(03) Prokaryotic Genetics Flashcards
T or F: bacteria only encode a single RNA polymerase
True
What is an operator?
- location relative to promoter.
Operator:
- Binds Proteins in involved in gene expression
- Located near the promoter
Why can transcription be paired with translation in bacteria?
- No Introns
2. No Nucleus
What is an operon?
- what bacterial type contains the lac operon?
- Cluster of Genes involved in a related function that is transcribed on a single mRNA (aka polycistronic)
Gram (-) bacillus Escherichia Coli
What 3 genes make up the lac operon?
- what is its function?
- Does gene transcription take place in unison or individually?
- Lac Z –> ß-galactosidase
- Lac Y –> Lactose Permease
- Lac A –> Lactose Transacetylase
- Allows Gram (-) Bacillus Escherichia Coli to use lactose
- Transcrption takes place together but RNA translation takes place individually
**Note: if this was not the case, then they would be translated together then cleaved apart
What are the reasons for an operon rather than genes located on different parts of the chromosome?
- Operons help to ward off loss of function
- if genes were in different part of the genome they could more easily be lost by evolutionary divergence - Use of a single promoter and operator provides co-regulation of genes in the operon. ALLOWS FOR COORDINATED EXPRESSION
What are the 3 types of gene regulation?
- which type is most rare?
- why?
- Constitutive
- no regulation, genes always expressed
* *Most rare because of ATP expenditure - Positive Regulation
- Activator Protein promotes RNA polymerase binding to promoters - Negative Regulation
- Repressor protein binds operator sequence and prevents transcription until it is removed
Is positive or negative regulation more common?
- why?
Negative regulation
- From an evolutionary standpoint loss of promoter leads to complete loss of the gene while loss of repressor, then the gene is still functional
What is a regulon?
Independently transcribed genes or multiple operons are controlled by the same regulator protein = Regulon
Steps of 2 component signaling?
- Extracellular Molecule Binds Receptor in membrane = SENSOR that has an intracellular TRANSDUCER
- SENSOR then TRANSDUCER get phosphorylated
- TRANSDUCER works on operator
***Important in Quorum Sensing
What processes use 2 component signaling?
- Changing Temperature
- Osmolarity
- Extracellular Iron availability
What is quorum sensing?
- what virulence factor relies on this hardcore?
- Bacteria Communication via releasing and measuring signaling molecules
- More signaling molecules released as population grows until CONCENTRATION THRESHOLD is reached
- All cells now begin expressing new gens
**Particularly important in biofilm formation
What are the signaling molecules in quarum sensing?
- Non-peptides
- Usually cyclic structures like lactones or quinolones
What challenges are posed to bacteria as a result of them being haploid?
- How are these challenges overcome?
- Mutations readily lead to LOSS of FUNCTION
- Overcome by SHEER NUMBERS, loss of millions of bacteria doesn’t matter much - Phenotypic Diversity could be limited because of only a single pair of alleles
- Phase Variation
- Antigenic Variation
What is phase variation?
- what is the consequence of phase variation?
a. Gene can be inverted into the ON position
- aka Aligned with its promoter
b. Gene could be inverted into OFF position
- aka Gene FLIPPED so that PROMOTER IS NOT LONGER ALIGNED WITH GENE
**Results in an on and off switch for the gene
Antigenic Variation
- what is the overall result
- what bacteria is known to use this?
- what is the clinical presentation for antigenic variation?
**SEE SCHEME for STEPS
Bacteria:
- Neisseria Gonorrheae
Overall Result:
- Large array of Surface Components
- Recurring infection because they are constantly changing their pilus
Requirements for Antigenic Variation?
- Large numbers of bacteria
- Lots of growth
**Because you need functional mutant to expand and take over
What are 4 ways that bacteria can actually acquire new genes?
- Transposition
- Conjugation
- Transduction
- Transformation
What is transposition?
- 4 basic elements?
- Genetic elements hop or put an additional copy of themselves into their genome
- *All happens in one cell, does not jump from one to the next
- Insertion sequences
- Transposase
- Repressor
- Beneficial host gene (ANTIBIOTIC RESISTANCE GENE)
What are the functions of the 4 basic transposon elements?
- Insertion Sequences
- Inverted DNA repeats - Transposase
- Recombinase that allows insertion of sequence - Repressor
- Regulates frequency of Transmission - Beneficial Host Gene
- Antibiotic Resistence Gene
- other virulence factor gene
How can transposons be detrimental to cell health?
- It could enter in a bad spot in the genome (e.g. before lacZ)
- Must insert before a gene
**Cause problems when they bind to genes or operons
What two ways do transposons increase cell diversity?
- Introduction of an antibiotic resistance gene or another gene
- Creation of mutants lacking non-essential wild-type functions
**all others will die
Plasmids
- what are they?
- How big are they?
- Gene types?
Plasmids - Extrachromosomal (episomal) circular DNAs found in MOST bacteria
2-500 kilobase pairs
Targets for antibiotic genes for transposons
T or F: smaller plasmids can replicated independently of the genome
True
Why are plasmids maintained in the genome if they don’t contain genes needed for cell survival?
- They many have transposons in them that contain Antibiotic Resistance Genes
While transposons in themselves are retained to the cell from which they orginated, how could they travel to another cell?
- Could go to another cell via a Plasmid during Conjugation
Conjugation
- how does it occur
- how is genetic diversity conferred as a result?
How:
- High Frequency Transfer (Hfr) or F+ cells transfer F factor plasmid from one cell to another that is F- via a sex pilus
Genetic Diversity:
- Results from the fact that genomic DNA can enter off of chromosomes that are Hfr that allow some of their own genetic material to enter the second cell.
- Cell2 can either put Hfr into genome or make a T or R factor plasmid
What are PAIs (pathogenicity-associated islands)?
- what leads to their formation?
- Large regions of a chromosome that encode various virulence factors, toxins, and iron acquisition components.
- Hfr are believed to be the major source of PAIs
What are two factors that help us to know that PAIs are from an external source?
- NON-pathogenic strains of the same bacterium lack PAIs
2. Also lack G+C
What is the difference between an F factor and an R factor?
F Factor:
- plasmid that does not contain resistance genes
R Factor:
- plasmid that has acquired antibiotic resistance genes LIKELY FROM A TRANSPOSON
Given that F factor is needed for formation of the sex pilus, how can R factor spread?
- R Factor may be conjugative (can form the pilus)
OR
- It may be dependent on F factor for conjugation
What is the major thing that confers antibiotic resistance of bacteria, and why is this?
- R Factor
why?
- It can spread through intERspecies conjugation and sometimes even across genera
**Bacteria with several different R factors may confer multidrug antibiotic resistance
What are viruses called that prey on bacteria?
- what are the two types?
- how do they differ?
- Phage
- Lytic
- Phage infects –> Replicates –> released from cell (usually by lysis) - Lysogenic (temperate)
- Phage genomes integrate at specific sites in the bacterial chromosome (aka lysogen)
- Lysogen remains in the cell until the cell undergoes stress (radiation etc.)
- Stress induces Lytic Replication
What is transduction?
- what processes can cause transduction?
- Transfer of genes from bacteria 1 (host) to bacteria 2 via a phage
- Phages moves from one cell to the next causes this
Generalized Transduction
- who carries it out
- how
- what are the consequences
(transfer of genes horizontally among bacteria via phage)
- ONLY carried out by LYTIC phage
How:
1. Lytic phage cleaves bacterial chromosome into small fragments
- Fragments may be similar size as virus (phage) genome and packages into Phage Particles
- Phage particles containing Bacteria 1 DNA will be inserted into Bacteria 2
Consequences:
- Any bacterial gene can be transduced to a recipient cell (resistance genes may be passed along)
Specialized Transduction
- who carries it out
- how
- what are the consequences
- Carried out by Temperate (lysogenic) phages
How:
- lysogen (viral genome) is inserted into the host genome
- when stress occurs ABERRANT EXCISION causes some of the viral DNA to be left behind and BACTERIAL DNA is included instead
- New phage is stable and will continue to carry bacterial DNA with it
Consequences:
**Original lysogen only inserts at certain parts of the genome, therefore only a few specific bacterial genes can be transduced by any particular phage
Of the two types of transduction, which is most important to generating genetic diversity?
Specialized transduciton
What is transformation?
- what is its role in nature
- what are cells called that can carry out transformation?
Example?
- Uptake of Naked DNA from one cell to another
- Insignificant role in creating genetic diversity in nature
- Cells that can carry out transformation = Competent
- e.g. Streptococcus Pneumoniae
Overall, what is the most common way that bacteria exchange information?
Conjugation
What is the paradigm case of transduction?
- what type?
Cholera Toxin (specialized transduction)
What is the rate of both generalized and specialized transduction?
1/1000 - 1/10,000
What is the rate of antigenic variation?
1/1000
