Genetics of Bacteria and Viruses Flashcards
Bacteria As Valuable Research Subjects
Small size
Rapid reproduction
Simple structures
Genetic variability (have only one chromosome)
Selective media (can grow on variety of different media such as antibiotics)
Bacterial Viruses
Bacteriophages
Reproduce by infecting bacterial cells
Clearances/Plaques
Dead colonies that were killed by viruses
Bacteriophage T4
Goes through lytic phase
168 000 base paris, 150 genes
Bacteriophage Lambda
48 502 base pairs, 50 genes
Lytic or lysogenic
Lambda Life Cycle
Lambda attaches to bacteria, inserts DNA into bacterial cell
Then undergoes lytic or lysogenic pathways
Lytic Pathway
Replication of viral chromosome, synthesis of viral proteins, lysis (viral proteins burst out and infect other cells)
Lysogenic Pathway
Viral DNA fuses into bacterial chromosome (now considered a prophage)
Inactive in this state
Whenever bacteria divide, viral DNA is also replicated
If there is not a dense population of bacteria at the time of infection, virus might not want to lyse itself because there aren’t very many hosts, so it waits until there are more hosts and then lyses itself
Genetics of Bacteria
Circular genomes of several million base pairs
Do not undergo meiosis, do not have chiasmata (but bacterial recombination is still possible)
Gene transfer in bacteria is unidirectional
Episome
Circular DNA that can integrate into bacterial chromosome for replication or remain separate
Type of plasmid
Plasmid
Small circular DNA that can replicate independently of bacterial chromosome
Antibiotic Resistance
When bacterial cells are stressed/dying, proof reading for DNA loosens, allows for mutation to occur
Auxotrophs
Missing gene that transcribes an essential protein
Ways Bacteria Exchange Genetic Material
Transformation
Conjugation
Transduction
Transformation
Uptake of free DNA
Recombination between bacterial chromosome and DNA segment (free DNA now in bacterial chromosome), can now be replicated
Sensitive to DNase
Conjugation
Direct transfer of DNA from one bacterial cell to another
Donor transfers bacterial chromosome into repent
Donor replicates its chromosome as it is donating (retains chromosome), transfers a copy to recipient
Transduction
Transfer of bacterial DNA by a bacteriophage (virus)
Uses a virus as a vehicle to transfer new genes into host
Head of virus contains viral and bacterial DNA (bacterial DNA accidentally got into virus head from previous host)
When virus infects new cell, it will also give segment of bacterial DNA
U-tube Experiment
DNase added to medium so free flowing DNA can pass through pores are destroyed
2 strains of bacteria (a-, a+) are physically separated by a barrier, barrier has pores small enough that liquid and virus can flow through, but prevents bacteria from flowing through (prevents conjugation)
Added virus to one side, found a- becomes a+, a+ becomes a-, proves that virus can infect cell on one side, carry genes to other side, infect cells on other side
Competent Cell
Cell that is able to undergo transformation
How To Become Competent
If bacterial cell is in environment where other cells are dying, then it wants to find a cell with an allele that allows it to combat the environment (cell with mutant allele will die eventually)
Turns on DNA receptor, allows gene to come in, then bacterial cell can recombine with free fragment
All cells have potential to become competent, but not every cell will unless it thinks that it needs to
Transformation and Distance Between Genes
If 2 genes are very close, they will be present on the same piece of transforming DNA
Fertility/F Factor
Episome
Mediates conjugation
Can exists as an extra chromosomal DNA or it can insert into the bacterial chromosome and replicate as part of it
Contains genes required to build conjugation channel and pilli
Donor cell must contain F+
Conjugation Channel
Connects donor and recipient cells
F+ x F-
F+ replicates itself and transfers F+ into recipient cell (so both cells are F+)
Hfr H Cell x F-
Hfr H (integrated F factor) integrated into bacterial chromosome
F factor allows cell to build pills, conjugation factor, wants to be transferred into F- cell, by doing so also transfers the rest of the chromosome because it is attached to it
Conjugation bridge usually breaks during transfer (very fragile)
Generalized Transduction
Random fragment of bacterial DNA is packaged in phage head by mistake along with phage DNA
Specialized Transduction
Prophage excises imprecisely from chromosome and produced a phage chromosome containing only adjacent bacterial genes
