Transcription

Recall:

• What is the Central Dogma of molecular genetics?

LE 17-7

Elongation Non-templatestrand of DNA

RNApolymerase

RNA nucleotides

3 end

3

5

5

Newly madeRNA

Templatestrand of DNA

Direction of transcription(“downstream”)

Promoter

53

RNA polymerase

Start pointDNA

Transcription unit

35

53

35

UnwoundDNA

RNAtran-script

Template strandof DNA

Initiation

Elongation

Termination

53

35

53

35

35

RewoundDNA

RNAtranscript

5

Completed RNA transcript

Transcription •is catalyzed by RNA polymerase•DNA strands are separated•RNA nucleotides are linked by RNA polymerase•mRNA grows in the 5’ 3’ direction•Only 1 DNA strand serves as template•U pairs with A

LE 17-7a-2

Promoter

53

35

35

53

Transcription unit

DNA

Initiation

Start pointRNA polymerase

UnwoundDNA

RNAtran-script

Template strandof DNA

Promoter•approx. 100 nucleotides long upstream from gene•have a lot of A’s and T’s in them—RNA polymerase recognizes these sequences

•DNA is unwound and separates into 2 strands•RNA polymerases bind to DNA at promoter region

LE 17-7a-1

Promoter Transcription unit

RNA polymeraseStart point

DNA

53

35

• nucleotide sequence on the template strand of DNA that is transcribed into a single RNA molecule by RNA polymerase

• includes initiation and termination sequences

LE 17-8

Promoter

53

35

TATA box Start point

Transcriptionfactors

53

35

Several transcriptionfactors

Additional transcriptionfactors

RNA polymerase IITranscription factors

RNA transcript

53

355

Transcription initiation complex

Eukaryotic promoters

TemplateDNA strand

ranscription factor: DNA binding proteins, bound specific DNA nucleotide sequence that help RNA polymerase recognize and bind the promoter

box •short nucleotide sequence rich in Ts and As•~ 25 nucleotides upstream from the initiation site

RNA polymerase II•Recognizes complex between TATA transcription factor and the DNA binding site•When RNA polymerase binds to promoter, the enzyme separates the two DNA strands

LE 17-7a-3

Promoter

53

Transcription unit

35DNA

Start pointRNA polymerase

Initiation

35

53

UnwoundDNA

RNAtran-script

Template strandof DNA

Elongation

RewoundDNA

35

53 3

5

RNAtranscript

-mRNA is synthesized using one strand of the DNA-Template strand is 3’ -5’ on the DNA strand

-Therefore mRNA is made in the direction of 5’ 3’-RNA sequence is complementary to the template strand and identical to the coding strand

LE 17-7a-4Promoter

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

-End of gene is reached when RNA polymerase reaches the terminator sequence--common sequence in eukaryotes is AAUAAA

-mRNA comes off the DNA strand (disassociates)

LE 17-7b

ElongationNon-templatestrand of DNA

RNApolymerase

RNA nucleotides

3 end3

5

5

Newly madeRNA

Templatestrand of DNA

Direction of transcription(“downstream”)

LE 17-9

5Protein-coding segment

5 Start codon Stop codon Poly-A tail

Polyadenylation signal

5 3Cap UTR UTR

RNA processing—Capping and Tailing

(pre-mRNA – transcript that will be processed to functional

mRNA)

Both the 5’ and 3’ ends are covalently modified

5’ cap - GTP is added – for protection

- easy recognition for small ribosomal subunit

- cap prevents mRNA from digestion

3’ end –poly-A polymerase adds a poly-A tail (30-200

nucleotides)

- may inhibit degradation of mRNA in cytoplasm

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

Split Genes and RNA splicing• Introns – noncoding regions (initially transcribed but not translated)

•Excised from transcript before mRNA leaves the nucleus

• Exons – coding regions--linked together by RNA splicing

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

•Small nuclear ribonucleoproteins (snRNPs) (complex of proteins and small nuclear RNAs found only in the nucleus) play role in RNA splicing

•Spliceosome – large molecular complex that catalyzes RNA splicing reactions

LE 17-12

Gene

Transcription

RNA processing

Translation

Domain 2

Domain 3

Domain 1

Polypeptide

Exon 1 Intron Exon 2 Intron Exon 3

DNA

Function/Importance of Introns• may control gene activity• may help regulate the export of mRNA• may allow a single gene to direct the synthesis of different proteins (if the same RNA transcript is processed differently i.e. One transcript remove all introns, in another leave introns and translate them

LE 17-26

TRANSCRIPTION

RNA PROCESSING

RNAtranscript

5

Exon

NUCLEUS

FORMATION OFINITIATION COMPLEX

CYTOPLASM

3

DNA

RNApolymerase

RNA transcript(pre-mRNA)

Intron

Aminoacyl-tRNAsynthetase

Aminoacid

tRNA

AMINO ACID ACTIVATION

3

mRNA

A

P

E Ribosomalsubunits

5

Growingpolypeptide

E A

Activatedamino acid

Anticodon

TRANSLATION

Codon

Ribosome