Genetics Chapter 17 Concept Notes
Gene Expression Control in Eukaryotes
Introduction to RNA Polymerase II Promoter Elements
Different DNA elements that might be seen on a gene:
- Exons
- Introns
- +1 Transcription start site
- AUG translation start codon
- Between +1 and AUG: 5' UTR
- Translation Stop and Poly A signal sequence
- Between Stop codon and Poly A: 3' UTR
Key Elements
- Upstream of +1 txn start site: core promoter
- -25 sequence and TATA box
- Proximal promoter
- Represents conserved DNA sequences that bind specific txn factors
- CCAAT box
- GC Box
- Enhancers and Silencers
- Can be located upstream of proximal promoter, or downstream of gene
- Can be found in introns
- Enhancers and silencers within one gene can affect the next gene downstream
- Insulator:
- Likely between genes
- Can block the activity of enhancers and silencers
- They function via DNA methylation and particular proteins can bind or fail to
bind to that particular methylated DNA sequence
- CPG Islands
- Regions of DNA that are methylated at the cytosine whenever we see a cytosine,
guanine location
- Found upstream of +1 and into the actual structural gene into exon 1
- Increase methylation, decrease expression
Eukaryotic Cis Regulatory Elements
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Initiator region (InR) flanks txn start site
TATA Box (-25)
Transcription initiation at the promoter requires prieinitiation complex (PIC)
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Effect of promoter and proximal promoter mutations on transcription
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Generic Proximal Promoter Elements
- Control proper level of txn and are located within first 200 nucleotides upstream
core promoter
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Bound by proteins that promote or repress transcription
Consensus Sequences
- CCAAT Box (Bound by CTF Protein)
- GC Box (Bound by SP1 Protein)
- When bound by proteins may facilitate binding of TFIID at the promoter
Proteins Bind Proximal Promoter Elements
Cell and Tissue-Specific Protein Activation
- Combinatorial Model of Gene Expression
- Cell or tissue-specific proximal promoter elements
- transcription factors can be expressed in different patterns (both
temporally and spatially), which will generate different patterns of gene
expression
Proximal Promoter Elements
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Enhancers
- DNA sequences that bind proteins to control transcription of a gene in a specific
cell or tissue
- Proteins that bind enhancers
- Activators, repressors
Difference between enhancers and promoters
Architectural Protein Binds Enhancers
- Binding of a protein to the enhancer can cause a conformational/architectural
change in the DNA
- This change in the conformation of the DNA can affect the ability of other
proteins to bind the DNA or the relative location of different DNA sequences that
linearly are separated by a long distance that brought close together
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How enhancers affect txn
Silencers
- Repress transcription for either a single gene or globally
- Single gene silencing
- Proteins bind silencers in front of a given gene
- Changes chromatin structure
- Global silencing
- Restrictive element sequence found in front of many genes
- Modifies histone proteins
- Alters chromatin structure
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Insulators
- Shield effects of enhancers or silencers from neighboring genes