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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