chapter 6 part 2 Flashcards
transformation
occurs when recipient cell takes up a fragment of donor DNA from the surrounding growth medium
is transformation naturally occurring or induced in a laboratory
both
how is transformation used in the laboratory
to introduce DNA into microbial cells, plant cells, and animal cells
length of transforming DNA
usually shorter than 100 kb
- in E. coli can contain up to 50 genes, representing at most only 1-2% of total genome of recipient
what is transformation useful for
mapping genes that are closely linked
cotransformation
simultaneous transformation of two or more genes
what precedes natural transformation
lysis
lysis
breakage of a donor cell and fragmentation of the DNA of the donor
how many steps in transformation
4
steps in natural transformation
- lysis
- passage of DNA into a recipient accompanied by degradation of one of the strands
- remaining strand aligns with complementary regions of the recipient chromosome
- alignment of donor/recipient DNA triggers excision of one strand of recipient DNA and replacement with donor DNA
heteroduplex
when alignment of donor/recipient DNA triggers excision of one strand of recipient DNA and replacement with donor DNA
products of transformation
- one daughter cell is transformed cell (transformant)
- other daughter cell retains the recipient chromosome original DNA strand (unchanged)
transformant
transformed cell
steps of lytic cycle
- attachment of phage to host cell
- injection of the phage chromosome into the host, followed by circularization of the phage chromosome
- replication of phage DNA using host proteins and enzymes
- transcription/translation of phage genes, production of phage components
- packaging of phage chromosomes into phage heads
- lysis of host cell, release of progeny phage particles
temperate phages
have alternate, temporary life cycle involving integration of the page chromosome into the bacterial chromosome
does the lysogenic or lytic cycle use temperate phages
lysogenic
lysogenic cycle steps
- attachment/injection same as lytic
- integration of phage chromosome into host chromosome via recombination at specific DNA sequence found in both chromosomes
- excision of prophage in response to an environmental signal, through reversal of site-specific recombination leading to integration
- resumption of lytic cycle, beginning with phage-chromosome replication
bacterial transduction mediated by bacteriophages
transfer of genetic material from donor to recipient by way of a bacteriophage
- phage accidentally incorporates a fragment of donor DNA rather than copy of phage chromosome
how small are bacteriophage particles
less than 1% the size of the bacteria they attack
composition of bacteriophage
- icosahedral head
- hollow protein sheath
- set of tail fibers
what does the head of a bacteriophage contain
single chromosome from 5,000 to 100,000 base pairs
what does replication and gene expression of bacteriophages require
enzymes and factors of the host cell
what does the transfer of genetic material from a donor bacterium to a recipient via a phage lead to the production of
transductant bacterial strain
2 types of transduction
- generalized transduction
- specialized transduction
generalized transduction
transfer of random bacterial genes, carried out by bacteriophages that can’t distinguish between phage and bacterial DNA during packing of DNA into new phage particles
specialized transduction
carried out only by temperate phages and the only genes transferred are those close to the integration site
generalized transduction steps
- normal P1attaches to donor bacterium and injects DNA into the cell
- replication of phage chromosome followed by transcription/translation to produce phage proteins
- some phages receive host DNA fragment instead of phage DNA
- host cell lysis releases all the progeny phages
- generalized transducing phages attach to new recipient cells and inject DNA
- homologous recombination may occur between donor fragment/recipient chromosome
- stable transductant strain results
generalized transacting phage
phages that receive host DNAA fragment instead of phage DNA
- (random selection of DNA fragments based upon size alone)
ex. of temperate phage
lambda phage
where do lambda phages integrate into host chromosomes
at specific sites
integration site in bacterial host chromosome
att site
- same 15-bp sequence
integration site in lambda phage
attP
integration site in E. coli host
attB
how similar are the attB and attP sites
identical
because attB/attP are identical, excision of prophage is usually…
exact reversal of the integration
possible error in excision of specialized transduction
only part of prophage is removed, and along with it, some of nearby bacterial DNA
what does aberrant excision produce
specialized transducing phage
what genes are found on either side of attB site in E. coli
galK and bioA
(lambda)dgal+ specialized transducing phage
includes galK gene
- can infect host cells but lacks genes needed to complete either lysis or lysogeny
(lambda)dbio+ specialized transducing phage
includes bioA gene
- can induce lysis but lacks genes needed for lysogeny
lateral gene transfer
transfer of genetic material between ind. bacteria or archaea and other organisms
studies suggest that more than ____% of genes in a genome are the result of LGT
12
what kind of genes are more likely to undergo LGT
genes encoding for cell surface proteins, DNA-binding proteins, and proteins related to pathogenicity
where does LGT occur
among bacteria and between bacteria and eukaryotes
evolutionary example of LGT
endosymbiosis that led to mitochondria and chloroplasts in eukaryotes
another ex. of LGT
LGT of 10,000-30,000 base pairs from bacterium Agrobacterium tumefaciens to plants
how is LGT identified
presence of DNA-sequence features in regions of a genome that are uniquely different from the rest of the genome
genomic islands
regions of genome that are uniquely dif from rest of genome
are genomic islands usually large or small
large (10-200 kb)
- often include multiple genes
2 common characteristics of genomic islands
- group of genes much more like genes of distantly related species than closely related species
- ratio of G-C and A-T substantially different than rest of genome
LGT medical importance
- allowed for rapid organism adaptation to changing environmental conditions by acquiring antibiotic resistance
- can lead to acquired pathogenicity islands
pathogenicity islands
contain genes for proteins that promote host invasion and genes that produce toxic compounds
ex. of pathogenicity islands in certain strains of E. coli
can cause diarrhea and meningitis
