Prokaryotic Gene structure an Function, Regulatory Regions

Question 1

What’s the structure of bacteria ?

Answer 1

Bacteria are generally haploid, with only of each gene on a single circular chromosome.

Question 2

What does the bacterium need to carry out to allow different functions ?

Answer 2

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.

Question 3

What are the 3 essential features that genes in bacteria have ?

Answer 3

1. A PROMOTOR -
2. A TRANSCRIBED REGION
   3 A TERMINATOR SEQUENCE

Question 4

What is a promoter ?

Answer 4

• All the sequences in the DNA required for expression and regulation of a gene
• ## These sequences are NOT all included in the mRNA

Question 5

What is a transcribed region ?

Answer 5

• Can include more than one ORF
• These sequences ARE included in the mRNA

Question 6

What is a terminator sequence ?

Answer 6

• Defines end of gene or operon
• Often a stem loop structure

Question 7

What is the structure of mRNA in a bacterial gene ( translated region , terminator sequence ) ?

Answer 7

1. 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

Question 8

What is mRNA synthesised by, how does the synthesis stops ?

Answer 8

-mRNA is synthesised by RNA polymerase
- Binds to promoter motif and initiates RNA synthesis
-mRNA synthesis ends at terminator site

Question 9

How many protein subunits does bacteria RNA polymerase holoenzyme consist of ?

Answer 9

• Córę enzyme ( five subunits )
• Sigma factor

Question 10

What does the core enzyme plus sigma factor equal to ?

Answer 10

Holoenzyme

Question 11

What does the sigma factor do ?

Answer 11

Binds to the -10 and -35 sequences in bacterial promoters and identifies the beginning of a gene

Question 12

What does regulatory proteins control ?

Answer 12

Binding RNA pol to promoters

Question 13

Explain what occurs when the regulatory protein (Lacl) binds to operator site ?

Answer 13

Prevents RNA pol transcribing full mRNA encoding structural genes

Question 14

What occurs when binding of the inducer occurs ?

Answer 14

Changes the lack repressor shape - allows transcription of full mRNA encoding structural genes.

Question 15

What does it lead to if RNA pol cannot bind to promoter without help of an activator ?

Answer 15

Leads to lots of varied mechanisms of gene control

Question 16

How is transcription in bacteria started ?

Answer 16

1. Closed complex formed as holoenzyme assembles on promoter.
2. Open complex formed by activity of core enzymes.
3. transcription initiated at +1 site.

Question 17

Whats the difference between DNA polymerase and RNA polymerase when it comes to transcription and what does it reflect ?

Answer 17

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

Question 18

What are the 2 types of transcription termination ?

Answer 18

1- INTRINSIC TERMINATION
2- RHO-DEPENDENT TERMINATION

Question 19

What is intrinsic termination ?

Answer 19

• 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

Question 20

Explain what Rho-dependent termination is ?

Answer 20

• 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.

Question 21

Where does translation occur and how does it start ?

Answer 21

• Translation occurs in the same compartment ( cytoplasm )
• Ribosome binds to ribosome binding site and initiates protein synthesis at start codon

Question 22

How many ORF can bacterial genes encode for ?

Answer 22

just 1

Question 23

How many promoters does each operon have ?

Answer 23

1

Question 24

What does each ORF have its own of to initiate translation ?

Answer 24

RBS

Question 25

What can occur if mutations arise in the 1st gene in an operon - what’s this called ?

Answer 25

Can switch off expression of downstream genes = polarity

Question 26

What allows polar mutation to occur ?

Answer 26

Gene structure

Question 27

What needs to be introduced for a shorter mRNA to be made ?

Answer 27

A new terminator

Question 28

Explain why bacterial mRNAs do not undergo any significant processing ?

Answer 28

The primary transcript synthesised by RNA POL IS THE MATURE mRNA

Question 29

Where does transcription and translation both occur ?

Answer 29

In the same cell compartment ( nucleoid )

Question 30

What is a polyribosome ?

Answer 30

Where multiple ribosomes can bind to one transcript in the nucleoid

Question 31

What does a polyribosome result in?

Answer 31

Results in multiple copies of the new protein being introduced from one RNA molecule

Question 32

What are the 3 different levels that regulation of bacterial genes can occur at ?

Answer 32

1- transcriptional control
2- Translational control
3- Post transcriptional control