Ch 17 Gene Expression I: Transcription

What is this?

Basic Principles of Gene Expression

DNA encodes hereditary information (genotype) ->

decoded into RNA -> protein (phenotype)

DNA

RNA

Protein

Transcription

Translation

LE 17-3-1

TRANSCRIPTIONDNA

Prokaryotic cell

LE 17-3-1

TRANSCRIPTIONDNA

Prokaryotic cell

LE 17-3-2

TRANSCRIPTIONDNA

Prokaryotic cellRibosome

Polypeptide

mRNA

Prokaryotic cell

LE 17-3-3

TRANSCRIPTION

TRANSLATION

DNA

mRNA

Ribosome

Polypeptide

DNA

Prokaryotic cell

Nuclearenvelope

TRANSCRIPTION

Eukaryotic cell

LE 17-3-4

TRANSCRIPTION

TRANSLATION

DNA

mRNA

Ribosome

Polypeptide

DNA

Pre-mRNA

Prokaryotic cell

Nuclearenvelope

mRNA

TRANSCRIPTION

RNA PROCESSING

Eukaryotic cell

LE 17-3-5

TRANSCRIPTION

TRANSLATION

DNA

mRNA

Ribosome

Polypeptide

DNA

Pre-mRNA

Prokaryotic cell

Nuclearenvelope

mRNA

TRANSLATION

TRANSCRIPTION

RNA PROCESSING

Ribosome

Polypeptide

Eukaryotic cell

• Promoter: DNA sequence where RNA polymerase binds to transcribe the gene

• Transcription start site: the nucleotide where RNA pol initiates transcription

• Transcription unit: the transcribed DNA

Transcription: DNA->RNA

Structure of a gene

Basic components for transcription

dsDNA with a promoter

RNA polymerase

rNTPs (ribonucleotides triphosphates)ATP, CTP, GTP, UTP

LE 17-7

Promoter Transcription unit

RNA polymeraseStart point

DNA

53

35

LE 17-7

ElongationNon-templatestrand of DNA

RNApolymerase

RNA nucleotides

3 end3

5

5

Newly madeRNA

Templatestrand of DNA

Direction of transcription(“downstream”)

Synthesis of an RNA Transcript

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

LE 17-7Promoter

35

Transcription unit

DNA

InitiationRNA polymerase

Start point

Template strandof DNA

RNAtran-script

UnwoundDNA

Elongation

3

3

53

5

5

3 5

RewoundDNA

5 3

35 35

RNAtranscript Termination

35

5 3Completed RNA transcript

Termination of Transcription

Different in prokaryotes and eukaryotes• In prokaryotes• RNA pol stops transcription at the end of the

terminator (DNA sequence)

• In eukaryotes• pre-mRNA is cleaved from the growing RNA chain• RNA pol eventually falls off the DNA

RNA processing in eukaryotes, not prokaryotes

1. Addition of methylated cap to 5’ end of messenger RNA (mRNA)-> increases stability and translation of mRNA

2. Addition of poly(A) tail to 3’ end (polyadenylation) -> increases stability and translation of mRNA

3. Splicingremoval of introns and joining together of exons

All processing events occur in nucleus

before transport to cytoplasm

Draw

LE 17-10

5 Exon Intron Exon Intron Exon 3Pre-mRNA

1 30 31 104 105 146

Codingsegment

Introns cut out andexons spliced together

1 146

5Cap

5Cap

Poly-A tail

Poly-A tail

5 3UTR UTR

(mature) mRNA

• RNA splicing:

carried out by spliceosomes

• Spliceosomes complex of proteins and several small nuclear

ribonucleoproteins (snRNPs)

Recognize splice sites (specific RNA sequences)

cleave out introns and splice together exons (coding region)

LE 17-11

Exon 15

Intron Exon 2

Other proteinsProtein

snRNA

snRNPs

RNA transcript (pre-mRNA)

Spliceosome

5

Spliceosomecomponents

Cut-outintron

mRNA

Exon 1 Exon 25

Ribozymes

• Catalytic RNAs molecules that function as enzymes; involved in splicing

• Non-protein biological catalyst

Can you think of a ribozyme with a different function?

Telomerase

Functional and Evolutionary Importance of Introns

• Some genes can encode more than one kind of polypeptide

-different combinations of exons can be spliced together

• Called alternative RNA splicing

• Increases the potential number of different proteins (and thus functions) in an organism

• Increased adaptive potentialDraw Splice Variants

• In many cases, different exons code for the different domains in a protein

• Protein domains– Distinct conformational regions often with discrete

functions

Exons and protein domains

LE 17-12

Gene

Transcription

RNA processing

Translation

Domain 2

Domain 3

Domain 1

Polypeptide

Exon 1 Intron Exon 2 Intron Exon 3

DNA

LE 17-9

5Protein-coding segment

5 Start codon Stop codon Poly-A tail

Polyadenylation signal

5 3Cap UTR UTR

Architecture of eukaryotic mRNA

UTR: untranslated regions