Lecture 9 Flashcards
What are some reasons why DNA take up can be beneficial to bacteria?
*nutrient source
*repair of damaged chromosomes
*drives genome evolution
_________is the transfer of genes/genetic information from cell to cell
Horizontal gene transfer
What is the rearrangement of genetic material, especially by crossing over in chromosomes or by the artificial joining of
segments of DNA from different organisms?
Recombination
What are 3 mechanisms of horizontal gene transfer?
1.Conjugation
2.Transduction
3. Transformation
Who discovered recombination
and conjugation in 1946?
Joshua Lederberg
What did Joshua Lederberg mix to obtain recombinant prototroph?
Auxotrophs
What is an auxotroph?
a mutant unable to synthesize an
essential nutrient, which has to be
supplemented externally for growth
What is a phototroph?
a strain able to synthesizes all
necessary nutrients
What is conjugation?
the transfer of DNA
from one bacterium to
another, requiring cell-to-cell
contact
What is typically used to initiate conjugation from the donor cell?
sex pilius
What does conjugation require?
Special transferable plasmids
What do transferable plasmids contain?
all the genes needed for pilus formation and DNA export: tra genes, relaxase
What are the two replication origins in the fertility factor (F factor) of E.coli?
- oriV: used in nonconjugating cells
- oriT: used during DNA transfer
What is the donor cell called?
F+
What is the recipient cell called?
F-
Characteristics of the Relaxosome complex include:
- Nicks one strand at the
nic site - composed of Tra
proteins encoded by the
tra genes on the F
plasmid
Nicked strand of the F
plasmid is transferred to
the F- recipient by what?
A mechanism resembling
rolling-circle plasmid
replication
What is the cell designated as when the F-factor plasmid is integrated into the chromosome?
Hfr (high frequency recombination) strain
An Hfr cell is capable of
transferring parts of the
chromosome into a
recipient cell:
*Genes are transferred in
order
* The entire chromosome
takes about 100 minutes
to transfer
True or False: Portions of the transferred
donor DNA can recombine
with the recipient chromosome
True
What does aberrant excision result in?
F’ factor or F’ plasmid that carries genes
True or False: An integrated F-factor can excise from the chromosome.
True
What does Agrobacterium tumefacians contain that causes tumors in plants?
a Ti plasmid that can
be transferred via conjugation to plant cells
True or False: Scientists also discovered that some genetic transfer also occurs from humans to bacteria
True
What is the process in which bacteriophages carry host DNA
from one cell to another.
Transduction
True or False: Transduction occurs accidentally as a result of the phage packaging host genomic DNA into the capsid
True
What type of transduction can transfer any gene from a donor to a recipient cell?
Generalized transduction
What type of transduction can transfer only a few closely linked to
the phage insertion site between cells?
Specialized transduction
What is the process of importing free DNA into the bacterial or archaeal cell?
Transformation
What are types of cells that can take up foreign DNA?
Competent cells
What bacterium can extend a type IV pilus, which can
actively take up free DNA in the environment?
Vibrio Cholerae
Characteristics of the type IV pilus in vibrio cholerae include:
- The pilus spans the
inner and outer
membranes to access
the environment. - Details of the
homologous
recombination event are
not well understood and
not shown
Other gene transfer methods include:
- Membrane vesicles
- Nanotubes
- Gene transfer agents
Bacteria evolved to have restriction and modification systems to prevent the free exchange of DNA. These systems include:
- Enzymatic cleavage (restriction)
of alien DNA, by restriction
endonucleases - Protective methylation
(modification) of host DNA
_________ is an adaptive phage immunity system found in bacteria and archaea.
CRISPR (clustered regularly interspaced short palindromic repeats)
CRISPRs consist of _________that do not encode proteins, but near them lie CRISPR-associated gene families that do encode proteins
spacers and repeats
The function of CRISPR can be divided into three stages:
- Foreign DNA (spacer) acquisition
- crRNA processing
- RNA-guided targeting (effector)
_________ move from one DNA molecule to another.
Transposons
What can transposons do?
- Exist in virtually all life forms
- Can move within and between chromosomes (transposition)
- Carry genes (cargo) other than those required for transposition
An ___________ contains simple transposable elements containing a transposase gene,
flanked by short, inverted repeat sequences that are targets of the
transposase enzyme.
Insertion sequence (IS)
Antibiotic resistance gene serves as a __________.
selected marker
True or False: A transposable element will randomly insert in the host genome.
True
Random transposon insertions generate a collection of mutant bacteria referred to as a ___________.
Transposon mutant library.
How do genomes evolve?
Through gain and loss of genes
The fundamental processes that contribute to genome
restructuring and evolution include:
- Horizontal gene transfer
- Duplication via homologous recombination followed by
functional divergence through mutation - Genome reduction
____________ are acquired by horizontal gene
transfer.
Genome islands
Examples of genome islands include:
- Pathogenicity islands
- Symbiosis islands
- Metabolic islands
- Resistance island
How can genomic islands be identified?
Guanine (G)/ Cytosine (C) content
What is the most important mechanism for generating new genes?
Gene duplication
What happens when a gene is duplicated?
frees a gene from its
previous functional constraints and allows divergent evolution through mutation
True or False: Superfamilies of proteins arising from divergent evolution share structural and functional features but may catalyze different reactions
True
What are genes with shared ancestry have sequence similarity, or homology?
Homologs
What are homologs found in different species that serve the same function?
Orthologs
What are homologs found in single genome, related but have distinct functions?
Paralogs
What is the large-scale loss of
genes through evolution?
Genome Reduction
Over half of the Mycobacterium leprae genome is composed of nonfunctional ________.
pseudogenes
________ appear to encode an
enzyme but are nonfunctional because a portion is missing as a result of mutation.
Pseudogenes
