91335 Lecture Notes - Lecture 2: Operon, Alternative Splicing, Myc
MB2 Lecture 2
26/3/18
Transcription and regulation of gene expression
Differential gene expression
Differences between cells are due to expression
Cells differentiation to perform different functions
Regulatory mechanisms turn specific genes on or off
Needs to be dynamic to maintain homeostasis in changing external environments
Various mechanisms
transcription: synthesis of RNA from DNA
translation: synthesis of polypeptide from mRNA
transcription
gene: entire sequence necessary for synthesis of functional product
gene= transcript unit
prokaryotes do not have introns and are much simpler
+1 = nucleotide that transcription starts from
oteplate= odig 5’-3’
teplate= oodig 3’-5’
RNA is same sequence as coding or nontemplate strand
Prokaryotes: transcription and translation are coupled and all happen in same compartment
Eukaryotes: not coupled and carried out in nucleus and cytoplasm
Stages
• Initiation
• Elongation
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• Termination
RNA polymerase
Eukaryotes have 3
- I= rRNA
- II= mRNA
- III= tRNA
Promotor
Sequence for transcription to start upstream of coding sequence
AT rich sequence
TATA box: energy efficient to break 2 H bonds
Factors
Bind to sequences (Specific proteins)
Each Transcription factor (TF) has a consensus sequence to which it will bind
Basal TF- are essential for initiation
Initiation
TF bind promotor first the recruit RNA polymerase
Transcription initiation complex= TF + RNA polymerase
control elements
core promotor: RNA polymerase II binding region
cis-acting DNA element: acts as binding site for protein, bound protein only acts upon the
same DNA molecule to which it is bound
Trans-acting DNA element: sequence controls gene at a separate location, sequence that
codes for a protein (trans-acting factor) that controls expression of a gene at a separate
location
Enhancers
Turn genes on
Up and down stream of promotor
Silencer
DNA sequence
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Turns gene off
Downstream of the promotor
Reduces transcription rate
Prokaryotes vs eukaryotes
Both require regulatory proteins
Pro-activator and repressor act on operators (downstream of promotor)
Eukaryotes: activator proteins act on enhancer sequences;
repressor proteins act on silencer sequences
– Enhancers can be found both down and upstream of promoter, in introns etc.
– Position of enhancer has profound effect on gene regulation
tryptophan operon
expressed when tryptophan levels are low
regulated by repressor protein
Myc transcription factor
- Binds to DNA sequence E box
- Enhancer
Activates genes involved in proliferation
Oncogene
Controlled by Myc-Mad-Max
Different combinations of these can cause transcriptional activation or repression
Proliferation or differentiation downstream
Elongation
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