Unit #3 Schedule:• Last Class:

– Sanger Sequencing– Central Dogma Overview– Mutation

• Today:– Homework 5– StudyNotes 8a Due– Transcription, RNA Processing, Translation

• Central Dogma Sculpting (Apr 2)

– StudyNotes 8b Due• Regulation of Gene Expression + Trivia (Apr 4)

– StudyNotes 9 Due• Tutorial (Apr 5)

• Review (Apr 9)

– Homework 6 Due• EXAM 3 (Apr 11)

Unpacking Central Dogma: Transcription

Learning Objectives:

By the end of today’s class you should be able to:1. Describe transcription.2. Compare and contrast eukaryotic +

prokaryotic transcription.3. Explain mRNA processing in eukaryotes.4. Describe the general structure of a tRNA

molecule.5. Discuss the three stages of translation.

Fig. 17-3a-2

(a) Bacterial cell

TRANSCRIPTIONDNA

mRNA

TRANSLATIONRibosome

Polypeptide

Fig. 17-3b-3

(b) Eukaryotic cell

TRANSCRIPTION

Nuclearenvelope

DNA

Pre-mRNARNA PROCESSING

mRNA

TRANSLATION Ribosome

Polypeptide

Synthesis of an RNA Transcript

• The three stages of transcription:– Initiation– Elongation– Termination

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Fig. 17-7a-4Promoter Transcription unit

DNAStart pointRNA polymerase

553

3

Initiation

33

1

RNAtranscript

5 5

UnwoundDNA

Template strandof DNA

2 Elongation

RewoundDNA

5

5 5 3 3 3

RNAtranscript

3 Termination

5

5 5 33

3Completed RNA transcript

RNA Polymerase and Promoter

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© 2011 Pearson Education, Inc.

Prokaryotes vs. Eukaryotes

Fig. 17-9

Transcription Termination in Eukaryotes:

• Transcribes the sequence AAUAAA• Signals for the RNA transcript to be cleaved.

Transcription Termination Sequences

• In Prokaryotes, transcription stops by way of as transcribed terminator. This is part of the transcription sequence that signals RNA polymerase to detach. (Hairpin theory)

• In Eukaryotes, transcription stops when a polyadenylation signal AAUAAA occurs. Proteins then cleave the RNA transcript off.

Transcript Processing

Fig. 17-3b-3

(b) Eukaryotic cell

TRANSCRIPTION

Nuclearenvelope

DNA

Pre-mRNARNA PROCESSING

mRNA

TRANSLATION Ribosome

Polypeptide

Fig. 17-9

Protein-coding segment Polyadenylation signal3

3 UTR5 UTR

5

5 Cap Start codon Stop codon Poly-A tail

G P PP AAUAAA AAA AAA…

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Fig. 17-10

Pre-mRNA

mRNA

Codingsegment

Introns cut out andexons spliced together

5 Cap

Exon Intron5

1 30 31 104

Exon Intron

105

Exon

146

3Poly-A tail

Poly-A tail5 Cap

5 UTR 3 UTR1 146

Fig. 17-11-1RNA transcript (pre-mRNA)

Exon 1 Exon 2Intron

ProteinsnRNA

snRNPs

Otherproteins

5

Fig. 17-11-2RNA transcript (pre-mRNA)

Exon 1 Exon 2Intron

ProteinsnRNA

snRNPs

Otherproteins

5

5

Spliceosome

Fig. 17-11-3RNA transcript (pre-mRNA)

Exon 1 Exon 2Intron

ProteinsnRNA

snRNPs

Otherproteins

5

5

Spliceosome

Spliceosomecomponents

Cut-outintronmRNA

Exon 1 Exon 25

Minute Paper:

• We have ~20,000 genes yet we can make ~100,000 different protein products. How is this possible if DNARNAProtein?

Fig. 17-3b-3

(b) Eukaryotic cell

TRANSCRIPTION

Nuclearenvelope

DNA

Pre-mRNARNA PROCESSING

mRNA

TRANSLATION Ribosome

Polypeptide

Alternative Splicing

• Some genes can encode more than one kind of polypeptide, depending on which segments are treated as exons during RNA splicing.

• Such variations are called alternative RNA splicing.

• Because of alternative splicing, the number of different proteins an organism can produce is much greater than its number of genes.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Translation

Fig. 17-3b-3

(b) Eukaryotic cell

TRANSCRIPTION

Nuclearenvelope

DNA

Pre-mRNARNA PROCESSING

mRNA

TRANSLATION Ribosome

Polypeptide

Fig. 17-3a-2

(a) Bacterial cell

TRANSCRIPTIONDNA

mRNA

TRANSLATIONRibosome

Polypeptide

© 2011 Pearson Education, Inc.

© 2011 Pearson Education, Inc.

© 2011 Pearson Education, Inc.

Translation

Transfer RNA (tRNA) bridges RNA language and amino acid language

Transfer RNA (tRNA)

• Single-stranded RNAs of about 74 - 95 nucleotides– On one “end”, they hold

an amino acid– On the other “end”, they

bind to mRNA• There are about 45

different tRNAs in eukaryotic cells

tRNA Structure

Campbell 8e, Fig. 17.14

“Charging” Transfer RNA

• Aminoacyl tRNA Synthetase

• Specific for particular pairs of amino acids and tRNA

• Requires ATP

Campbell 8e, Fig. 17.15

© 2011 Pearson Education, Inc.

Ribosomes and the Mechanism of Translation

Ribosomes and Translation

Three Stages of Translation

InitiationElongation

Termination

© 2011 Pearson Education, Inc.

© 2011 Pearson Education, Inc.

Elongation

© 2011 Pearson Education, Inc.

Termination

Campbell 8e, Fig. 17.21

Signal Peptides

Coming Up:

Tuesday:- Molecular Sculpting:

- Translation, transcript processing, translation

- StudyNotes 8b is due.