Lecture 13 Flashcards
What are mobile genetic elements?
nucleic acid segments (DNA or RNA) that can move in and out of cells or between different regions of the cell genome
How do mobile genetic elements affect host cells?
- some of these elements become integrated into the “host” genome and are replicated and expressed along with host cell genes
- some replicate independently of the host genome
- some can do both
Which mobile genetic elements require host cell of replication?
all, they exploit the host cells’ machinery and metabolism to propagate
How can mobile genetic elements be used as tools to study cell machinery?
Used as vectors
Are Mobile genetic elements evolutionarily related or not
Yes they are
3 types of mobile genetic elements?
Plasmids, viruses, transposable elements (transposons)
Describe plasmids
self-replicating (using host proteins), extra-chromosomal double-stranded DNA circles found in bacteria, yeast and fungi
plasmids lack a protein coat and generally cannot move independently from cell to cell (but can move from one cell to another by cell “conjugation”)
Describe viruses?
self-replicating (using host proteins), infectious DNA- or RNA-containing elements that possess a protein coat and can move from cell to cell
Describe transposable elements (transposons)
mobile DNA elements that lack a coat and can insert into and move around the host genome via recombination
Are plasmids circular or linear DNA?
circular dna
How big are plasmids
Small <10kilobase pairs
Where are plasmids commonly found
Bacteria
How many copies of plasmids are there per cell?
2-50 (low vs.high #)
Are plasmids chromosomal or not?
normally do not integrate into the host genome (are extra- chromosomal) – replicate independently but use host proteins for replication
What do plasmids encode?
encode functions that are dispensable to the host, but may impart a selective advantage, e.g., antibiotic resistance
can carry genes for toxins, antibiotic resistance, catabolism
of unusual substrates
What is plasmid-mediated conjugation?
conjugative transfer of a plasmid from one cell to another - requires F-pilus to bring cell membranes together
How is replication of plasmid similar to that of host chromosome replication
plasmids have one origin of replication - sequence that allows initiation of replication
Which of these can plasmids do: sometimes plasmids can recombine with each other or with host cell chromosome, can integrate into host cell chromosome
Both
What four cell properties can be conferred by plasmid genes?
Drug (antibiotic resistance), virulence factors, metabolic activities, chromosome transfer
Describe drug (antibiotic resistance)
gene for an enzyme that can inactivate a drug (e.g., B-lactamase inactivates
B-lactam antibiotics like ampicillin and penicillins that inhibit bacterial growth)
gene for a variant protein which is unaffected by a drug (i.e., can substitute for the protein that is inactivated by a given antibiotic but is not itself inactivated)
Describe virulence factors
may contribute to or be essential for the virulence of a pathogenic bacterium
e.g., Bacillus anthracis and Clostridium tetanus toxins are encoded on plasmid genes
Describe metabolic activities
e.g., nitrogen fixation (N2->NH3) by
Klebsiella
e.g., degradation of octane by
Pseudomonas
Describe chromosome transfer?
conjugative plasmid integrates into the host chromosome and mobilizes the chromosome or parts of the chromosome for transfer to another cell
How do bacteriophage (phage) genomes comparable in size to plasmids
Bacteriophage (phage) genomes can be comparable in size to plasmids or can be much larger.
Phage genomes and plasmids possess many of the same properties, suggesting?
A close evolutionary relationship, plasmids are related to bacterial virus (bacteriophage)
Major difference between bacteriophage and plasmids?
phage/viral nucleic acid encodes viral coat proteins that allow it to be “packaged” and released from the host cell to infect another host cell. Plasmids are only found intracellularly – the do not get packaged as infections agents
What are viruses?
Small intracellular parasites
What do viruses require to propagate?
Host metabolic and biosynthetic machinery
What can viruses infect?
Bacteria (these viruses called bacteriophage), plants, animals
What allows viruses to move from cell to cell?
an RNA or DNA genome surrounded by a protective, virus-encoded protein coat
What can happen when viruses overtake cellular machinery for their own reproduction?
- often cause disease –
e. g., polio virus, influenza virus, human immunodeficiency virus (HIV)
How big are viruses?
can be extremely small (e.g., <100 nm in diameter)
What happens to the cell when viruses multiply
often lethal to the cell – overtakes, lyses cell
How do viruses vary?
- type of nucleic acid in genome, structure of viral chromosome
- structure of coat
- presence of membrane “envelope”
- mode of entry into or exit from the host cell
- site and mechanism of replication
What kind of genome types can viruses have?
DNA, RNA, single-stranded, double-stranded, linear, circular, segmented.
How do viruses structures vary in terms of nucleic acid?
Either RNA or DNA genome
What is the coat protein on viruses fxn?
Surrounds and protects the nucleic acid
What is the fxn of nucleoprotein in viruses? How do they vary
Packages the viral genome
- Not on all viruses
How do viruses structures vary in terms of envelope
- some viral coats are surrounded by a lipid bilayer (envelope)
- envelope is derived from the host cell membrane
- envelope may contain viral-encoded glycoproteins on its surface
Viral proteins are encoded by ___ but synthesized by ____
- the viral genome
- the cellular machinery
The virus is packaged in the cell (genome + proteins) – enveloped viruses bud off the cell and get coated in the cell membrane -> envelope.
.
What is lytic phage genome?
does not integrate into the host chromosome. It is replicated extra-chromosomally. Assembled phage
often lyse their host cell when they are released, killing the cell.
13: Mobile genetic elements
What is lysogenic phage genome?
becomes integrated (lysogenizes) into the host cell chromosome (via site specific recombination), is replicated along with the bacterial (or other host) chromosome, and is passed on to daughter cells. (Integration/excision occurs via site-specific recombination, utilizes integrases/recombinases.)
Lytic phage that infect E. coli?
T4 phage
Nickname for transposable elements
“jumping genees”
What are transposable elements?
mobile DNA segments that can “jump” or transpose from one site on the chromosome to another
Size of transposable elements?
transposons range in size from 100’s to ~104 bps
How many transposable element per cell?
Multiple copies
What do transposable elements encode?
multiple proteins including transposases - enzymes that catalyze transposition
What is transposition
transposition moves transposable elements from one site to another random site in the genome
How often does transposition occur?
transposition events are tightly regulated, occur infrequently (random transposition can be lethal to cell)
How does transposition differ from homologous or site-specific recombination?
transposition is a form of genetic recombination that is distinct from homologous or site-specific recombination - DNA homology is not usually required
What cells are transposons found in?
All cell types
How do transposons spread
transposons can spread from one cell to another, carried on plasmids, to confer new properties on the acceptor cell (e.g., antibiotic resistance in bacteria)
What are the 2 types of transposons in bacteria?
Direct (simple) and replicative
What is Direct (simple) transposition
Direct (simple) transposition - the transposon segment is moved from the donor site to the target site
Role of transpose enzymes in direct (simple transposition)
Transposase enzymes recognize inverted repeat segments in DNA (terminal repeats), brings them together, excising the transposon that lies between the repeats.
What is replicative transposition
Replicative transposition – the transposon is copied and inserted at a separate site in the DNA
- the entire transposon is replicated to produce a “cointegrate” intermediate, in which the donor and target DNA are covalently linked
- the cointegrate is resolved with the aid of a separate site-specific recombination system
