Genetics of Bacteria Flashcards
What is a gene?
smallest region of DNA (or RNA) that encodes a polypeptide or is transcribed (RNA)
What is an allele?
an alternate form of a gene
What is a Locus?
Location of a gene on the chromosome and often refers to a group of related genes
What is a wild type organism?
an organism that carries standard/reference gene which is usually but not always functional
What is a mutant organism?
carries altered form of gene(s)
What is a genotype?
genetic or allelic composition of a strain
What is a phenotype?
the observable properties of a strain
What is the structure of the bacterial genome?
Circular chromosome (usually one)
No nucleus, histones, mitosis, meiosis
Genome is highly dense in coding sequences (lack introns)
Number of self replicating plasmids
What is sequencing used for?
- understanding how genomes work
- identification of pathogenicity and resistant genes
- epidermiology
What is the core genome?
the group of essential genes and other genes found in all representatives of the species
What is the accessory genome?
genes present in some members of a species but not all e.g antibiotic resistance on some plasmids
What are the 3 stages of chromosome replication in bacteria?
Initiation
Elongation
Termination
Describe how the initiation occurs in replication of the bacterial chromosome
Initiation:
- replication is always initiated at the same segment of DNA: oriC which is fixed to the cell membrane
- initiation is started by the accumulation of initiation protein DnaA to oriC
- DnaA melts the `DNA then DnaB is recruited (a helicase) to unwind DNA
- DnaB opens replication forks in both directions (bidirectional replication)
Describe elongation in chromosome replication
the process of elongation involves the movement of replication forks and synthesis of new DNA complementary to the melted template strands
- primase (DnaG) lays down an RNA primer and DNA polymerase III copies the DNA
- okazaki fragments allow the lagging strand to be synthesised
- DNA ligase joins Okzaki fragments, together in the lagging strand
Describe termination in chromosome replication
it occurs when the replication forks reach the part of the chromosome opposite oriC at ter sites (which prevent helicase activity)
-completed chromosomes are interlinked and need to be resolved by topoisomerase to separate them
How are bacterial genes often organised?
in polycistronic operons consisting of genes of related function
What is the structure of a bacterial gene?
Operator Promotor (-35, -10) Transcription start site Ribosome binding site Gene A Gene B Gene C Terminator
How does bacteria control gene expression?
by controlling transcription
In what ways can you influence the rate of transcription?
- promoter structure (strength)
- operators/enhancers
- operons/regulons
- transcriptional regulators (+ve and -ve)
What are promotors?
Upstream of a genes coding sequences that are binding sites for RNAP
Consist of -10 and -35 sites
Binding is via sigma factor
What does the sequence of a promoter region influence?
How well RNAP binds (binding strength) and the rate of transcription
How do transcriptional regulators effect the rate of transcription?
-by helping/hindering the binding of DNA to turn genes on or off in response to environmental change
What is a regulon?
When genes with similar functions and therefore similar sequences can be turned on or off together
Why do bacteria evolve fast?
- rapid generation rates
- very large populations
- programmed mutations
How does evolution occur?
when an alteration to the genome occurs, followed by selection
What do DNA changes result from?
- mutation
- acquisition of new DNA
What is a programmed mutation?
When some genes are turned on or off at high frequency
genes known as phase variable
What are the mechanisms of phase variation?
- genomic variation
- strand slippage
- methylation
What are the three mechanisms that DNA can be exchanged by? (horizontal gene transfer)
conjugation
transduction
transformation
What are the two ways that transposition can occur?
- copy and paste mechanism (class II transposition) sequence coped to RNA and then into DNA (by reverse transcriptase) and is inserted elsewhere
- cut and paste mechanism (class II transposition) transposon cut out of DNA and inserted elsewhere by resolvase
How are transposons related to antibiotic resistance?
-capture resistance genes and transfer them between chromosomes/plasmids and therefore between species/strains
How does transduction occur?
- phage infects bacterial cell
- DNA injected and inserts randomly in host chromosome
- when lytic cycle s initiated phage DNA excised +/- some host DNA
- new phage particles synthesised
- bacterial cell lyses releasing particles
- infect new cell and may carry DNA from previous host
What is transformation?
-uptake of naked DNA from the environment by competent bacteria
How does conjugation occur?
- Plasmid replicates in the cytoplasm
- F pili promote cell-cell contact between 2 bacteria of the same generation
- A the new copy of the plasmid, generated by replication is transferred to a recipient cell along the conjugation tube
- A second copy of plasmid remains in the donor cell
