bacterial genetics Flashcards
what are three common misconceptions about bacterial genetics?
that bacteria don’t have genes/quantized portions of hereditary substance
that bacteria don’t have sex
that bacterial genetics has nothing to do with prokaryotes
when the truth is ‘everything that is true for E.coli is true for the elephant???’
what is transformation (and relevant experiment)?
its when exogenous DNA is imported and incorporated into a genome
demonstrated by the smooth vs rough bacteria experiment by Avery and McCarthy, when they discovered the dead smooth bacteria could pass DNA to the live rough bacteria
why do harsh conditions encourage transformation?
transformation is risky business - few bacteria have the ‘competence’ to take up this exogenous DNA
the bacteria has no indication as to whether or not the DNA will be beneficial, so harsh conditions increase likelihood that the bacteria ‘risks’ it
how does transformation work?
1) DNA binding proteins on the bacterial membrane bind to the extracellular DNA
2) a nuclease on the membrane nicks the DNA, one strand is degraded and the other is internalised using a translocase enzyme
3) The internalised strand is bound by RecA - a protein that prevents degradation
4) RecA also binds to the bacterial ell’s dsDNA, stretching it to increase it’s complementarity and allow for recombination
how does transduction work?
its transfer of DNA from one bacterium to another via a viral vector
a phage infects a bacterium and inserts DNA into the host’s, makes copies etc…
but in viral assembly of new phages, some/one idk take up some of the host DNA, and when they infect another bacterium and inject their DNA it now contains some of the previous host bacterium’s genes which get inserted into the new host’s genome
these phages that take up host DNA are called transducing particles
who discovered bacterial sex/conjugation?
Lederberg and Tatum
how did Lederberg and Tatum discover conjugation, including their controls?
use two strains of E.coli - A and B - deficient in producing different amino acids/molecules so that neither could survive alone - but when together it turned out they could grow
to ensure this wasn’t any kind of transformation, a U-tube was used with a filter that only allowed DNA through the middle - growth was not observed for whether strain leading to the conclusion that contact is required
how does conjugation work?
the donor cell requires a fertility plasmid - F+ cell, which has the transfer genes required for conjugation
the recipient is F - meaning it doesn’t have the fertility plasmid
the plasmid has genes required to form a sex pilus - like a grappling hook that pulls two cells closer together and forms a channel between them
the F+ cell transfers a single strand of the F+ plasmid to the recipient cell, where it is converted into dsDNA (using the rolling circle I think) the OG single strand is also made back into dsDNA, giving two F+ cells
what is an Hfr cell?
Hfr means nigh frequency recombination
the F+ plasmid can have insertion genes that result in the transferred DNA being integrated into the recipient cell chromosome instead of remaining as a plasmid
the recipient cell is considered Hfr if integration occurs
if the donor cell in conjugation is Hfr what usually happens?
If the donor is Hfr, it has all the genes needed for conjugation and will form a sex pillus etc… but the F+ genes are now integrated into the donor cell’s chromosome
The origin of transfer - where the nick in the donor DNA is - is in the middle of the integrated F+ genes, so in order for the entire plasmid of F+ genes to be passed on now, the entire chromosome will have to be transferred (single strand, then replicated but still)
This is very unlikely as it takes much longer and any mechanical disruption separates the cells, so when the donor is Hfr the recipient doesn’t usually get all the F+ genes and is therefore considered F-
However the cell has managed to transfer some of it’s chromosomal genes
how has the wide variety of Hfr strains allowed for the identification of arrangement and orientation of almost all genes before the genome sequence of E.coli became known?
Hfr strains transfer genes in different sequences
Genes further along on the chromosome taker longer to transfer
Experiments mixing bacteria that for example cant make tryptophan with those that can, and measuring how long it takes for the transfer to occur by seeing when tryptophan can be produced by the cells that originally could not,
have been used to map out where lots of genes are on the chromosome
eukaryotic vs prokaryotic DNA?
prokaryotes don’t ‘waste space’, almost all the sequence codes for proteins
eukaryotes have introns and VNTRs
what is a psuedogene?
a gene that has mutated/decayed into an inactive form/cannot produce a protein
what is an operon?
what is a regulon?
used by bacteria, it’s a group of genes coding for proteins often needed at the same time/for the same thing and so are grouped and transcribed together
same principle just a step up - a group of operons often needed at the same time/for same process, controlled by a common regulatory protein
do bacteria have monocistronic genes?
yes, but it’s rare, usually polycistronic
