L2-4:Intro to Bacterial Genetics Flashcards
What are wild type genes?
Unmodified natural isolate, isolated from the environment
What is a mutant?
An organism that differs from the wild type as a result of change
What is a mutation?
A specific change in DNA sequence that is different to the wild type DNA
What is an allele?
Alternative forms of a gene that arise by mutation that are found at the same place on a chromosome in wild type and mutant
What is phenotype?
Identifiable/observable trait
What is genotype?
The nucleotide sequence of a region of DNA, expressed in terms of alleles of genes
Why are bacteria used as model organisms?
Quickly divide (increases exponentially)
Easy to genetically manipulate
What is an advantage of using bacteria?
They are haploid (only one copy of genes/alleles easier to identify mutations)
What is vertical gene transfer?
When changes in heritable properties are passed onto progeny
What is horizontal gene transfer?
When bacteria not only inherit DNA from parents but also inherit it from other bacteria and viruses in their environment
What is genetic transformation?
When heritable properties can be transferred from one bacterium to another
What is conjugation?
When 2 different strains are mixed with different characteristics they can isolate progeny that have a combination of characteristics
What is transduction?
When bacterial viruses (bacteriophage) can carry DNA from one bacterium to another
What is the Griffith Experiment?
Tested bacteria in rats (Streptococcus pneumoniae) and was shown that when the virulent bacteria was head-treated and mixed with the avirulent bacteria a transformation took place which changed the avirulent to virulent bacteria
What is the difference between capsulated and non-capsulated Streptococcus pneumoniae?
Capsulated: virulent, capsule helps evade immune system
Non-capsulated: avirulent, recognised by immune system so they are killed
What did the Avery-MacLeod-McCarty experiment find?
That DNA is the only molecule with transformation ability, not RNA or protein
Why is competence needed in bacteria?
In order for it to be transformed meaning it is in an appropriate physiological state
How is natural competence advantageous?
It is important to horizontal gene transfer in natural environments
Prevents the spread of antibiotic resistance
How is conjugation mediated and how does it work?
By a conjugative plasmid that encodes a sex pilus that establishes a link that pulls the cells together establishing cell-cell junction so transferred DNA can pass
What is a Hfr strain?
High Frequency recombinant strains (when host chromosomal genes are used)
How bacteriophage infection and growth shown?
Results in zones of lysis on a lawn of bacteria
How do phages transfer genes from bacterium to bacterium?
When they make mistakes when packaging DNA into their phage particles
What are transducing particles?
When phage particles become filled with either host chromosomal DNA or a mixture of host and phage DNA
What components make up the ribosome?
Ribosomal RNA and protein
What is the central dogma?
DNA can be replicated or transferred to RNA and RNA can be replicated and transferred to protein but protein cannot be replicated
What is a characteristic of protein synthesis in bacteria?
Transcription and translation are coupled
What are the 3 stages in transcription?
Initiation, elongation and termination
What are the common types of RNA?
Messenger RNA (mRNA)
Ribosomal RNA (rRNA)
Transfer RNA (tRNA)
How is DNA used in transcription?
DNA unwinds as one strand is the template and the other is the coding strand
How is the template strand used in transcription?
Used by RNA polymerase to generate a complementary strand by RNA (T swapped for Us)
How many reading frames are there?
6
How are genes organised on the chromosome?
Open reading frame (ORF) encodes genes on coding strand starting at the promotor and ending on the terminator
How are one of the six reading frames chosen?
Determined by ATG/AUG start codon
How is translation initiated?
One 1 of the 6 ORFs are chosen immediately in front is Ribosome Binding Site (RBS) which is complementary to 3’ end of 16S rRNA in the small subunit which allows the ribosome to start transcription
What are the enzymes responsible for DNA synthesis?
DNA-dependent DNA polymerases
What are the precursors for DNA synthesis?
Deoxynucleoside triphosphates
How is DNA synthesis initiated?
By pre-existing RNA or DNA to primer DNA synthesis (RNA nucleotides removed and replaced with RNA ones)
What is the bond that DNA polymerase generates?
Phosphodiester
What way do DNA polymerises synthesise?
5’ to 3’
How are chromosomes replicated in bacteria?
As it is circular initiation occurs at the oriC (origin of replication), it occurs bi-directionally so there are 2 sites of replication called replication forks
What are the 6 types of mutation?
- Base-pair changes
- Frameshifts
- Deletions
- Inversions
- Duplications
- Insertions
What are the 2 different base-pair changes that take place?
Transitions and transversions
What are the 3 potential consequences of base change mutations?
Silent mutation - codes for same amino acid
Missense mutation - codes for different amino acid
Nonsense mutation - codes for stop
How do frameshift mutations take place?
By insertion or deletion
What are the consequences of a frameshift mutation?
Leads to invariably truncated protein, 3/64 codons are stop codons and 1/20 randomly generated codons will be a stop codon
What is the calculation for mutation frequency?
Mutation frequency (MF) = m/N
m - number of mutants
N - total number of bacteria
How does mutation frequency increase?
Overload repair systems by treating cell with chemicals (mutagens) or radiation that damages the DNA
What are the 3 ways mutants can be selected?
- Negative selection - selects against the mutant growing
- Enrichment - use negative selection to inhibit growth of mutants and kill wild-type using an antibiotic
- Positive selection - Uses selective conditions where the mutant will grow. In the form of resistance to a phage, chemical compound or intermediate metabolic product
What is genomics?
The acquisition, storage, retrieval and analysis of DNA sequence data
What are the characteristics of the E.coli genome?
Replicates at a rate of ~850 bases per sec per replication fork so it takes 40 mins to replicate the entire genome, cell divides 20 mins after replication is complete
Genes are present as 1 copy per cell (exemption of rRNA genes which have 7 copies)
How does evolution mainly occur in bacteria?
Vertical and Horizontal gene transfer
