Genetic diversity in bacterial genomes and populations Flashcards
What type of chromosomes are usually present in bacterial genomes?
Mostly singular, circular chromosomes
What are the exceptions
Multiple chromosomes and linear chromosomes
What are episcopal elements
Plasmids and phage
Where are coding strands located
Coding sequences are preferentially located on the leading strand
- essential genes are more likely to be encoded on leading strand
- consequence of selection against collisions between the replication fork and transcriptional machinery due to toxicity of truncated products
As genome size increases what happens to the gene number?
It also increases - in a linear relationship.
Most bacterial genomes contain little non-coding or junk DNA
Presence of mobile DNA
Transducing phage (integrative and non-integrative) Conjugative plasmids - mobilisation of the self or the chromosome Transposons, insertion sequences, integrons
What is meant by open genomes
Widespread occurrence of genetic exchange, closely related bacteria can differ greatly in genetic complements - open genome
What is the pangenome
The genes available to a particular bacterium. Increases in size according to power law as more genomes are sampled - Heaps Law
What are the elements of a genome
Core genome (DNA replication, ribosomes, cell envelope, key metabolic pathways)
Acessory genome (alternative metabolic pathways, transport systems)
Parasitic elements (toxins, restriction/modification systems)
Gene pool (antibiotic resistance, degrative metabolism)
What are the three mechanisms of genetic change
Point mutation (small scale change)
Insertion or deletion (small or large scale)
Rearrangement (small or large scale)
Ideas about bacterial populations dominated by:
- Bacteria are asexual
- Reproduce by binary fission
- Genetic change accumulated by vertical inheritance
What are some examples of genetically monomorphic pathogens
Mycobacterium leprae
Mycobacterium tuberculosis
Yersinia pestis
What is the Muller’s Ratchet?
Small, asexual populations are vulnerable to this
Accumulation of slightly harmful mutations – purifying selection too weak to remove all new deleterious mutations. In clonal reproducing populations, there is a substantial probability that all the fittest individuals will acquire slightly deleterious mutation and go extinct - so only “second fittest” individuals survive. Leads to reductive evolution .
e.g. Mycobacterium leprae and Yersinia pestis
What is the impact of recombination on bacterial population structure
Recombination disrupts clonal structure, disrupting tree-like phylogeny, linkage disequilibrium and congruence
