Bacterial genome Flashcards
(37 cards)
Describe the typical bacterial genome:.
• Single circular chromosome – single origin of replication (ori).
• Small genome size – very little non-coding DNA.
• Genome is often augmented by extrachromosomal DNA molecules called plasmids:
o Small circular DNA molecules with their own ori.
o Plasmids don’t normally carry essential genes.
Describe bacterial Chromosomes:.
Describe bacterial Chromosomes:
What is the bacterial genome size:.
- Free living: 0.5Mb- 10Mb
* Symbionts: as small as: 160kb.
What does genome size correlate to?
- Genome size correlates well with gene number and in turn with morphology, physiology and metabolic complexity.
- Small genomes = small no. of genes = restricted growth to relatively few restricted niches – often pathogens
- Large genomes = large no. of genes = higher metabolic diversity.
How big is one gene roughly?
• 1kb = est. 1 gene. (average protein = aa 333 = average gene = 1000bp = 1kb.
How are related genes relative to eachother in the genome?:
• genes encoding enzymes in the same metabolic pathway are often organised into polycistronic operons.
What is the name used to describe a section of DNA/RNA that codes for a specific polypeptide?
• Cistron is the section of DNA/RNA that codes for a specific polypeptide in protein synthesis.
What is the name of an area that codes for more than one gene?
• Polycistronic operons code for more than one gene.
o Destroying the activity of any one of the gene products will block the pathway.
What are Plasmids? Draw one.
see doc or google.
• Non-essential DNA molecules with their own ori.
• Most are circular but some linear.
How big is a plasmid?
How big is a plasmid?
What are plasmids called when they occur in high/low levels?
• Number of copies of different plasmids vary:
o High copy number (small) plasmids = 50 copies/cell.
o Low copy number (large) plasmids = 1-2 copies/cell.
• Found in most bacterial species.
What are Conjugative plasmids? What do they do? What are the key words in this process?
Many large plasmids encode genes that enable the transfer of the plasmid between cells.
o DNA transferred through mating bridge whose formation depends on pilus that connects the two cells.
o Transfer process is called conjugation and the recipient is a transconjugant.
What does conjugation occur between?
• Conjugation occurs in and between species and between bacteria and plants.
What word is used to describe the replication in conjugation? Why is this? How does this affect how long it takes?
• Replication in conjugation is unidirectional = 1 rep fork. Therefore take twice as long as normal replication..
What are Non-conjugative plasmids?
• Most small plasmids are non-conjugative.
• Maintained in both daughter cells following cell division.
• Cannot transfer to another cell on their own.
o They can be mobilised by hitching a ride on a system provided by a conjugative plasmid.
• Have been employed in gene cloning technology as vectors for introducing and maintaining DNA bacteria.
What is Horizontal gene transfer (HGT)? What are the mechanisms in bacteria? SKETCH THEM ALL.
• Genetic exchange between organisms.
• Three mechanisms in bacteria:
o Transformation – naked DNA.
o Transduction – phage coated DNA.
o Conjugation – transfer through mating bridge.
• Almost all bacteria utilise at least one of the above mechanisms for genetic exchange.
What are plasmids called that allow spread into species where recombination may not occur?
• Broad Host Range plasmids allow spread into species where recombination may not occur.
What genes can be transferred from one bacterium to another? What are the most common? DRAWWWW.
• Any gene can be transferred from one bacterium to another but the most common examples are:
o Antibiotic resistance
o Pathogenicity.
o Ability to survive in different environments.
• New traits spreading through populations.
What is the The F-Plasmid? Why is it unusual?
• The F (fertility)- plasmid is an e.g. of a conjugative plasmid
• 100kb low & copy number – replicates once /cell cycle & segregates to both daughter cells.
• F+ cells contain the F-plasmid.
• F+ plasmids act as donors in the transfer via conjugation of the F plasmid to F- cells, which then become the wildtype.
o Transmission only takes a few minutes.
o F+ cells cannot transfer the plasmid to an already wild type cell due to surface exclusion and compatibility.
• Unusual because depressed for transfer:
o Not all plasmids have their genes switched on all the time.
o For F this is due to repressor gene being inactivated.
o Picking out this mutant may have occurred because of the selection experiment where it was first located.
Sketch how f factors work.
google.
What is transformation?:
- integration of exogenous DNA into genome by 2 crossovers.
What is Transduction?
- integration of exogenous DNA introduced by phage by 2 crossovers.
What are High frequency of recombination (Hfr) cells?
• F plasmid can integrate into the bacterial genome via recombination.
• Resulting cells are called Hfr.
• Results in larger bacterial genome (E.coli = 2%).
• Position and orientation of integration is (sort of) random.
o Once integrated, position is fixed.
o Different strains can have F plasmid integrated at different positions and orientations.
• Integration involves one crossover.
Can Hfr cells conjugate?
• It is possible.
