bacteria Flashcards
structure of a bacterium (10)
Fimbria, chromosome, storage granules, 70S ribosome, flagellum, plasmid, cytoplasm, plasma membrane, peptidoglycan cell wall, capsule.
Outline the process of binary fission
DNA replication begins at the origin of replication where DNA is unzipped by breaking hydrogen bonds between bases of the 2 strands to form a replication bubble
DNA replicates by semiconservative replication where each original strand serves as a template for synthesis of daughter strands by CBP
2 newly formed ori move to opp poles of the cell and attach to the plasma membrane
the cell elongates to prepare for division
DNA is circular with no free ends, and the two Daughter DNA molecules will be interlocked with the completion of replication.
enzyme topoisomerase cuts separates and reseals the 2DNA molecules
invagination of the plasma membrane and the deposition of new cell wall eventually divide the parent cell into two daughter cells
comparison of binary fission and mitosis:
end product
amount of DNA
DNA replication
behaviour of chromosomes
spindle fibres
both form 2 genetically identical cells
both have the same amt of DNA in daughter cell as compared to parent cell
DNA replication occurs during binary fission, occurs during S phase of interphase b4 mitosis
attachment of chromosomes to plasma membrane vs no
entangled vs not entangled
no condensation vs condensation
no specific positioning vs specific positioning
no spindle fibres involved vs yes
definition of transformation
transformation is the uptake naked foreign DNA from the surrounding, resulting in a change of the bacterial cell’s genotype and phenotype
process of transformation
fragments of naked foreign DNA in the surrounding medium are taken up by competent bacterial cells via cell surface proteins
foreign DNA is incorporated into host cell chromosome through crossing over at homologous regions on the bacterial chromosme
resulting in a recombinant cell
if different alleles of a gene were exchanged and the new allele is expressed, this results in a permanent change in genotype and phenotype and the cell is said to be transformed.
definition of conjugation
Direct transfer of genetic material from one bacterial cell to another through a mating bridge between the two cells via the transfer of F plasmid from an F+ donor to F- recipient cell
process of conjugation
sex pilus of F+ bacterial cells makes contact with a F- cell and retracts to bring the 2 cells closer
the hollow pilus then acts as a cytoplasmic mating bridge between the 2 cells
one of the 2 strands of the plasmid DNA is nicked and transferred from the F+ cell to the F- cell through the mating bridge. the other strand is used as a template for synthesis of a complementary strand and thus forming a double stranded plasmid in the donor cell
the single stranded F plasmid DNA circularises in the F- cell and is used as a template to synthesise a complementary strand in order to form a double stranded plasmid DNA. the F- recipient cell is now a F+ cell
replication of the plasmid occurs via rolling circle DNA replication
how does the rolling circle DNA replication occur?
a strand of DS plasmid is nicked by a nuclease –> free 3’OH end is then used as a primer for strand elongation by DNA polymerase using the intact strand as a template –> elongation process is facilitated by the displacement of the 5’ end of the nicked strand which is transferred across the cytoplasmic mating bridge to the recipient bacterium. upon completion of a unit length of the plasmid DNA, another nick occurs to release the og strand.
in the recipient cell, the single strand of F plasmid DNA re-circularises and serves as a template for the synthesis of a complementary daughter strand to form a DS circular DNA
definition of transduction
the process by which bacterial DNA from one host cell is introduced into another bacterial host cell by bacteriophage due to aberrations in the phage reproductive cycle
process of transduction: generalised
Generalised: a phage infects a bacterium by injecting its viral genome into the host cell; thje bacterial DNA is degraded into small fragments, one of which may be randomly packaged into a capsid head during the spontaneous assembly of new viruses
upon lysis of the donor bacterium, the defective phage will infect a recipient bacterium by injecting the bacterial DNA from the donor bacterium into it
the foreign bacterial DNA can replace the homologous region in the recipient cell’s chromosome through homologous recombination, allowing the expression of a different allele from the donor bacterium
process of specialised transduction
temperate phage (lambda) infects a bacterium, injecting its viral genome into the host cell
the viral DNA is integrated into bacterial chromosome forming a prophage which may be improperly excised to include the adjacent segment of bacterial DNA and not the entire phage DNA during an induction event
the phage-bacterium hybrid DNA may be packaged into a capsid head during the spontaneous assembly of new viruses
upon cell lysis, the defective recombinant phage will infect a recipient bacterium by injecting the phage-bacterium hybrid DNA into it
new alleles from the previous bacterial cell can be incorporated into the genome of the new bacterial cell by homologous recombination or integration of phage-bacterium hybrid DNA as defective phage enters the lysogenic cycle.
comparison between mechanisms of transformation, transduction, conjugation:
type of donor cell / source of DNA
agent mediating DNA transfer
type of DNA transferred to recipient cell
homologous recombination needed for permanent expression of foreign genes?
Broken down DNA from lysed bacterial cells vs bacteria cell infected by virulent (temperate) phage vs F+ cell containing F plasmid
cell surface proteins make cells competent vs bacteriophage vs F factor on F plasmid which codes for proteins involved in formation of sex pili
random fragments of the bacterial genome vs random fragments of the bacterial genome small enough to fit into phage capsid vs F plasmid
yes vs yes vs no
what is an operon
a cluster of functionally related genes which are under the control of the same promoter and operator, allowing for functionally related proteins to be synthesized tgt. when the genes are transcribed, a polycistronic mRNA is produced.
whats the promoter
RNA polymerase binding site, upstream of structural genes
whats the operator
repressor protein binding site to prevent RNA polymerase from binding to the promoter and initiating transcription
