17 ge lecture presentation

BIOLOGYA Global Approach

Campbell • Reece • Urry • Cain • Wasserman • Minorsky • Jackson

TENTH EDITION

Global Edition

Lecture Presentation by Nicole Tunbridge andKathleen Fitzpatrick

17Expression of

The Flow of Genetic Information

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

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

c) Proteins are the links between genotype and phenotype

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

Figure 17.1

Figure 17.1a

An albino racoon

Concept 17.1: Genes specify proteins via transcription and translation

a) How was the fundamental relationship between genes and proteins discovered?

Evidence from the Study of Metabolic Defects

a) In 1902, British physician Archibald Garrod first suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions

b) He thought symptoms of an inherited disease reflect an inability to synthesize a certain enzyme

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

Nutritional Mutants in Neurospora: Scientific Inquiry

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

b) Using crosses, they and their coworkers identified three classes of arginine-deficient mutants, each lacking a different enzyme necessary for synthesizing arginine

c) They developed a one gene–one enzyme hypothesis, which states that each gene dictates production of a specific enzyme

Figure 17.2

Precursor

Enzyme A

Enzyme B

Enzyme C

Ornithine

Citrulline

Arginine

No growth:Mutant cellscannot growand divide

Growth:Wild-typecells growingand dividing

Control: Minimal medium

Results Table

Wild type

Minimalmedium(MM)(control)

MM +ornithine

MM +citrulline

MM +arginine(control)

Summaryof results

Can grow withor without anysupplements

Gene(codes forenzyme) Wild type

Precursor

Ornithine

Gene A

Gene B

Gene C

Enzyme A

Enzyme B

Enzyme C

Enzyme A

Enzyme B

Enzyme C

Citrulline

Arginine

Precursor

Ornithine

Citrulline

Arginine

Precursor

Ornithine

Citrulline

Arginine

Precursor

Ornithine

Citrulline

Arginine

Enzyme A

Enzyme B

Enzyme C

Enzyme A

Enzyme B

Enzyme C

Class I mutants(mutation in

gene A)

Class II mutants(mutation in

gene B)

Class III mutants(mutation in

gene C)

Can grow onornithine,citrulline,or arginine

Can grow onlyon citrulline orarginine

Require arginineto grow

Class I mutants Class II mutants Class III mutants

Classes of Neurospora crassa

Figure 17.2a

Precursor

Enzyme A

Enzyme B

Enzyme C

Ornithine

Citrulline

Arginine

Figure 17.2b

No growth:Mutant cellscannot growand divide

Growth:Wild-typecells growingand dividing

Control: Minimal medium

Figure 17.2c

Results Table

Wild type

Minimalmedium(MM)(control)

MM +ornithine

MM +citrulline

MM +arginine(control)

Summaryof results

Can grow withor without anysupplements

Can grow onornithine,citrulline,or arginine

Can grow onlyon citrulline orarginine

Require arginineto grow

Class I mutants Class II mutants Class III mutants

Classes of Neurospora crassaC

Figure 17.2d

Gene(codes forenzyme) Wild type

Precursor

Ornithine

Gene A

Gene B

Gene C

Enzyme A

Enzyme B

Enzyme C

Citrulline

Arginine

Class I mutants(mutation in

gene A)

Class II mutants(mutation in

gene B)

Class III mutants(mutation in

gene C)Precursor

Ornithine

Enzyme A

Enzyme B

Enzyme C

Citrulline

Arginine

Precursor

Ornithine

Enzyme A

Enzyme B

Enzyme C

Citrulline

Arginine

Precursor

Ornithine

Enzyme A

Enzyme B

Enzyme C

Citrulline

Arginine

The Products of Gene Expression:A Developing Story

a) Some proteins aren’t enzymes, so researchers later revised the hypothesis: one gene–one protein

b) Many proteins are composed of several polypeptides, each of which has its own gene

c) Therefore, Beadle and Tatum’s hypothesis is now restated as the one gene–one polypeptide hypothesis

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

Basic Principles of Transcription and Translation

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

b)Transcription is the synthesis of RNA using information in DNA

c) Transcription produces messenger RNA (mRNA)

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

e) Ribosomes are the sites of translation

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

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

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

Figure 17.3

Nuclear envelope

CYTOPLASM

Pre-mRNA

RibosomeTRANSLATION

(b) Eukaryotic cell

NUCLEUS

RNA PROCESSING

TRANSCRIPTION

(a) Bacterial cell

Polypeptide

mRNARibosome

CYTOPLASM

TRANSCRIPTION

TRANSLATION

Polypeptide

Figure 17.3a-1

(a) Bacterial cell

mRNACYTOPLASM

TRANSCRIPTION

Figure 17.3a-2

(a) Bacterial cell

Polypeptide

mRNARibosome

CYTOPLASM

TRANSCRIPTION

TRANSLATION

Figure 17.3b-1

Nuclear envelope

CYTOPLASM

Pre-mRNA

(b) Eukaryotic cell

NUCLEUS

TRANSCRIPTION

Figure 17.3b-2

Nuclear envelope

CYTOPLASM

Pre-mRNA

(b) Eukaryotic cell

NUCLEUS

RNA PROCESSING

TRANSCRIPTION

Figure 17.3b-3

Nuclear envelope

CYTOPLASM

Pre-mRNA

RibosomeTRANSLATION

(b) Eukaryotic cell

NUCLEUS

RNA PROCESSING

TRANSCRIPTION

Polypeptide

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

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

Figure 17.UN01

DNA RNA Protein

The Genetic Code

a) How are the instructions for assembling amino acids into proteins encoded into DNA?

b) There are 20 amino acids, but there are only four nucleotide bases in DNA

c) How many nucleotides correspond to anamino acid?

Codons: Triplets of Nucleotides

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

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

c) These words are then translated into a chain of amino acids, forming a polypeptide

Figure 17.4

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

ACTTTT CGGGGT

U G G U U U G G C CU A

SerGlyPheTrp

CodonTRANSLATION

TRANSCRIPTION

Protein

mRNA 5′

Amino acid

DNAtemplatestrand

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

b) The template strand is always the same strandfor a given gene

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

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

Cracking the Code

a) All 64 codons were deciphered by the mid-1960s

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

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

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

Figure 17.5Second mRNA base

GCG GAG

AlaGlu

Tyr Cys

UGUUAU

UAG Stop

Stop Stop

Met orstart

U C A G

Evolution of the Genetic Code

a) The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals

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

Figure 17.6

Pig expressing a jellyfishgene

(b)Tobacco plant expressinga firefly gene

Figure 17.6a

Tobacco plant expressinga firefly gene

Figure 17.6b

Pig expressing a jellyfishgene

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

a) Transcription is the first stage of gene expression

Molecular Components of Transcription

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

b) The RNA is complementary to the DNA template strand

c) RNA polymerase does not need any primer

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

Figure 17.7-1Promoter Transcription unit

RNA polymeraseStart point

Template strand of DNARNAtranscript

UnwoundDNA

Initiation

5′3′

UnwoundDNA

RewoundDNA

RNAtranscript

Direction oftranscription(“downstream”)

Initiation

Elongation2

5′3′

5′3′ 3′

5′3′

UnwoundDNA

RewoundDNA

RNAtranscript

Direction oftranscription(“downstream”)

Completed RNA transcript

Initiation

Elongation

Termination

5′3′

5′3′ 3′

5′3′

Animation: Transcription

a) The DNA sequence where RNA polymerase attaches is called the promoter; in bacteria, the sequence signaling the end of transcription is called the terminator

b) The stretch of DNA that is transcribed is called a transcription unit

Synthesis of an RNA Transcript

a) The three stages of transcription

a)Initiation

b)Elongation

c)Termination

Figure 17.8 Promoter Nontemplate strand

15′3′

5′3′

Start point

RNA polymerase II

Templatestrand

TATA box

Transcriptionfactors

DNA3′5′

3′5′

Transcription factors

RNA transcript

Transcription initiation complex

3′5′5′3′

A eukaryoticpromoter

Severaltranscriptionfactors bindto DNA.

Transcriptioninitiationcomplexforms.

T A T AAAATA A T T T T

Elongation of the RNA Strand

a) As RNA polymerase moves along the DNA, it untwists the double helix, 10 to 20 bases at a time

b) Transcription progresses at a rate of 40 nucleotides per second in eukaryotes

c) A gene can be transcribed simultaneously by several RNA polymerases

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

Figure 17.9

Nontemplatestrand of DNA

3′3′ end

Direction of transcription

RNApolymerase

Templatestrand of DNA

Newly madeRNA

RNA nucleotides

G G T TT

C C CU

Concept 17.3: Eukaryotic cells modify RNA after transcription

a) Enzymes in the eukaryotic nucleus modify pre-mRNA (RNA processing) before the genetic messages are dispatched to the cytoplasm

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

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

Alteration of mRNA Ends

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

a)The 5′ end receives a modified nucleotide 5′ cap

b)The 3′ end gets a poly-A tail

b) These modifications share several functions

a)They seem to facilitate the export of mRNA to the cytoplasm

b)They protect mRNA from hydrolytic enzymes

c)They help ribosomes attach to the 5′ end

Figure 17.10

A modified guaninenucleotide added tothe 5′ end

Region that includesprotein-coding segments

5′ Cap

5′ UTR Startcodon

Stopcodon

G P P P

3′ UTR

3′AAUAAA AAA AAA

Poly-A tail

Polyadenylationsignal

50–250 adeninenucleotides addedto the 3′ end

Split Genes and RNA Splicing

a) Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides that lie between coding regions

b) These noncoding regions are called intervening sequences, or introns

c) The other regions are called exons because they are eventually expressed, usually translated into amino acid sequences

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

Figure 17.11

Pre-mRNA Intron Intron

Introns cut outand exonsspliced together

Poly-A tail5′ Cap

5′ Cap Poly-A tail

1–30 31–104 105–146

1–1463′ UTR5′ UTR

Codingsegment

Figure 17.12

Spliceosome Small RNAs

Exon 2

Cut-outintron

Spliceosomecomponents

Exon 1 Exon 2

Pre-mRNA

Exon 1

Intron

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

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

Molecular Components of Translation

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

b) tRNAs transfer amino acids to the growing polypeptide in a ribosome

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

Figure 17.14

PolypeptideAminoacids

tRNA withamino acidattached

Ribosome

Anticodon

Codons

U U U UG G G G C

The Structure and Function of Transfer RNA

a) Molecules of tRNA are not identical

a)Each carries a specific amino acid on one end

b)Each has an anticodon on the other end; the anticodon base-pairs with a complementary codon on mRNA

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

b) Flattened into one plane to reveal its base pairing, a tRNA molecule looks like a cloverleaf

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

b) tRNA is roughly L-shaped

Figure 17.15

Amino acidattachmentsite Amino acid

attachment site5′

3′ACCACGCUUA

GAUUUA

AGAA CC

G U UGC*

*C CUA G

*U* G A

Hydrogenbonds

Anticodon Anticodon

Symbol usedin this book

Three-dimensionalstructure

(a) Two-dimensional structure (b) (c)Anticodon

5′3′A A G

3′5′

Figure 17.15a

Amino acidattachmentsite 5′

Hydrogenbonds

(a) Two-dimensional structureAnticodon

GCUUAA

GGAUUUAA GCC

CA * C CU

A G **G

GGAGAGC

* G AGGU

Figure 17.15b

Amino acidattachment site

Hydrogenbonds

Anticodon Anticodon

Symbol usedin this book

Three-dimensionalstructure

(b) (c)

5′3′

3′5′

Video: Stick and Ribbon Rendering of a tRNA

Ribosomes

a) Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis

b) The two ribosomal subunits (large and small) are made of proteins and ribosomal RNA (rRNA)

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

Figure 17.17

Exit tunnel

Largesubunit

Smallsubunit

mRNA 3′5′

tRNAmolecules

Growingpolypeptide

(a) Computer model of functioning ribosome

Growingpolypeptide Next amino

acid to beadded topolypeptidechain

tRNA3′

Amino end

Codons

(c) Schematic model with mRNA and tRNA(b) Schematic model showing binding sites

Smallsubunit

Largesubunit

Exit tunnel

A site (Aminoacyl-tRNA binding site)

P site (Peptidyl-tRNA binding site)

E site(Exit site)

mRNAbinding site

Figure 17.17a

Exit tunnel

Largesubunit

Smallsubunit

mRNA 3′5′

tRNAmolecules

Growingpolypeptide

(a) Computer model of functioning ribosome

Figure 17.17b

(b) Schematic model showing binding sites

Smallsubunit

Largesubunit

Exit tunnel

A site (Aminoacyl-tRNA binding site)

P site (Peptidyl-tRNA binding site)

E site(Exit site)

mRNAbinding site

Figure 17.17c

Growingpolypeptide

Next aminoacid to beadded topolypeptidechain

Amino end

Codons

(c) Schematic model with mRNA and tRNA

Elongation of the Polypeptide Chain

a) During elongation, amino acids are added oneby one to the C-terminus of the growing chain

b) Each addition involves proteins called elongation factors and occurs in three steps: codon recognition, peptide bond formation, and translocation

c) Energy expenditure occurs in the first andthird steps

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

Figure 17.19-1Amino endof polypeptide

Codonrecognition

P Asitesite

P iGDP +

Codonrecognition

P Asitesite

Peptide bondformation

Codonrecognition

P Asitesite

23 GTP

Translocation

Peptide bondformation

Ribosome ready fornext aminoacyl tRNA

Termination of Translation

a) Termination occurs when a stop codon in the mRNA reaches the A site of the ribosome

b) The A site accepts a protein called a release factor

c) The release factor causes the addition of a water molecule instead of an amino acid

d) This reaction releases the polypeptide, and the translation assembly comes apart

Figure 17.20-1

Releasefactor

3′5′

Stop codon(UAG, UAA, or UGA)

Ribosome reaches astop codon on mRNA.

Figure 17.20-2

Releasefactor

3′5′5′

Release factorpromotes hydrolysis.

Freepolypeptide

Figure 17.20-3

Releasefactor

3′5′5′

Release factorpromotes hydrolysis.

Ribosomal subunitsand other componentsdissociate.

Freepolypeptide

2 GDP +2 P i

Completing and Targeting the Functional Protein

a) Often translation is not sufficient to make a functional protein

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

Protein Folding and Post-Translational Modifications

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

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

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

Targeting Polypeptides to Specific Locations

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

b) Free ribosomes mostly synthesize proteins that function in the cytosol

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

d) Ribosomes are identical and can switch from free to bound

a) In eukaryotes, the nuclear envelop separates the processes of transcription and translation

b) RNA undergoes processes before leavingthe nucleus

Figure 17.24DNA

RNApolymerase

RNA transcript(pre-mRNA)Intron

Aminoacyl-tRNAsynthetase

AminoacidtRNA

Aminoacyl(charged)tRNA

CYTOPLASM

NUCLEUS

RNAtranscript

5′ Poly-

5′ Cap

TRANSCRIPTION

RNAPROCESSING

Poly-A

AMINO ACIDACTIVATION

TRANSLATION

Ribosomalsubunits

E AA C C

U U U U U GGGA A A Anticodon

CodonRibosome

Figure 17.24a

TRANSCRIPTION DNA

Poly-A

5′ Cap

RNApolymerase

RNA transcript(pre-mRNA)Intron

Aminoacyl-tRNAsynthetase

AminoacidtRNA

AMINO ACIDACTIVATION

CYTOPLASM

NUCLEUS

RNAPROCESSING

RNAtranscript

Figure 17.24b

mRNA Growingpolypeptide

Ribosomalsubunits

Anticodon

TRANSLATION

Poly-A

5′ Cap

U U U U U GGG AAAA

A C CC

CodonRibosome

5′ Cap

E PA 3′

BioFlix: Protein Synthesis

Animation: Translation

Figure 17.UN10

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