Chapter 17: From Gene to Protein

Chapter 17: From Gene to Protein

Chapter 16 answered the question “What is the genetic material?”

Chapter 17 answers the question “How do genes give us our traits?”

Chapter 17: From Gene to Protein

1. What is the “Central Dogma of Molecular Biology?

DNA RNA Protein

2. How is this different in prokaryotes & eukaryotes?

Transcription Translation

TRANSLATION

TRANSCRIPTION

TRANSCRIPTION

RNA PROCESSING

TRANSLATION

DNA

mRNA

mRNA

DNA

Pre-mRNA

Ribosome

Polypeptide

Polypeptide

Ribosome

Nuclearenvelope

Prokaryotic cell. In a cell lacking a nucleus, mRNAproduced by transcription is immediately translatedwithout additional processing.

(a)

Eukaryotic cell. The nucleus provides a separatecompartment for transcription. The original RNAtranscript, called pre-mRNA, is processed in various ways before leaving the nucleus as mRNA.

(b)

Figure 17.3 Overview: the roles of transcription and translation in the flow of genetic information

Chapter 17: From Gene to Protein1. What is the “Central Dogma of Molecular Biology?2. How is this different in prokaryotes & eukaryotes?3. What are the stages of transcription?

- Initiation- Elongation- Termination


- Initiation- At promoter – promotes transcription- Transcription factors bind to TATA box- RNA polymerase makes RNA

TRANSCRIPTION

RNA PROCESSING

TRANSLATION

DNA

Pre-mRNA

mRNA

Ribosome

Polypeptide

Eukaryotic promoters

T AT AAA A

ATAT T T T

TATA box Start point TemplateDNA strand

5

3

3

5

Several transcriptionfactors

Additional transcriptionfactors

Transcriptionfactors

5

3

3

5

1

2

3

Promoter

5

3

3

55

RNA polymerase IITranscription factors

RNA transcript

Transcription initiation complex


- Initiation- Elongation

- Bases added to 3’ end- RNA polymerase – 60 nts/sec- opens DNA 10-20 bases at

a time

Elongation

RNApolymerase

Non-templatestrand of DNA

RNA nucleotides

3 end

C A U G C A AU

T A G G T TA

AC

G

U

AT

CA

T C C A A TT

GG

3

5

5

Newly madeRNA

Direction of transcription(“downstream) Template

strand of DNA

PromoterTranscription unit

RNA polymerase

Start point

53

35

35

53

53

35

53

35

5

5

Rewound

RNA

RNA

transcript

3

3Completed RNA transcript

Unwound

DNARNA

transcript

Template strand of DNA

1 Initiation. After RNA polymerase binds to the promoter, the DNA strands unwind, and the polymerase initiates RNA synthesis at the start point on the template strand.

3 Termination. Eventually, the RNAtranscript is released, and the polymerase detaches from the DNA.

2 Elongation. The polymerase moves downstream, unwinding theDNA and elongating the RNA transcript 5 3 . In the wake of transcription, the DNA strands re-form a double helix.

DNA



- AAUAAA – stop sequence- 10 – 35 bases later

Chapter 17: From Gene to Protein1. What is the “Central Dogma of Molecular Biology?2. How is this different in prokaryotes & eukaryotes?3. What are the stages of transcription?4. How is the mRNA altered (processed)?

- 5’ cap – modified Guanine added- 3’ poly-A tail

A modified guanine nucleotideadded to the 5 end

50 to 250 adenine nucleotidesadded to the 3 end

Protein-coding segment Polyadenylation signal

Poly-A tail3 UTRStop codonStart codon5 Cap 5 UTR

AAUAAA AAA…AAA

TRANSCRIPTION

RNA PROCESSING

DNA

Pre-mRNA

mRNA

TRANSLATIONRibosome

Polypeptide

G P P P5 3

Chapter 17: From Gene to Protein1. What is the “Central Dogma of Molecular Biology?2. How is this different in prokaryotes & eukaryotes?3. What are the stages of transcription?4. How is the mRNA altered (processed)?

- 5’ cap- 3’ poly-A tail- Splicing exons together & removing introns

TRANSCRIPTION

RNA PROCESSING

DNA

Pre-mRNA

mRNA

TRANSLATION

Ribosome

Polypeptide

5 CapExon Intron

1

5

30 31

Exon Intron

104 105 146

Exon 3Poly-A tail

Poly-A tail

Introns cut out andexons spliced together

Codingsegment

5 Cap1 146

3 UTR5 UTR

Pre-mRNA

mRNA

Chapter 17: From Gene to Protein1. What is the “Central Dogma of Molecular Biology?2. How is this different in prokaryotes & eukaryotes?3. What are the stages of transcription?4. How is the mRNA altered (processed)?5. How is the mRNA read to go from NA language to aa language?

- Triplet code – codon- 3 bases = 1 aa- Codons do not overlap- Reading frame

- The red dog ate the cat.- The red cat ate the dog.- There dc ata tet hed og.

- “The Rosetta Stone of Molecular Biology”

DNAmolecule

Gene 1

Gene 2

Gene 3

DNA strand(template)

TRANSCRIPTION

mRNA

Protein

TRANSLATION

Amino acid

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

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

Trp Phe Gly Ser

Codon

3 5

35

Figure 17.5 The dictionary of the genetic code

Second mRNA baseU C A G

U

C

A

G

UUU

UUCUUA

UUG

CUU

CUC

CUA

CUG

AUU

AUC

AUA

AUG

GUU

GUC

GUA

GUG

Met orstart

Phe

Leu

Leu

lle

Val

UCU

UCCUCA

UCG

CCU

CCC

CCA

CCG

ACU

ACC

ACA

ACG

GCU

GCC

GCA

GCG

Ser

Pro

Thr

Ala

UAU

UAC

UGU

UGCTyr Cys

CAU

CAC

CAA

CAG

CGU

CGC

CGA

CGG

AAU

AAC

AAA

AAG

AGU

AGCAGA

AGG

GAU

GAC

GAA

GAG

GGU

GGC

GGA

GGG

UGG

UAA

UAG Stop

Stop UGA Stop

Trp

His

Gln

Asn

Lys

Asp

Arg

Ser

Arg

Gly

U

CA

G

UCAG

UCAG

UCAG

Firs

t mR

NA

base

(5 e

nd)

Third

mR

NA

base

(3 e

nd)

Glu

Let’s translate:- UGG- ACC- GAA

- There is redundancy- Only the 1st 2 bases matter

- AUG – start - UAA, UAG, UGA – stop

- The overview of translation

- Trp- Thr- Glu

Chapter 17: From Gene to Protein1. What is the “Central Dogma of Molecular Biology?2. How is this different in prokaryotes & eukaryotes?3. What are the stages of transcription?4. How is the mRNA altered (processed)?5. How is the mRNA read to go from NA language to aa language?6. Where does translation take place?

TRANSCRIPTION

TRANSLATION

DNA

mRNARibosome

Polypeptide

PolypeptideAminoacids

tRNA withamino acidattachedRibosome

tRNA

Anticodon

mRNA

Trp

Phe Gly

A GC

A A A

C C G

U G G U U U G G C

Codons5 3

Chapter 17: From Gene to Protein1. What is the “Central Dogma of Molecular Biology?2. How is this different in prokaryotes & eukaryotes?3. What are the stages of transcription?4. How is the mRNA altered (processed)?5. How is the mRNA read to go from NA language to aa language?6. Where does translation take place?7. What does tRNA do?

- Transfers amino acids to protein being made3ACCACGCUUAAGACACCU

GC * G U GU

CUGAGG

U

AA A G

UCA

GACC

C G A GA G GG

GACUCAUUUAGGCG5

Amino acidattachment site

Hydrogenbonds

Anticodon

*

**

**

***

* **

Amino acidattachment site

Hydrogen bonds

Anticodon Anticodon

A A G

53

3 5

Wobble – 3rd base doesn’t matter

P site (Peptidyl-tRNAbinding site)

E site (Exit site)

mRNAbinding site

A site (Aminoacyl-tRNA binding site)

Largesubunit

Smallsubunit

E P A

Schematic model showing binding sites. A ribosome has an mRNA binding site and three tRNA binding sites, known as the A, P, and E sites. This schematic ribosome will appear in later diagrams.

(b)

Ribosomes are made of rRNA & proteins in the nucleolus (Ch 6)

Chapter 17: From Gene to Protein1. What is the “Central Dogma of Molecular Biology?2. How is this different in prokaryotes & eukaryotes?3. What are the stages of transcription?4. How is the mRNA altered (processed)?5. How is the mRNA read to go from NA language to aa language?6. Where does translation take place?7. What does tRNA do?8. What are the steps of translation?


Figure 17.17 The initiation of translation

2

Initiator tRNA

mRNA

mRNA binding site Smallribosomalsubunit

Largeribosomalsubunit

Translation initiation complex

P site

GDPGTP

Start codon

1

Met MetU A CA U G

E A

3

5

5

3

35 35

Small subunitmRNAInitiator tRNA with Met

Energy from GTP allows large subunit to bind

Figure 17.18 The elongation cycle of translation

Amino endof polypeptide

mRNA

Codon recognition. The anticodon of an incoming aminoacyl tRNA base-pairs with the complementary mRNA codon in the A site. Hydrolysisof GTP increases the accuracy andefficiency of this step.

Ribosome ready fornext aminoacyl tRNA

Translocation. The ribosome translocates the tRNA in the A site to the P site. The empty tRNA in the P site is moved to the E site, where it is released. The mRNA moves along with its bound tRNAs,bringing the next codon to be translated into the A site.

Peptide bond formation. An rRNA molecule of the large Subunit catalyzes the formation of a peptide bond between the new amino acid in the A site and the carboxyl end of the growing polypeptide in the P site. This step attaches the polypeptide to the tRNA in the A site.

E

P A

E

P A

E

P A

E

P A

GDPGTP

GTP

GDP

2

2

site site5

3

3

2

1TRANSCRIPTION

TRANSLATION

DNA

mRNARibosome

Polypeptide

Figure 17.19 The termination of translation

Release factor

Freepolypeptide

Stop codon(UAG, UAA, or UGA)

When a ribosome reaches a stop codon on mRNA, the A site of the ribosome accepts a protein called a release factor instead of tRNA.

The release factor hydrolyzes the bond between the tRNA in the P site and the last amino acid of the polypeptide chain. The polypeptide is thus freed from the ribosome.

1 2 3

5

3

5

3

5

3

The two ribosomal subunits and the other components of the assembly dissociate.

Chapter 17: From Gene to Protein1. What is the “Central Dogma of Molecular Biology?2. How is this different in prokaryotes & eukaryotes?3. What are the stages of transcription?4. How is the mRNA altered (processed)?5. How is the mRNA read to go from NA language to aa language?6. Where does translation take place?7. What does tRNA do?8. What are the steps of translation?9. What are polyribosomes?

Figure 17.20 Polyribosomes

Growingpolypeptides

Completedpolypeptide

Incomingribosomalsubunits

Start of mRNA(5 end)

End of mRNA(3 end)

Polyribosome

An mRNA molecule is generally translated simultaneously by several ribosomes in clusters called polyribosomes.

(a)

Ribosomes

mRNA

This micrograph shows a large polyribosome in a prokaryotic cell (TEM).

0.1 µm(b)

Figure 17.22 Coupled transcription and translation in bacteria

DNA

Polyribosome

mRNA

Direction oftranscription

0.25 mRNApolymerase

Polyribosome

Ribosome

DNA

mRNA (5 end)

RNA polymerase

Polypeptide(amino end)

Upon further review……- Test – Tuesday November 29- Essay questions & Ch 16 & 17 ppts are available

Chapter 17: From Gene to Protein1. What is the “Central Dogma of Molecular Biology?2. How is this different in prokaryotes & eukaryotes?3. What are the stages of transcription?4. How is the mRNA altered (processed)?5. How is the mRNA read to go from NA language to aa language?6. Where does translation take place?7. What does tRNA do?8. What are the steps of translation?9. What are polyribosomes?10. How are proteins targeted to the ER?

- SRP - signal recognition particle- Binds to signal peptide (1st couple amino acids)

Figure 17.21 The signal mechanism for targeting proteins to the ER

The rest ofthe completedpolypeptide leaves the ribosome andfolds into its finalconformation.

Polypeptidesynthesis beginson a freeribosome inthe cytosol.

An SRP binds to the signal peptide, halting synthesismomentarily.

The SRP binds to areceptor protein in the ERmembrane. This receptoris part of a protein complex(a translocation complex)that has a membrane poreand a signal-cleaving enzyme.

The SRP leaves, andthe polypeptide resumesgrowing, meanwhiletranslocating across themembrane. (The signalpeptide stays attachedto the membrane.)

The signal-cleaving enzymecuts off thesignal peptide.

1 2 3 4 5 6

Ribosome

mRNASignalpeptide

Signal-recognitionparticle(SRP) SRP

receptorprotein

Translocationcomplex

CYTOSOL

Signalpeptideremoved

ERmembrane

Protein

Chapter 17: From Gene to Protein1. What is the “Central Dogma of Molecular Biology?2. How is this different in prokaryotes & eukaryotes?3. What are the stages of transcription?4. How is the mRNA altered (processed)?5. How is the mRNA read to go from NA language to aa language?6. Where does translation take place?7. What does tRNA do?8. What are the steps of translation?9. What are polyribosomes?10. How are proteins targeted to the ER?11. Let’s consider mutations……

- Sickle cell disease

Figure 17.23 The molecular basis of sickle-cell disease: a point mutation

In the DNA, themutant templatestrand has an A where the wild-type template has a T.

The mutant mRNA has a U instead of an A in one codon.

The mutant (sickle-cell) hemoglobin has a valine (Val) instead of a glutamic acid (Glu).

Mutant hemoglobin DNAWild-type hemoglobin DNA

mRNA mRNA

Normal hemoglobin Sickle-cell hemoglobin

Glu Val

C T T C A T

G A A G U A

3 5 3 5

5 35 3

Chapter 17: From Gene to Protein1. What is the “Central Dogma of Molecular Biology?2. How is this different in prokaryotes & eukaryotes?3. What are the stages of transcription?4. How is the mRNA altered (processed)?5. How is the mRNA read to go from NA language to aa language?6. Where does translation take place?7. What does tRNA do?8. What are the steps of translation?9. What are polyribosomes?10. How are proteins targeted to the ER?11. Let’s consider mutations……12. How do mutations alter the genotype & phenotype of organisms?

- Mutation – any change in the genetic material of a cell (heritable)- Point mutation – a change in 1 or only a few bases

- Base-pair substitution- Insertions or deletions result in a frameshift

Figure 17.24 Base-pair substitution

Wild type

A U G A A G U U U G G C U A AmRNA 5Protein Met Lys Phe Gly Stop

Carboxyl endAmino end

3

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

Met Lys Phe Gly

Base-pair substitutionNo effect on amino acid sequence

U instead of C

Stop

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

Met Lys Phe Ser Stop

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

Met Stop

Missense A instead of G

NonsenseU instead of A

Figure 17.25 Base-pair insertion or deletion

mRNAProtein

Wild type

A U G A A G U U U G G C U A A5

Met Lys Phe Gly

Amino end Carboxyl end

Stop

Base-pair insertion or deletionFrameshift causing immediate nonsense

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

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

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

Met Stop

U

Met Lys Leu Ala

Met Phe GlyStop

MissingA A G

Missing

Extra U

Frameshift causing extensive missense

Insertion or deletion of 3 nucleotides:no frameshift but extra or missing amino acid

3

Figure 17.26 A summary of transcription and translation in a eukaryotic cell

TRANSCRIPTION RNA is transcribedfrom a DNA template.

DNA

RNApolymerase

RNAtranscript

RNA PROCESSING In eukaryotes, theRNA transcript (pre-mRNA) is spliced andmodified to producemRNA, which movesfrom the nucleus to thecytoplasm.

Exon

Poly-A

RNA transcript(pre-mRNA)

Intron

NUCLEUSCap

FORMATION OFINITIATION COMPLEX

After leaving thenucleus, mRNA attachesto the ribosome.

CYTOPLASM

mRNA

Poly-A

Growingpolypeptide

Ribosomalsubunits

Cap

Aminoacyl-tRNAsynthetase

AminoacidtRNA

AMINO ACID ACTIVATION

Each amino acidattaches to its proper tRNAwith the help of a specificenzyme and ATP.

Activatedamino acid

TRANSLATION A succession of tRNAsadd their amino acids tothe polypeptide chainas the mRNA is movedthrough the ribosomeone codon at a time.(When completed, thepolypeptide is releasedfrom the ribosome.)

Anticodon

A CC

A A AUG GUU UA U G

UACE A

Ribosome

1

Poly-A

5

5

3

Codon

2

3 4

5

Chapter 17: From Gene to Protein

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