07 - microbial genetics - finished slide 26

add slide 15, 17, 26; finish card 54, and 55; ADD NOTION

what do resistance (R) plasmids contain?

they contain resistance (R) genes that confer resistance to multiple antibiotics and heavy metals

how are resistance plasmids transferred between bacteria?

they are transferred quickly, often via conjugation, allowing rapid spread of antibiotic resistance

what additional genetic elements may resistance plasmids contain?

they may contain transposons, which also carry antibiotic resistance genes

can resistance plasmids transfer between different bacterial genera?

yes, they can transfer between closely related genera such as Escherichia, Klebsiella, Salmonella, Serratia, Shigella, and Yersinia

what is vertical gene transfer?

gene transfer from mother to daughter cells

what is horizontal (lateral) gene transfer?

gene transfer between unrelated cells

why does lateral gene transfer complicate prokaryotic taxonomy?

because organisms with similar genes may not have evolved from a common ancestor

does prokaryotic reproduction require gene exchange?

no, prokaryotes reproduce asexually by binary fission or budding

how does eukaryotic reproduction differ from prokaryotic reproduction?

many eukaryotes reproduce sexually, requiring meiosis and fusion of gametes

how much DNA is transferred during prokaryotic gene exchange compared to eukaryotic sexual reproduction?

only a relatively small amount of DNA is transferred in prokaryotes; in eukaryotic sexual reproduction, all DNA mixes during gamete fusion

when is transferred DNA in prokaryotes propagated to progeny cells?

only if it integrates into a chromosome, integrates into a plasmid, or is a plasmid itself

what must happen for transferred DNA in prokaryotes to be inherited by progeny?

it must integrate into a chromosome or plasmid, or be a plasmid itself

what is a double cross-over (double recombination) event?

a type of homologous recombination that requires the RecA protein

what happens to DNA that is not part of a stable replicon in prokaryotes?

it is degraded by nucleases in the cytoplasm

what is the ploidy difference between most higher eukaryotes and prokaryotes?

most higher eukaryotes are diploid, but prokaryotes are haploid.

how does haploidy in prokaryotes affect the expression of alleles?

recessive alleles or mutations are expressed, which streamlines genetic studies.

how does genetic exchange occur in eukaryotes?

only through gamete fusion.

what are the three mechanisms of gene exchange in prokaryotes?

transformation, transduction, and conjugation

what is transformation in prokaryotes?

gene transfer by taking up soluble DNA from the environment (from lysed cells)

what type of cells are able to take up dna from the environment? (transformation)

competent cells

what is conjugation in prokaryotes?

gene transfer that requires cell-to-cell contact; includes R plasmid, F plasmid, F’ plasmid, and Hfr transfer

what is transduction in prokaryotes?

gene transfer by bacteriophages; can be generalized or specialized

what did Griffith discover in his 1928 experiment with pneumococci?

something from the dead S cells “transformed” R cells, making them smooth and virulent.

what did Avery, MacLeod, and McCarty discover in 1944?

the “transforming” material in Griffith’s experiment was DNA

what are competent cells in prokaryotes (transformation)?

cells that can take up DNA from the environment

how common is natural transformation in prokaryotes?

its rare

which bacteria are known to undergo natural transformation?

Streptococcus pneumoniae, Bacillus subtilis, and Haemophilus influenzae

all bacteria can go through the ___ cycle but only some can go through the ___ cycle

lytic; lysogenic

what conditions favor natural transformation?

naturally competent cells found in dense cultures; DNA likely comes from dead or lysed cells

what are two methods for artificial transformation in the lab?

chemically competent cells: Ca²⁺ ions make E. coli take up DNA; electroporation: transient holes in membranes allow DNA entry via high-voltage pulse

why is artificial transformation important?

it is a fundamental technique in recombinant DNA technology

what is transduction in prokaryotes?

gene exchange that occurs due to mistakes by bacteriophages (viruses that infect bacteria)

what is generalized transduction?

any of the host bacterium’s genes can be transferred

what is specialized transduction?

only certain genes from the host bacterium can be transferred

what are virulent phages (transduction)?

phages that always replicate using the lytic cycle

what are temperate phages?

phages (e.g., phage lambda) that can replicate using either the lytic or lysogenic cycle

what happens in generalized transduction?

any bacterial gene can be transferred to a recipient due to an error in phage replication, where bacterial DNA is accidentally packaged into a phage head instead of phage DNA

what is the first step in generalized transduction?

a bacteriophage injects its genome into a host bacterium

what happens after the phage genome enters the host? (what is step 2?)

the phage directs the host to synthesize new phage genomes and coats. sometimes bacterial DNA is mistakenly packaged into phage heads, forming “transducing phages.”

what happens after transducing phages are produced? (what is step 3?)

mature phage particles are released and can infect another bacterial cell

how are genes from a transducing phage inherited by the recipient bacterial cell?

only if the transferred genes are integrated into the recipient’s genome by homologous recombination

why can generalized transduction occur?

errors during phage genome packaging can accidentally include bacterial DNA in phage heads

how does P22 phage package DNA in Salmonella?

multiple P22 genomes are synthesized as long concatemers, then cut at a “pac” site and additional sites to create slightly oversized DNA segments for packaging

why are certain bacterial genes more likely to be transferred by P22?

“pac”-like sites in the bacterial genome make genes near these sites more likely to be packaged into phage heads and transferred

how is a transducing phage produced in specialized transduction? (specialized transduction part 1)

it results from a rare mistake in prophage excision from the bacterial chromosome

what happens in specialized transduction after a transducing phage is produced? (specialized transduction part 2)

the transducing phage infects a recipient bacterial cell, transferring specific genes from the original host

what is a key feature of specialized transduction by lambda phage in E. coli?

the same bacterial genes are always excised during the process

where does lambda phage integrate in the E. coli genome?

attB site

which bacterial genes are adjacent to the lambda phage integration site?

biotin synthesis genes (bio) on one side and galactose degradation genes (gal) on the other

what is required for conjugation in gram-negative bacteria?

direct cell-to-cell contact facilitated by sex pili (conjugation pili) and the tra (transfer) genes

how does conjugation occur in gram-positive bacteria?

pili are not involved; pheromone secretion and cell clumping seem to facilitate gene transfer; mechanism for conjugation is not known

how does the amount of DNA transferred by conjugation compare to transformation or transduction?

conjugation transfers a much larger quantity of DNA

what are common types of conjugation in bacteria?

F plasmid (F factor or Fertility plasmid), Hfr (High-frequency recombination), F’ plasmid, R plasmid (R factor or Resistance plasmid), Vvrulence plasmids

what is the lytic cylce?

what is the lysogenic cycle?

what is the generalized transduction and specialized transduction of mediators?

virulent or temperate phages; only temperate phages

what is the generalized transduction and specialized transduction of dna in transducting particle?

only in host cell dna; phage and host cell dna

what is the generalized transduction and specialized transduction of genes transducted?

any host gene; only genes close to insertion site of prophage

what is step 4 of generalized transduction?

recipient bacterial cell infected by transducing phage acquires genes that originated from the first host bacteriam. these genes will be inherited by progeny cells only if the new genes are integrated into the recipient’s genome by homologous recombination

why might generalized transduction occur?

when a phage accidentally packages fragments of bacterial DNA instead of phage DNA during assembly

what phage provides an example of generalized transduction?

the P22 phage infecting salmonella

how are P22 genomes synthesized inside Salmonella?

as long concatemers

what are concatemers?

tandem copies of the phage genome linked together

what is the “head-full” packaging mechanism used by P22?

the concatemer is first cut at a “pac” site, then at other sites to make DNA segments slightly longer than one genome length; each segment is packaged into a phage head

what happens if the packaging machinery recognizes a “pac-like” site in the bacterial chromosome?

a segment of bacterial DNA near that site can be mistakenly packaged into a phage head, leading to generalized transduction

which bacterial genes are most likely to be transferred during generalized transduction?

genes located close to “pac-like” sites in the bacterial genome

what is specialized transduction

a process where specific bacterial genes are transferred by a temperate phage due to an error in prophage excision

what causes the production of a transducing phage in specialized transduction (Part I)?

a are mistake during prophage excision from the bacterial chromosome that removes adjacent bacterial genes along with the phage DNA

what does the resulting transducing phage carry?

a hybrid DNA molecule containing both phage and specific bacterial genes.

what happens in specialized transduction Part II?

the transducing phage infects a new recipient cell and injects the hybrid DNA

what can happen to the bacterial genes carried by the transducing phage after infection?

they can recombine with the recipient cell’s chromosome, introducing new genetic material

how does specialized transduction differ from generalized transduction?

specialized transduction transfers only specific bacterial genes near the prophage insertion site, while generalized transduction can transfer any gene

the Discovery of Conjugation (1946): Who conducted the experiment?

lederberg and Tatum

what did Lederberg and Tatum mix in their experiment? what was recovered

two different auxotrophs; phototrophs

what is an auxotroph?

a cell that lost its ability to synthesize a certain nutrient due to a mutation

what is a phototroph?

an organism that converts light energy to chemical energy through photosynthesis to create its own food from inorganic substances

when a phage inserts itself into a genome in specialized transduction by lambda phage in e coli, where does it insert

at the attB site

what happens in abnormal excision in specialized transduction by lambda phage

only genes next to the attB site could be potentially transferred using specialized transduction

in generalized transduction, what genes can be transferred?

any of them

F plasmid conjugation in E. coli involves what type of replication?

rolling circle replication

during F plasmid conjugation, what enters the F⁻ cell?

a single strand of the F plasmid

what region on the F plasmid enters the recipient cell first?

oriT

what does an Hfr cell have integrated into its chromosome?

the F plasmid

where does integration of the F plasmid occur in E. coli?

the insertion sequence (IS) site in the chromosome

what can an Hfr strain mediate?

transfer of chromosomal genes into a recipient F⁻ cell

amount of Hfr that is transferred is dependent on what? what is the order of transfer?

time dependent; order of transfer is always the same

in Hfr conjugation, which genes are transferred to the recipient cell first?

genes immediately following oriT

what determines the amount of DNA transferred during Hfr conjugation?

the amount of time the conjugation lasts

what were interrupted mating studies using Hfr conjugation used for?

to map the position of genes on the E. coli chromosome

where are tra genes located?

on the F plasmid

what do tra genes encode for?

conjugation

what is being transferred in F plasmid conjugation?

linear dna (oriT and the closest genes behind it)

in Hfr conjugation, is the whole F plasmid transferred

no; the recipients stay F-

in F plasmid conjugation, is the whole F plasmid transferred

it can if conjugation completes (often does); recipients are F+

what causes F’ conjugation?

imprecise excision of the F plasmid from the chromosome of an Hfr cell

what is an F’ plasmid?

an F plasmid that also carries some chromosomal genes

what happens during conjugation with an F’ plasmid?

the genes on the F’ plasmid are transferred to a recipient F⁻ cell

what is a mutation?

any change in a cell’s DNA sequence

what types of changes can mutations involve?

mall changes (point mutations) or large changes (e.g. deletions, insertions) in DNA

how can mutations be described?

by their phenotypes