campbell gene expression biology - napa … 120 fall...11/18/2014 2 what structure assembles the...

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Copyright © 2009 Pearson Education, Inc.

CAMPBELL

BIOLOGY Reece • Urry • Cain • Wasserman • Minorsky • Jackson

© 2014 Pearson Education, Inc.

TENTH

EDITION

CAMPBELL

BIOLOGY Reece • Urry • Cain • Wasserman • Minorsky • Jackson


TENTH

EDITION

17 Gene to Protein

Lecture Presentation by

Dr Burns

NVC Biol 120

Gene Expression

The process in which the information

coded in DNA is used to make proteins

A gene is the part of the DNA molecule

that codes for a specific protein

Gene expression, the process by which

DNA directs protein synthesis, includes

two stages: transcription and translation

Figure 17.1

Protein Synthesis Overview

Remember back to the lecture on the cell

we briefly went through the steps of

protein synthesis

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

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What structure assembles the polypeptide chains?

1. Nucleus

2. Rough

Endoplasmic

reticulum

3. Ribosomes

4. Lysosomes

Nucl

eus

Rough

Endop

lasm

ic ..

.

Rib

osom

es

Lys

osom

es

25% 25%25%25%

The Nature of Genes

The central dogma of molecular biology

states that information flows in one

direction:

DNA RNA protein

Transcription is the flow of information from

DNA to RNA.

Translation is the flow of information from

RNA to protein.

1.Transcription: DNA

unwinds and nucleotides

form base pairs to

produce a single strand of

mRNA

2. mRNA leaves nucleus

3. Translation: mRNA docks

with ribosomes. tRNA

brings amino acids to the

ribosome to be assembled

into polypeptide

RNA

During the transcription: part of DNA is copied

to make mRNA

mRNA is only a single strand – remember that

DNA is two strands.

RNA has same “handrail” structure with the

phosphates covalently bound to the sugars.

The sugars are bound covalently to bases

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Differences between RNA and DNA

1. RNA is single stranded, DNA is double

stranded

2. The sugar is different = ribose

3. RNA has four bases, but one base is

different from DNA: CGAU, the U is uracil

During transcription uracil is paired with adenine

Types of RNA

There are different types of RNA:

Messenger RNA (mRNA) – single strand, carries

information for making a protein from the nucleus to

the cytosol

Transfer RNA (tRNA) – single strand, folds back on

itself. Each tRNA carries one specific amino acid

and brings it to the ribosome

Ribosomal RNA (rRNA) – globular structure, forms

the catalytic part of ribosomes. Catalyzes the

formation of the peptide bonds between amino

acids

Types of RNA

There are different types of RNA:

small nuclear RNA (snRNA) are involved in

processing pre-mRNA, involved in splicing mRNA

signal recognition particle (SRP) is composed of

protein and RNA and involved in directing ribosome

to the RER

micro-RNA (miRNA) are very small and their role is

not clear yet, control gene expression, protect cells

from viral attack

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

(a) Tobacco plant expressing a firefly gene gene

(b) Pig expressing a jellyfish

Figure 17.4

DNA template strand

TRANSCRIPTION

mRNA

TRANSLATION

Protein

Amino acid

Codon

Trp Phe Gly

5

5

Ser

U U U U U

3

3

5 3

G

G

G G C C

T

C

A

A

A A A A A

T T T T

T

G

G G G

C C C G G

DNA molecule

Gene 1

Gene 2

Gene 3

C C

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Translation

Codon 1 Codon 2 Codon 3 Codon 4 Codon 5 Codon 6

Polypeptide

Coding strand

Transcription DNA

Template strand

mRNA

(complementary

copy of template

DNA strand)

Molecular Components of Transcription

RNA synthesis is catalyzed by RNA

polymerase, which pries the DNA strands

apart and hooks together the RNA nucleotides

The RNA is complementary to the DNA

template strand

RNA synthesis follows the same base-pairing

rules as DNA, except that uracil substitutes for

thymine


The DNA sequence where RNA polymerase

attaches is called the promoter

The stretch of DNA that is transcribed is called

a transcription unit


Transcription Transcription Transcription

RNA polymerase act in a similar manner as

DNA polymerase

Build the RNA molecule from 5’ end of the RNA

to the 3’ end

Nucleotides are added to build the

complementary RNA strand, use the hydrolysis

of phosphates to provide the energy to build the

new strand

RNA is antiparallel to DNA

Transcription

Upstream Downstream

Template strand: 3’-A-C-C-A-5’

Coding strand: 5’-T-G-G-T-3’

RNA 5’-U-G-G-U-3’

Figure 17.7-1 Promoter

RNA polymerase

Start point DNA

5 3

Transcription unit

3 5

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5


RNA polymerase

Start point DNA

5 3

Transcription unit

3 5

Initiation

5 3

3 5

Coding (Nontemplate) strand of DNA

Template strand of DNA RNA transcript

Unwound DNA

1


RNA polymerase

Start point DNA

5 3

Transcription unit

3 5

Elongation

5 3

3 5

Coding (Nontemplate) strand of DNA


Unwound DNA

2

3 5 3

5 3

Rewound DNA

RNA transcript

5

Initiation 1


RNA polymerase

Start point DNA

5 3

Transcription unit

3 5

Elongation

5 3

3 5

Nontemplate strand of DNA


Unwound DNA

2

3 5 3

5 3

Rewound DNA

RNA transcript

5

Termination 3

3

5

5 Completed RNA transcript

Direction of transcription (“downstream”)

5 3

3

Initiation 1


Animation: Transcription

Right-click slide / select “Play”


Which DNA strand is copied to make RNA?

1 2

50%50%1. Coding

2. Template

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Transcription

http://vcell.ndsu.nodak.edu/animations/tran

scription/index.htm

Transcription

RNA polymerase binds to a promoter region of

the DNA = initiation stage

The RNA polymerase acts to bind nucleotides

bound together to form the complementary

RNA strand = elongation stage

The RNA polymerase continues to add

nucleotides until it comes to a stop signal, a set

of three nucleotides on the DNA that signals

the end = termination stage

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 17.8

Transcription initiation

complex forms

3

DNA

Promoter Nontemplate strand

5

3

5

3

5

3

Transcription

factors

RNA polymerase II

Transcription factors

5

3

5

3

5

3

RNA transcript

Transcription initiation complex

5 3

TATA box

T

T T T T T

A A A A A

A A

T

Several transcription

factors bind to DNA

2

A eukaryotic promoter 1

Start point Template strand

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

Nucleotides are added to the 3 end of the

growing RNA molecule


Nontemplate

strand of DNA

RNA nucleotides

RNA

polymerase

Template

strand of DNA

3

3 5

5

5

3

Newly made

RNA

Direction of transcription

A A A

A

T

T T T G

C

C C

G

C C C A A U

end

Figure 17.9

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If the DNA sequence was 3’-ATCG-5’ then the

complementary mRNA sequence would be:

1 2 3 4

25% 25%25%25%1. 3’-TAGC-5’

2. 5’-TACG-3’

3. 3’-UAGC-5’

4. 5’-UAGC-3’

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

Figure 17.10

Protein-coding

segment

Polyadenylation

signal 5 3

3 5 5 Cap UTR Start

codon

G P P P

Stop

codon UTR

AAUAAA

Poly-A tail

AAA AAA …

Protein synthesis in Prokaryotes

In bacteria the transcription and translation

steps are coupled

Remember that there is no nucleus so the

mRNA does not leave the nucleus to go to

the cytosol to bind with the ribosomes

The bacterial mRNA is not modified after it is

transcribed, it is used immediately after

transcription

Figure 17.3

DNA

mRNA

Ribosome

Polypeptide

TRANSCRIPTION

TRANSLATION

TRANSCRIPTION

TRANSLATION

Polypeptide

Ribosome

DNA

mRNA

Pre-mRNA

RNA PROCESSING

(a) Bacterial cell

(b) Eukaryotic cell

Nuclear

envelope

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 some interior parts of the

molecule are cut out, and the other parts

spliced together


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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

Protein-coding

segment

Polyadenylation

signal 5 3

3 5 5 Cap UTR Start

codon

G P P P

Stop

codon UTR

AAUAAA

Poly-A tail

AAA AAA …

Split Genes and RNA Splicing

The mRNA contains regions that do not code

for amino acids that are found between coding

regions

Areas of the gene that are noncoding = introns

Coding areas of the gene = exons

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

Figure 17.11

5 Exon Intron Exon

5 Cap Pre-mRNA Codon numbers

130 31104

mRNA 5 Cap

5

Intron Exon

3 UTR

Introns cut out and

exons spliced together

3

105

146

Poly-A tail

Coding

segment

Poly-A tail

UTR 1146

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The Functional and Evolutionary Importance of

Introns

Some introns contain sequences that may

regulate gene expression

Some genes can encode more than one kind of

polypeptide, depending on which segments are

treated as exons during splicing

This is called alternative RNA splicing

Consequently, the number of different proteins an

organism can produce is much greater than its

number of genes


Fig. 16.17

RNA splicing

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


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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



mRNA modifications

http://vcell.ndsu.nodak.edu/animations/mrn

aprocessing/index.htm

Translation is the RNA-directed synthesis of

a polypeptide: a closer look

Genetic information flows from mRNA to

protein through the process of translation


Translation

http://vcell.ndsu.nodak.edu/animations/tran

slation/index.htm

Codons

Three mRNA bases code for one amino acid

The three mRNA bases together are called a

codon

So when CCU are next to each other as a

codon then that will be read as proline

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Figure 17.5 Second mRNA base

Fir

st m

RN

A b

ase

(5 en

d o

f co

do

n)

Th

ird

mR

NA

base

(3

en

d o

f co

do

n)

UUU

UUC

UUA

CUU

CUC

CUA

CUG

Phe

Leu

Leu

Ile

UCU

UCC

UCA

UCG

Ser

CCU

CCC

CCA

CCG

UAU

UAC Tyr

Pro

Thr

UAA Stop

UAG Stop

UGA Stop

UGU

UGC Cys

UGG Trp

G C

U

U

C

A

U

U

C

C

C A

U

A

A

A

G

G

His

Gln

Asn

Lys

Asp

CAU CGU

CAC

CAA

CAG

CGC

CGA

CGG

G

AUU

AUC

AUA

ACU

ACC

ACA

AAU

AAC

AAA

AGU

AGC

AGA

Arg

Ser

Arg

Gly

ACG AUG AAG AGG

GUU

GUC

GUA

GUG

GCU

GCC

GCA

GCG

GAU

GAC

GAA

GAG

Val Ala

GGU

GGC

GGA

GGG Glu

Gly

G

U

C

A

Met or

start

UUG

G

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


tRNA

mRNA codes for which amino acids go in

what order,

DNA also copied to make tRNA

tRNA (transfer RNA) brings the amino acids

to the ribosomes

One side of tRNA attaches to an amino acid,

the other side will attach to the mRNA

tRNA

Each tRNA is covalently bound to an amino

acid

tRNA molecules have an anticodon region =

complementary to the mRNA region

Figure 17.14

Polypeptide

Ribosome

tRNA with

amino acid

attached

Amino

acids

tRNA

Anticodon

Codons

U U U U G G G G C

C

G

C

G

5 3

mRNA

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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

© 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc.

BioFlix: Protein Synthesis

A tRNA molecule consists of a single RNA

strand that is only about 80 nucleotides long


The Structure and Function of Transfer RNA Figure 17.15

Amino acid

attachment

site

3

5

Hydrogen

bonds

Anticodon

(a) Two-dimensional structure (b) Three-dimensional structure (c) Symbol used

in this book

Anticodon Anticodon

3 5

Hydrogen

bonds

Amino acid

attachment

site 5

3

A A G

Because of hydrogen bonds, tRNA actually

twists and folds into a three-dimensional

molecule

tRNA is roughly L-shaped


The Structure and Function of Transfer RNA

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

Translation

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Flexible pairing at the third base of a codon is

called wobble and allows some tRNAs to bind to

more than one codon

Translation Aminoacyl-tRNA

synthetase (enzyme)

Amino acid

P P P Adenosine

ATP

Figure 17.16-1

Aminoacyl-tRNA

synthetase (enzyme)

Amino acid

P P P Adenosine

ATP

P

P

P

P P i

i

i

Adenosine

Figure 17.16-2 Aminoacyl-tRNA

synthetase (enzyme)

Amino acid

P P P Adenosine

ATP

P

P

P

P P i

i

i

Adenosine

tRNA

Adenosine P

tRNA

AMP

Computer model

Amino

acid

Aminoacyl-tRNA

synthetase

Figure 17.16-3

Aminoacyl-tRNA

synthetase (enzyme)

Amino acid

P P P Adenosine

ATP

P

P

P

P P i

i

i

Adenosine

tRNA

Adenosine P

tRNA

AMP

Computer model

Amino

acid

Aminoacyl-tRNA

synthetase

Aminoacyl tRNA

(“charged tRNA”)

Figure 17.16-4

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)


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Ribosomes

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

tRNA

molecules

Growing

polypeptide Exit tunnel

E P A

Large

subunit

Small

subunit

mRNA 5

3

(a) Computer model of functioning ribosome

Figure 17.17b

Exit tunnel

A site (Aminoacyl-

tRNA binding site)

Small

subunit

Large

subunit

P A

P site (Peptidyl-tRNA

binding site)

mRNA

binding site

(b) Schematic model showing binding sites

E site

(Exit site)

E

Figure 17.17c

Amino end

mRNA

E

(c) Schematic model with mRNA and tRNA

5 Codons

3

tRNA

Growing polypeptide

Next amino

acid to be

added to

polypeptide

chain

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


Ribosomes Building a Polypeptide

The three stages of translation

Initiation

Elongation

Termination

All three stages require protein “factors” that

aid in the translation process


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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


Ribosome Association and Initiation of Translation

First, a small ribosomal subunit binds with mRNA and a special initiator tRNA (carrying the amino acid methionine)

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

Initiator

tRNA

mRNA

5

5 3

Start codon Small

ribosomal

subunit mRNA binding site

3

Translation initiation complex

5 3

3 U

U

A A G

C

P

P site

i

GTP GDP

Large

ribosomal

subunit

E A

5

Elongation of the Polypeptide Chain

During the elongation stage, amino acids are

added one by one to the preceding 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


Amino end of polypeptide

mRNA

5

E

P site

A site

3

Figure 17.19-1 Amino end of polypeptide

mRNA

5

E

P site

A site

3

E

GTP

GDP P i

P A

Figure 17.19-2

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Amino end of polypeptide

mRNA

5

E

P site

A site

3

E

GTP

GDP P i

P A

E

P A

Figure 17.19-3 Amino end of polypeptide

mRNA

5

E

A site

3

E

GTP

GDP P i

P A

E

P A

GTP

GDP P i

P A

E

Ribosome ready for next aminoacyl tRNA

P site

Figure 17.19-4

Protein Synthesis

Protein synthesis proceeds from the N-

terminus to the C-terminus of the protein.

The ribosomes "read" the mRNA in the 5'

to 3' direction.

Fig. 15.18

Created by Dr. Joachim Frank

https://www.rpi.edu/dept/bcbp/molbiochem

/MBWeb/mb2/part1/translate.htm

http://cen.acs.org/articles/85/i8/Protein-

Factory-Reveals-Secrets.html

https://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/translate.htm

https://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/translate.htm

http://cen.acs.org/articles/85/i8/Protein-Factory-Reveals-Secrets.html







11/18/2014

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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

© 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc.

Animation: Translation

Right-click slide / select “Play”

Figure 17.20-1

Release factor

Stop codon (UAG, UAA, or UGA)

3

5

Figure 17.20-2

Release factor


3

5

3

5

Free polypeptide

2 GTP

2 GDP 2 i

P

Figure 17.20-3

Release factor


3

5

3

5

Free polypeptide

2 GTP

5

3

2 GDP 2 i

P

Initiation

1. The mRNA binds with the small subunit of the

ribosome

2. The beginning of the coding region of mRNA is

AUG

3. The tRNA with the anticodon UAC has

methionine attached to it.

4. The tRNA with met attached binds to the P site

of the ribosomes.

5. This requires energy in the form of GTP

6. Large subunit joins the small subunit

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Elongation

This is the stage where amino acids are added to the growing polypeptide chain

7. A tRNA with the next amino acid comes into the A site, requires GTP for energy

8. The amino acids are bound by a peptide bond

9. The bond is between the carboxyl end of the P site amino acid and the amino side of the A amino acid

Elongation

10. The now ribosome moves so the free tRNA is at

the E site and the tRNA with the polypeptide

chain moves is at the P site = translocation

11. This requires GTP

12. The free tRNA exits the ribosome

Termination

13. The end of the coding region will have a

stop codon that signals the end of the

polypeptide chain

14. No tRNA binds to this codon, instead a

release factor binds, requires GTP for

energy

15. The polypeptide chain is released

What molecules are produced in

transcription?

1. Amino acids

2. Proteins

3. RNA

4. DNA

Am

ino a

cids

Pro

tein

s

RNA

DNA

25% 25%25%25%

Polyribosomes

A number of 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 17.21 Completed

polypeptide

Incoming

ribosomal

subunits

Start of

mRNA

(5 end)

End of

mRNA

(3 end) (a)

Ribosomes

mRNA

(b) 0.1 m

Growing

polypeptides

11/18/2014

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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 and after synthesis, a polypeptide chain

spontaneously coils and folds into its three-

dimensional shape

Proteins may also require post-translational

modifications before doing their job

Some polypeptides are activated by enzymes

that cleave them

Other polypeptides come together to form the

subunits of a protein


Post-translational Modifications

Folding, either spontaneously or with the aid of chaperone proteins

Proteolysis – some polypeptide chains are cut into smaller chains

Glycosylation – sugars are added to some proteins, some of these sugar chains are important to “address” the protein

Phosphorylation – Protein Kinases phosphorylate proteins to activate them

Methionine is often removed

Targeting Polypeptides to Specific Locations

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

Free ribosomes mostly synthesize proteins that function in the cytosol

Bound ribosomes make proteins of the endomembrane system and proteins that are secreted from the cell

Ribosomes are identical and can switch from free to bound


Polypeptide synthesis always begins in the

cytosol

Synthesis finishes in the cytosol unless the

polypeptide signals the ribosome to attach to

the ER

Polypeptides destined for the ER or for

secretion are marked by a signal peptide


Targeting Polypeptides to Specific Locations Targeting Polypeptide Chains

Polypeptide chains that need to be brought into

the RER will start (after MET) with a short

sequence of amino acids = signal sequence

Proteins, called signal recognition particles

(SRPs), in the cytoplasm recognize this

sequence during translation and bind to the

amino acid sequence.

11/18/2014

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A signal-recognition particle (SRP) binds

to the signal peptide

The SRP brings the signal peptide and its

ribosome to the ER

Signal recognition particle (SRP) is composed of

protein and RNA


Targeting Polypeptides to Specific Locations Targeting Polypeptide Chains

The signal sequence and SRP complex binds

to a receptor on the RER membrane

The complex docks with the RER

As the polypeptide chain is built, the chain is

brought into the RER

Once inside the RER the polypeptide chain can

be folded, modified

Then the protein is transported via a vesicle to

the Golgi

Figure 17.22

Ribosome

mRNA

Signal peptide

SRP

1

SRP receptor protein

Translocation complex

ER LUMEN

2

3

4 5

6

Signal peptide removed

CYTOSOL

Protein

ER membrane

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


Figure 17.23

Wild-type hemoglobin

Wild-type hemoglobin DNA

3

3

3 5

5 3

3 5

5

5 5 3

mRNA

A A G

C T T

A A G

mRNA

Normal hemoglobin

Glu

Sickle-cell hemoglobin

Val

A

A

A U G

G

T

T

Sickle-cell hemoglobin

Mutant hemoglobin DNA

C

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


11/18/2014

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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


Wild type

DNA template strand

mRNA5

5

3

Protein

Amino end

A instead of G

(a) Nucleotide-pair substitution

3

3

5

Met Lys Phe Gly Stop

Carboxyl end

T T T T T

T T T T T A A A A A

A A A A C C

C

C

A

A A A A A

G G G G

G C C

G G G U U U U U G

(b) Nucleotide-pair insertion or deletion

Extra A

3

5

5

3

Extra U

5 3

T T T T

T T T T

A

A A A

A

A T G G G G

G A A A

A C

C C C C A

T 3 5

5 3

5 T T T T T A A A A C C A A C C

T T T T T A A A A A T G G G G

U instead of C

Stop

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

Met Lys Phe Gly

Silent (no effect on amino acid sequence)

T instead of C

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

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

A instead of G

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

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

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

Met Lys Phe Ser

Stop

Stop Met Lys

missing

missing

Frameshift causing immediate nonsense

(1 nucleotide-pair insertion)

Frameshift causing extensive missense

(1 nucleotide-pair deletion)

missing

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

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

Leu Ala

Missense

A instead of T

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

A

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

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

Met

Nonsense

Stop

U instead of A

3

5

3 5

5

3

3

5

5

3

3 5 3

Met Phe Gly

No frameshift, but one amino acid missing

(3 nucleotide-pair deletion)

missing

3

5

5

3

5 3

U

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

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

T T C

A A G

Met

3

T

A

Stop

3

5

5

3

5 3

Figure 17.24

Figure 17.24a

Wild type

DNA template strand

mRNA5

5

Protein

Amino end Stop Carboxyl end

3

3

3

5

Met Lys Phe Gly

A instead of G

(a) Nucleotide-pair substitution: silent

Stop Met Lys Phe Gly

U instead of C

A

A

A A

A A A A

A A T

T T T T T

T T T T

C C C C

C

C

G G G G

G

G

A

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

5

3

3

5 A

A A

A A A A

A A T

T T T T T

T T T T

C C C C

G G G G

A

A

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

T

U 3 5

Figure 17.24b

Wild type

DNA template strand

mRNA5

5

Protein


3

3

3

5

Met Lys Phe Gly

T instead of C

(a) Nucleotide-pair substitution: missense

Stop Met Lys Phe Ser

A instead of G

A

A

A A

A A A A

A A T

T T T T T

T T T T

C C C C

C

C

G G G G

G

G

A


5

3

3

5 A

A A

A A A A

A A T

T T T T T

T T T T

C C T C

G

G

G A

A G A A A A A G G U U U U U 3 5

A C

C

G

Figure 17.24c

Wild type

DNA template strand

mRNA5

5

Protein


3

3

3

5

Met Lys Phe Gly

A instead of T

(a) Nucleotide-pair substitution: nonsense

Met

A

A

A A

A A A A

A A T

T T T T T

T T T T

C C C C

C

C

G G G G

G

G

A


5

3

3

5 A

A

A A A A

A A T

T A T T T

T T T T

C C C

G

G

G A

A G U A A A G G U U U U U 3 5

C

C

G

T instead of C

C

G T

U instead of A

G

Stop

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


11/18/2014

22

Figure 17.24d

Wild type

DNA template strand

mRNA5

5

Protein


3

3

3

5

Met Lys Phe Gly

A

A

A A

A A A A

A A T

T T T T T

T T T T

C C C C

C

C

G G G G

G

G

A


(b) Nucleotide-pair insertion or deletion: frameshift causing

immediate nonsense

Extra A

Extra U

5

3

5

3

3

5

Met

1 nucleotide-pair insertion

Stop

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

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

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

A T

A

A G

Figure 17.24e

DNA template strand

mRNA5

5

Protein


3

3

3

5

Met Lys Phe Gly

A

A

A A

A A A A

A A T

T T T T T

T T T T

C C C C

C

C

G G G G

G

G

A


(b) Nucleotide-pair insertion or deletion: frameshift causing

extensive missense

Wild type

missing

missing

A

U

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

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

A G A A G U U U C A A G G U 3

5

3

3

5

Met Lys Leu Ala

1 nucleotide-pair deletion

5

Figure 17.24f

DNA template strand

mRNA5

5

Protein


3

3

3

5

Met Lys Phe Gly

A

A

A A

A A A A

A A T

T T T T T

T T T T

C C C C

C

C

G G G G

G

G

A


(b) Nucleotide-pair insertion or deletion: no frameshift, but one

amino acid missing

Wild type

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

T T C missing

missing

Stop

5

3

3

5

3 5

Met Phe Gly

3 nucleotide-pair deletion

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

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

A A G

Mutagens

Spontaneous mutations can occur during DNA

replication, recombination, or repair

Mutagens are physical or chemical agents that

can cause mutations to the DNA


While gene expression differs among the domains

of life, the concept of a gene is universal

Archaea are prokaryotes, but share many

features of gene expression with eukaryotes


Comparing Gene Expression in Bacteria, Archaea,

and Eukarya

Bacteria and eukarya differ in their RNA polymerases, termination of transcription, and ribosomes; archaea tend to resemble eukarya in these respects

Bacteria and archaea can simultaneously transcribe and translate the same gene

In eukarya, transcription and translation are separated by the nuclear envelope


11/18/2014

23

Figure 17.25

RNA polymerase

DNA mRNA

Polyribosome

RNA

polymerase DNA

Polyribosome

Polypeptide

(amino end)

mRNA (5 end)

Ribosome

0.25 m Direction of

transcription

If the mRNA sequence is: AUGCCCAAGUAA then the amino

acid sequence would be:

1. Start-Pro-Lys

2. Met-Pro-Lys

3. Met-Pro-Lys-Stop

4. Start-Pro-Lys-Stop

Sta

rt-P

ro-L

ys

Met

-Pro

-Lys

Met

-Pro

-Lys

-Sto

p

Sta

rt-P

ro-L

ys-S

top

25% 25%25%25%

Which of the following processes occur in the

nucleus?

1. DNA replication

transcription, and

translation

2. DNA replication and

transcription

3. DNA replication only

4. Transcription only

DNA rep

licat

ion

tran

s...

DNA rep

licat

ion

and t.

..

DNA rep

licat

ion

only

Tra

nscript

ion o

nly

25% 25%25%25%


Which molecule is produced during

translation?

1 2 3 4

25% 25%25%25%1. Amino acids

2. Proteins

3. RNA

4. DNA

Important concepts

Know the vocabulary for this lecture

Know what monomers are bound together to make

a protein and what kind of bonds hold them

together

Differences between DNA and RNA

Know the types of RNA, their role and where they

work in the cell

Be able to determine the complementary mRNA

sequence from a DNA sequence.

Important concepts

Steps of transcription and translation, know which direction mRNA is built and which direction it is read

Be able to “read” the mRNA to make a protein, given the table of codons.

Know how the peptide bond is formed

Know the role of RNA polymerase

Know the three stages of transcription (initiation, elongation, and termination stage

Know the structure of ribosomes – what it is made of, how many subunits, what are the binding sites, which part of the ribosome is the catalytic region

11/18/2014

24

Important concepts

Know the steps of translation, what powers

translation (know which steps need the energy)

Know how the processes are different in

prokaryotes

Know the post-transcriptional and post-

translational modifications to eukaryotic mRNA

and proteins

Know how polypeptide chains are brought into

the RER

Know the types of mutations discussed in

lecture, be able to recognize examples of each

campbell gene expression biology - napa … 120 fall...11/18/2014 2 what structure assembles the...

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