viruses and bacteria genetics Flashcards
what makes bacteria valuable research objects
- small size
- rapids reproduction
- selective media that can easily identify the presence of an active allele
- simple physiology (monoploid)
- genetic variability
bacteriophages
- bacterial viruses
-reproduce by infecting bacterial cells
colony
population that derives from a single bacterium, all have the same genotype
dilution series
help pick out a single colony, easier to do when they are more spread out
- will isolate same species of bacteria that has genetic variation
plaques
bacteriophages produce clearances on plates with dense bacterial cultures within hours of infection
Bacteriophage T4
docs to surface of bacteria and injects its genetic material into the bacterium
- virus itself does not enter the cell
- protein coat (especially in virus head) envelopes genetic material
- goes through a lytic phase - lyses the cell to infect other cells
Bacteriophage lambda
- may be lytic or lysogenic
- genome contains 48,502 base pairs and about 50 genes
retrovirus
- has RNA genome
- less stable than DNA and more prone to degradation since it is single-stranded
- more adaptable because higher rate of mutations
retrovirus cycle
- virus attaches to host cell at receptors in the membrane and viral core enters host cell
- RNA degrades and reverse transcriptase synthesizes second DNA strand
- viral DNA enters the nucleus and is integrated in host cell chromosome forming a provirus
- proviral DNA is transcribed to viral RNA which is exported to cytoplasm where it is translated and new material is assembled
lytic cycle
- phage binds to the bacterium and its DNA enters host cell
- host DNA is digested
- phage DNA replicates
- host cell transcribes and translates phage DNA into proteins
- assembly of new phages is complete and are released to start the cycle again
sensor proteins
stimulus that tells prophage from the lysogenic cycle to enter the lytic cycle
- can detect if bacteria is healthy and wants to infect them
lysogenic cycle
- phage binds to the bacterium and its DNA enters the host cell
- phage DNA integrates into the bacterial chromosome (through crossing over) and becomes a prophage
- prophage is replicated as part of the bacterial chromosome
- the prophage may separate from the chromosome and the cell will enter the lytic cycle
prophage
inactive phage that is dormant in the host genome
Genomes of bacteria
- genomes are circular, called bacterial chromosomes
- additional genetic material resides in plasmids and can replicate independently
- episomes are large circular DNA that can integrate into the bacterial chromosome for replication or stay separate
- bacteria do not have meiosis, therefore no chiasmata or homologous pairs
Gene transfer in bacteria
unidirectional - from donor cell to recipient
- never an exchange
how can gene mutations be observed in bacteria
- colony colour and morphology
- nutritional mutants (cannot metabolize certain sugars)
- prototrophs (can make AAs needed) and auxotrophs (can’t make AAs needed)
- antibiotic resistance
how can you isolate a bacterial strain that cannot synthesize leucine
- plate bacteria on a medium containing leucine, both leu+ and leu- grow
- replica plate the colonies by pressing a velvet surface to the plate, cells adhere to velvet
- Press onto new petri plates,
- leu- are recovered from colony
what does a missing colony state
a colony that grows only on the supplemented medium has a mutation in a gene that encodes the synthesis of an essential nutrient
how do bacteria exchange genetic material
through parasexual processes
- conjugation
- transformation
- transduction
transformation
transfer of a free (out of the cell) piece of DNA from one bacterium into another
- sensitive to DNase
- does not require cell contact
conjugation
direct transfer of DNA from one cell to another via the establishment of a cytoplasmic bridge
- not sensitive to DNase (since DNA never touches outside environment)
- requires cell contact
transduction
transfer of genes from one cell to another via bacteriophage
- not sensitive to DNase (since virus head is protecting DNA)
- does not require cell contact
U-tube experiment
- asks how did the genetic exchange of leucine take place
- 2 different strains of bacteria separated by a barrier that allowed mixing of medium but not bacteria
- since no prototrophic bacteria were produced we conclude genetic exchange requires direct contact between bacteria cells
plasmid
an extra-chromosomal circular piece of DNA that can replicate independently of the chromosome
- not required for the survival of host cell and serve as auxiliary genetic elements
R plsdmid
resistant plasmids
F factor
fertility factor - an episome
epsiome
a genetic element that is not essential to the most and that can either replicate autonomously or be integrated into the bacterial chromosome