Bacteria Flashcards
Characteristics
Circular DNA
70S RIbosomes
Peptidoglycan cell wall
F plasmid vs bacterial chromosome
F plasmid has significantly fewer base pairs than bacterial chromosome
F plasmid codes for non-essential genes which may confer advantages which enhance survival such as antibiotic resistance, while bacterial chromosome codes only for genes essential to the cell’s survival such as enzymes for metabolism.
F plasmid contains genes that code for proteins necessary for formation of sex pili and subsequently mating bridge for conjugation, allowing for bacterial genes to be transferred between bacteria, increasing genetic variation.
Transformation
Cell-surface proteins on the surface of bacterial cell wall bind to foreign DNA fragments from neighbouring dead, lysed bacteria and take them in. Foreign DNA is incorporated into the bacterial genome via crossing over at homologous regions.
If foreign DNA contains different alleles that are now expressed in the bacterial cell. Bacteria is now transformed.
Conjugation
Sex pilus of bacteria F+ makes contact with that of another bacteria F- and retracts to bring the F- cell closer, forming a mating bridge between the cells.
One of the two strands of plasmid is nicked and transferred across the mating bridge by rolling circle DNA replication, as the other intact strand is used as a template for elongation.
The single strand of F plasmid in F- cell circularises and is used as a template for synthesis of complementary strand for a double-stranded F plasmid DNA resulting in F+ cell.
Rolling-circle (DNA) replication
One of the double-stranded circular DNA is nicked by breaking the phosphodiester bond. The 3’OH end is used for elongation by DNA polymerase using intact strand as a template while the 5’ end of the nicked strand is displaced and transferred to the recipient cell by the mating bridge.
Upon completion of a unit length of the plasmid DNA, another nick occurs to release the original strand which recircularises.
In the recipient cell, a complementary daughter strand is synthesised using the received F strand as a template, producing a double-stranded circular DNA
Benefits of conjugation
1) Gain new alleles that, when expressed, confers advantages to the bacterial cell that enhances its chances of survival in a different environment, e.g. antibiotic resistance
2) Gain ability to use new metabolites and resources e.g. new carbon source by producing the relevant enzymes
Generalised transduction
A lytic phage infects a bacterial cell, injecting its viral genome into the cell. During the lytic cycle, phage enzymes degrade the host bacterial genome into small fragments, one of which may be randomly packaged into a capsid head during assembly of new viruses.
After cell lysis, defective phage is released and infects another bacterial cell, injecting bacterial DNA from previous host cell into new bacterium. Foreign bacterial DNA can replace homologous regions in the recipient cell’s chromosome if crossing over takes place, allowing expression of different allele from previous host.
Specialised transduction
A temperate phage infects a bacterial cell, injecting its viral DNA into the cell which is integrated into the bacterial chromosome as a prophage. During induction, segments of the bacterial chromosome adjacent to the prophage is improperly excised along with the prophage and packaged in a capsid during assembly of new viruses.
Defective phages are released after cell lysis and infect another bacterial cell, injecting foreign bacterial DNA into the new cell. Foreign bacterial DNA can replace homologous regions in the recipient cell’s chromosome if crossing over takes place, allowing expression of different allele from previous host.
In lac operon, why glucose decrease first then lactose
Glucose is used in preference to lactose as respiratory substrate. No growth in bacteria → glucose is depleted
Lac operon and lactose
When lactose is of high concentration, lactose, converted to its isomer allolactose, acts as an inducer and binds to the active lac repressor protein and changes the conformation of the DNA-binding site of the repressor so it is longer complementary to operator. Since repressor can no longer bind to operator, RNA polymerase and GTFs can bind to promoter and assemble transcription initiation complex, hence allowing it to transcribe the lac structural genes to produce B-galactosidase, permease and transacetylase for metabolism of lactose.
Lac operon and glucose
In the absence of lactose, a basal level of B-galactosidase, permease and transacetylase is produced since repression at promoter of lac repressor is leaky. Small amount of permeases can transport lactose from surrounding medium into cell which is converted to inducer allolactose by B-galactosidase which binds to DNA-binding site of lac repressor changing confm…
When glucose is of high concentration, cAMP levels are low.
Catabolite Activator Protein is inactive since few cAMP to bind to CAP hence no CAP binding to the CAP binding site of the promoter and there is no upregulation of transcription of the lac operon.
No beta-galactosidase produced to hydrolyse lactose
No permease produced to take in lactose
Trp operon
When tryptophan levels are high, tryptophan binds to trp repressor at its allosteric site to change its conformation to active form complementary to operator.
Repressor binds to operator which prevents binding of RNA polymerase and GTFs to promoter, preventing assembly of transcription initiation complex, hence preventing expression of operon.
Lac operon vs Trp operon
Lac is inducible operon that is switched off in presence of inducer lactose, while trp is repressible operon switched off in excess end-product tryptophan
Effector molecule: inducer allolactose, substrate vs corepressor tryptophan, end-product
Repressor synthesised in active vs inactive form
Lac codes for 3 enzymes, Trp, 5
Lac operon default off until lactose, Trp default on until
Lac is catabolic, breaks down lactose. Trp is anabolic, synthesises tryptophan
Advantages of operons
In bacteria, functionally related genes are organised into operons with an operator, promoter and the structural genes.
Allows for control of expression of many genes with one promoter. Faster response to environmental changes bc bacteria are unicellular and exposed to fluctuating environment
Functionally related genes are expressed as a set so economical use of resources and energy.
Bacterial species able to regulate their own gene expression have an advantage over those without.
DNA arrangement
Eukaryotic DNA coils around histones to form nucleosomes, followed by coiling around itself to form a solenoid which associates with scaffold proteins to form looped domains.
Supercoiling of the loops condenses the DNA into metaphase chromosome.
Prokaryotic DNA is coiled around relatively fewer histone-like proteins into loops, and then undergoes supercoiling for further compaction.
