Prokaryotic Gene structure an Function, Regulatory Regions Flashcards
What’s the structure of bacteria ?
Bacteria are generally haploid, with only of each gene on a single circular chromosome.
What does the bacterium need to carry out to allow different functions ?
The bacterium needs to regulate gene expression
- Switches on sets of genes that are required at different times
- Switches on sets of genes that are required at different times
- Switches off other sets of genes that are not needed.
What are the 3 essential features that genes in bacteria have ?
- A PROMOTOR -
- A TRANSCRIBED REGION
3 A TERMINATOR SEQUENCE
What is a promoter ?
- All the sequences in the DNA required for expression and regulation of a gene
- ## These sequences are NOT all included in the mRNA
What is a transcribed region ?
- Can include more than one ORF
- These sequences ARE included in the mRNA
What is a terminator sequence ?
- Defines end of gene or operon
- Often a stem loop structure
What is the structure of mRNA in a bacterial gene ( translated region , terminator sequence ) ?
- A 5’ UTR
- 5’ untranslated region lying before the AUG start codon
- A TRANSLATED REGION
- each ORF starts with the start codon and is preceded by a ribosome binding site
- A TERMINATOR SEQUENCE
- Stem loop structure
- Give mRNA stability
What is mRNA synthesised by, how does the synthesis stops ?
-mRNA is synthesised by RNA polymerase
- Binds to promoter motif and initiates RNA synthesis
-mRNA synthesis ends at terminator site
How many protein subunits does bacteria RNA polymerase holoenzyme consist of ?
- Córę enzyme ( five subunits )
- Sigma factor
What does the core enzyme plus sigma factor equal to ?
Holoenzyme
What does the sigma factor do ?
Binds to the -10 and -35 sequences in bacterial promoters and identifies the beginning of a gene
What does regulatory proteins control ?
Binding RNA pol to promoters
Explain what occurs when the regulatory protein (Lacl) binds to operator site ?
Prevents RNA pol transcribing full mRNA encoding structural genes
What occurs when binding of the inducer occurs ?
Changes the lack repressor shape - allows transcription of full mRNA encoding structural genes.
What does it lead to if RNA pol cannot bind to promoter without help of an activator ?
Leads to lots of varied mechanisms of gene control
How is transcription in bacteria started ?
- Closed complex formed as holoenzyme assembles on promoter.
- Open complex formed by activity of core enzymes.
- transcription initiated at +1 site.
Whats the difference between DNA polymerase and RNA polymerase when it comes to transcription and what does it reflect ?
unlike DNA polymerases, RNA polymerase can initiate transcription WITHOUT the need for a primer.
- Probably reflects the fact that RNA was the first nucleic acid molecule that evolved
What are the 2 types of transcription termination ?
1- INTRINSIC TERMINATION
2- RHO-DEPENDENT TERMINATION
What is intrinsic termination ?
- This type of termination occurs in most bacterial genes
- involves an inverted repeat in gene followed by a polyA region
- Transcription stalls due to this structure and RNA pol and RNA separate
Explain what Rho-dependent termination is ?
- This involves an additional terminator protein
- RNA does not slow down - transcript pulled out of transcription complex by rho which acts as a molecular motor and runs along the new RNA molecule towards RNA Pol.
Where does translation occur and how does it start ?
- Translation occurs in the same compartment ( cytoplasm )
- Ribosome binds to ribosome binding site and initiates protein synthesis at start codon
How many ORF can bacterial genes encode for ?
just 1
How many promoters does each operon have ?
1
What does each ORF have its own of to initiate translation ?
RBS
What can occur if mutations arise in the 1st gene in an operon - what’s this called ?
Can switch off expression of downstream genes = polarity
What allows polar mutation to occur ?
Gene structure
What needs to be introduced for a shorter mRNA to be made ?
A new terminator
Explain why bacterial mRNAs do not undergo any significant processing ?
The primary transcript synthesised by RNA POL IS THE MATURE mRNA
Where does transcription and translation both occur ?
In the same cell compartment ( nucleoid )
What is a polyribosome ?
Where multiple ribosomes can bind to one transcript in the nucleoid
What does a polyribosome result in?
Results in multiple copies of the new protein being introduced from one RNA molecule
What are the 3 different levels that regulation of bacterial genes can occur at ?
1- transcriptional control
2- Translational control
3- Post transcriptional control
