

phenotypes through enzymes that catalyze specific

chemical reactions

He thought symptoms of an inherited disease

(alkaptonuria) reflect an inability to synthesize a certain

enzyme

Note: The term gene wasn’t actually coined until 1909

George Beadle and Edward Tatum (1940s) disabled genes

in bread mold one by one and looked for phenotypic

changes

Used 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© 2014 Pearson Education, Inc.

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


The researchers started a collection of Neurospora

mutant strains, catalogued by their defects

For example, one set of mutants all required arginine

for growth

Different classes of these mutants were blocked at a

different step in the biochemical pathway for arginine

biosynthesis

Precursor

Gene A

Ornithine Citrulline Arginine

Gene B Gene C

Enzyme

AEnzyme

B

Enzyme

C


The Products of Gene Expression: A Developing Story

Beadle and Tatum’s hypothesis, basically one gene

= one protein.

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

Beadle and Tatum’s hypothesis is now restated as

one gene–one polypeptide

It is common to refer to gene products as proteins

rather than polypeptides

2


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 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


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


The Genetic Code

Instructions for assembling amino acids into proteins

from DNA

There are 20 amino acids, but there are only four

nucleotide bases in DNA

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

3


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

A codon codes for an amino acid (one of 20) 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 correct

polypeptide to be produced


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

mR

NA

base

(5en

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

base

(3en

d o

f co

do

n)


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

(a) Tobacco plant expressinga firefly gene

(b) Pig expressing a jellyfishgene


Figure 14.UN02

DNA

Pre-mRNA

mRNA

Ribosome

Polypeptide

TRANSLATION

TRANSCRIPTION

RNA PROCESSING

4


Molecular Components of Transcription

Transcription is the first stage of gene expression

RNA synthesis is catalyzed by RNA polymerase, joins

together the RNA nucleotides

RNA polymerases assemble polynucleotides in the 5 to

3 direction

RNA polymerases can start a chain without a primer

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


Figure 14.8-3Transcription unit

RNA polymerase

Promoter

Template strand of DNA

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


RNA Polymerase Binding and Initiation of Transcription

Promoters 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 14.9

Transcription factors

TATA box

PromoterNontemplate strand

Start point

Transcriptioninitiationcomplex forms.

Transcription initiation complex

DNA

RNA transcript

A eukaryoticpromoter

Several transcriptionfactors bind to DNA.

35

5 335

35

35

3

2

1

Templatestrand

Transcription factors

RNA polymerase II

35

35 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

5


Figure 14.10

Nontemplate strand of DNA

Direction of transcription

RNA polymerase

3

53

5

RNA nucleotides

Template strand of DNA

Newly made RNA

3 end

5

U

G

A

A

A

A

AA

T T T

T CC

CC

G


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 (AAUAAA) signal sequence; the

RNA transcript is released 10–35 nucleotides past

this sequence


DNA

Pre-mRNA

mRNA

Ribosome

Polypeptide

TRANSLATION

TRANSCRIPTION

RNA PROCESSING

• Enzymes in the

eukaryotic

nucleus modify

pre-mRNA (RNA

processing)

before the genetic

messages go to

the cytoplasm

Eukaryotic cells

modify RNA after

transcription


Alteration of mRNA Ends

Each end of a pre-mRNA molecule is modified

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

Poly A tail length varies 50 – 250, As


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

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

6


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

Ribozymes can cut on own


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


Figure 14.UN04

DNA

mRNA

Ribosome

Polypeptide

TRANSLATION

TRANSCRIPTION

7


Figure 14.14

5

tRNA

Polypeptide

Ribosome

Anticodon

mRNA

Codons 3

tRNA withamino acidattached

Amino acids

Gly

A A A

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

One end of a tRNA contains the anticodon that

base-pairs with an mRNA codon


Figure 14.15

5

Anticodon

3

Amino acidattachment site

Anticodon

Anticodon

A A G

53

Hydrogenbonds

5

3

Amino acidattachment site

Hydrogenbonds

A

A

G

A

CG

C

C

C

C

U

A

C G

U

U

A

A A

A

C

G U

A

CG U

A

C

G

U

AC

G

U

*

C

G

*

G

G

U

A

AA

A

C C

C

C

C

GG

GG

G

U

UU

U

G

G

G

G

A

A

**

*

*

*

*

**

*

(b) Three-dimensionalstructure

(c) Symbol usedin this book(a) Two-dimensional structure

*


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 14.16-3Tyrosyl-tRNAsynthetase

Tyrosine (Tyr)(amino acid)

Amino acidand tRNAenter activesite.

Tyr-tRNA

ComplementarytRNA anticodon

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

8


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 A site holds the tRNA that carries the next

amino acid to be added to the chain

The P site holds the tRNA that carries the growing

polypeptide 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

9


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

Pi

mRNA binding site

Start codonSmallribosomalsubunit

InitiatortRNA

53

5

3

Large ribosomal subunit completes the initiation complex.

U G

C

5

3

Translation initiation complex

Largeribosomalsubunit

AE

GDP

1 2


The start codon 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

Added at 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

mRNA

Psite

Pi

5

3E

GTP

Asite

GDP

Codon recognition

E

P A

1



mRNA

Psite

Pi

5

3E

GTP

Asite

GDP

Peptide bondformation

Codon recognition

E

P A

E

P A

2

1

10



mRNARibosome ready fornext aminoacyl tRNA

Psite

Pi

5

3E

GTP

Asite

GDP

Peptide bondformation

Codon recognition

Translocation

E

P A

E

P A

Pi

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

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

11


Targeting Polypeptides to Specific Locations

Two populations of ribosomes are are in cells:

free ribosomes (in the cytosol)

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

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 m

Start of mRNA(5 end)

End of mRNA(3 end)

Growingpolypeptides

Completedpolypeptide


multiple mRNAs can be transcribed from the same

gene

In bacteria, the transcription and translation can take

place simultaneously

In eukaryotes, the nuclear envelope separates


12


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

3

mRNA

Ribosomalsubunits

Aminoacyl(charged)tRNA

PE

G

5

3

Ribosome

Codon

Anticodon

TRANSLATION

Aminoacid

Aminoacyl-tRNAsynthetase

CYTOPLASM

NUCLEUS

AMINO ACIDACTIVATION

Intron

TRANSCRIPTION

RNAPROCESSING

RNAtranscript

RNA transcript(pre-mRNA)

RNApolymerase

Exon

DNA

U GUG U U

A A A

AE


Concept 14.5: Mutations of one or a few nucleotides can affect protein structure andfunction

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

35

3

TC C

TG G

AC 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

Stop

Carboxyl end

Protein

Amino 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)

13


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

Stop

Carboxyl end

Protein

Amino end

Phe GlyMet Lys

3 55 3

5 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 G

TA 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 A

TA 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

Stop

Carboxyl end

Protein

Amino end

Phe GlyMet Lys

3 55 3

5 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 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

Stop

Carboxyl end

Protein

Amino end

Phe GlyMet Lys

3 55 3

5 3

Wild type

Nucleotide-pair substitution: nonsense

3 55 3

5 3

MetStop

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

14


Figure 14.26d

DNA template strand

mRNA

Stop

Carboxyl end

Protein

Amino end

Phe GlyMet Lys

3 55 3

5 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

Stop

Carboxyl end

Protein

Amino end

Phe GlyMet Lys

3 55 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 A

G

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

Stop

Carboxyl end

Protein

Amino end

Phe GlyMet Lys

3 55 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 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

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

Documents

CH 14: Gene to Protein Concept 14.1: Genes specify proteins via …bstephen.weebly.com/uploads/7/8/8/1/7881286/ch14_gento... · 2020. 1. 26. · UGG CUU CUC CUA CUG CCU CCC CCA CCG