(CHAPTER 2- Brooker Text) Linked Genes & Probability Nov 13 & 18, 2008 BIO 184 Dr. Tom Peavy.
(CHAPTER 12- Brooker Text) Transcription Sept 18 & 23, 2008 BIO 184 Dr. Tom Peavy.
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Transcript of (CHAPTER 12- Brooker Text) Transcription Sept 18 & 23, 2008 BIO 184 Dr. Tom Peavy.
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(CHAPTER 12- Brooker Text)
Transcription
Sept 18 & 23, 2008BIO 184
Dr. Tom Peavy
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![Page 3: (CHAPTER 12- Brooker Text) Transcription Sept 18 & 23, 2008 BIO 184 Dr. Tom Peavy.](https://reader036.fdocuments.net/reader036/viewer/2022062300/56649d7f5503460f94a6319d/html5/thumbnails/3.jpg)
• Bacterial mRNA may be polycistronic, which means it encodes two or more polypeptides
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• Transcription occurs in three stages– Initiation– Elongation– Termination
• These steps involve protein-DNA interactions – Proteins such as RNA polymerase interact with DNA
sequences
The Stages of Transcription
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• promoter = recognition site for transcription factors
• transcription factors + RNA polymerase to bind to the promoter
(initial phase) = closed promoter complex
• then DNA is denatured (bubble) = open promoter complex
Initiation
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• E. coli RNA polymerase = holoenzyme – Core enzyme (Four subunits = 2’)
– Sigma factor (One subunit =
Initiation stages involve RNA pol holoenzyme• Binding loosely to the DNA• Scaning for promoter region• Forming Open promoter complex• Synthesizing Short stretch of RNA• Releasing Sigma factor
Prokaryotic Transcription
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Sequence elements that play a key role in
transcription
(pribnow box)
Bacterial Promoter
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• The RNA transcript is synthesized during ELONGATION step
• The DNA strand used as a template for RNA synthesis is termed the template or noncoding strand
• The opposite DNA strand is called the coding strand– It has the same base sequence as the RNA transcript
• Except that T in DNA corresponds to U in RNA
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• short RNA-DNA hybrid is forced to separate
= release of newly made RNA
• E. coli has two different mechanisms for termination– 1. rho-dependent termination
• Requires a protein known as (rho)
– 2. rho-independent termination• Does not require but uses stem loop structure and U–rich
sequence region
Termination of Bacterial Transcription
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rho utilization site
-dependent terminationFigure 12.8
Rho protein is a helicase
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-dependent terminationFigure 12.8
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Types of RNA
mRNA = messenger RNA encodes for the sequence of amino acids within a polypeptide
tRNA = transfer RNA carries amino acids at the amino acyl terminus for incorporation into growing polpeptides during translation at ribosome
rRNA = ribosomal RNA is a major component of the ribosomal complex essential for coordinating protein sythesis using the mRNA as a template (translation)
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• Similar but more complex– Larger organisms– Cellular complexity– Multicellularity
TRANSCRIPTION IN EUKARYOTES
• Three Different RNA polymerase needed
1) RNA pol I = rRNA genes (except 5S)
2) RNA pol II = mRNAs (structural genes)
3) RNA pol III = tRNAs and 5S rRNA
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• The core promoter is relatively short = TATA box
- determines start point for transcription
- basal transcription by itself (low level)
• Regulatory Elements (e.g. GC and CAAT boxes)
- Enhancers (stimulate transcription)
- Silencers (inhibit transcription)
* often found nearby (-50 to -100) but can also be
found great distances away in either direction
Figure 12.11
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• cis-acting elements• DNA sequences that exert their effect only on nearby
genes• Example: TATA box, enhancers and silencers
Factors that Control Gene Expression
• trans-acting elements• Regulatory proteins that bind to such DNA sequences
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• In Bacteria:– DNA coding strand = mRNA sequence directly– Corresponds to amino acid sequence in the polypeptide
= colinearity of gene expression
• In Eukaryotes, RNA is modified:
- coding sequences, called exons, are interrupted by
intervening sequences or introns (which are removed)
= RNA splicing
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Figure 12.16
• In eukaryotes, the transcription of structural genes, produces a long transcript known as pre-mRNA– Also as heterogeneous
nuclear RNA (hnRNA)
• RNA is altered by splicing and other modifications, before it leaves the nucleus
• Spliceosome required for splicing = multicomponent structure
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• Most mature mRNAs have a 7-methyl guanosine covalently attached at their 5’ end = capping
• Occurs as pre-mRNA is being synthesized (RNA pol II)
• Cap structure is recognized by cap-binding proteins
Role of Cap-binding proteins– Movement of some RNAs into the cytoplasm– Early stages of translation– Splicing of introns
Capping
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• Most mature mRNAs have a string of adenine nucleotides at their 3’ ends = polyA tail
• polyA tail is not encoded in the gene sequence– added enzymatically after the gene is completely
transcribed
Addition of poly A tail
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Figure 12.20
Consensus sequence in higher eukaryotes
Appears to be important in the stability of mRNA and the
translation of the polypeptide
Length varies between species
From a few dozen adenines to several hundred
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Alternative splicing:
• pre-mRNA with multiple introns can be spliced in
different ways
• Generates mature mRNAs with different
combinations of exons = different polypeptides
• Organism can carry fewer genes within genome
• Variation in splicing can occur in different cell types
or during different stages of development
Intron Advantage?