Microbio Test 2 Study Guide

chromosome

main genetic element in prokaryotes and eukaryotes

plasmids

- nonessential genetic material

- circular, double stranded DNA

- 1-5% the size of a bactierla chormosome

- replicate "independently" of the chromosomes

- widely exploited in genetic engineering

vertical gene flow

binary fission or asexual reproduction

dna from parent cell to offspring

binary fission steps

1) DNA is replicated

2) cell membrane elongates, DNA is separated

3) cell wall forms, separating "daughter cells"

4) cell wall is complete

5) 2 daughter cells with identical DNA

how is DNA transferred between different microbes?

1) transformation

2) conjugation (horizontal gene flow)

3) transduction

transformation (not common)

- prokaryotes can take up "free" DNA from the environment

- genome is released during death of previous host

transformation steps

1) donor cell lyses and DNA fragments are released

2) recipient cell takes up the donor cell DNA

3) donor DNA aligns with complementary bases

4) donor cell DNA is recombined into recipient cell DNA

5) unrecombined donor cell DNA is degraded

conjugation key requirements

1) required a donor cell and recipient cell

2) direct cell-to-cell contact

this mechanism transfers a plasmid, not the genome

process is very common - more common in gram neg tahn gram pos

occurs betwen closely related bacteria

types of plasmids - conjugative plasmids

carry genes for sex pili (used by gram neg. bacteria) and transfer of the plasmid between different bacteria (F and R plasmids)

- F Plasmid

- R Plasmid

F plasmid

can carry genes crucial to cell survival/growth in diverse, extreme or specialized enivronemnts

2 examples of conjugative plasmids

F plasmid

R plasmid

R plasmid

contain a resistance factor (R factor) that encodes antibiotic resistance

all r plasmids contain R-determinant factors

- genes that code for antibiotic resistance to one specific antibiotic (ex. enzyme production that inactivates certain components within an antibiotic)

What do all conjugative plasmids share?

- transfer genes (TraJ)

- origin of transfer (OriT)

transfer genes (TraJ)

activation of these genes results in :

- growth of pilus (gram neg)

- fusion of outer membranes (cell to cell contact - needed to transfer genetic info)

F+ cell

donor cell

contains F or R plasmid

F- cell

doesn't contain F or R plasmid

conjugation in gram neg bacteria

1) cell to cell contact must be achieved (plasmid carries transfer genes that code for the synthesis of a sex pilus)

donor cell (F+) produces a sex pilus and binds to the receptor on the recipient cell (F-)

sex pilus retracts in cell-to-cell contact between the F+ and F- cell

2) preparing for the transfer of DNA

a channel forms between F+ and F- cell

one strand of the plasmid (F+ cell) is nicked, and this strand begins to pass through the channel to the F- cell

3) transferring the DNA

the remaining single plasmid strand in the F+ plasmid cell is replicated

once the single strand enters the F- cell it is also replicated

4) cells separate

after the DNA is transferred to the F- cell, the two cells will separate

the F- recipient cell will now be an F+ cell (if all DNA was transferred and replication)

conjugation in gram pos bacteria

1) cell to cell contact must be achieved

"pheromone induced conjugation"

- recipient cell release sex pheromones

- donor cells produce surface proteins that cause cells to "stick" together

2-4 (same as gram neg)

closer look at replication in conjugation F+ cell

transfers one strand of the plasmid to the F- cell

DNA polymerase will bind to the primer and replicate the DNA, so it will become double stranded

closer look at replication in conjugation F- cell

after it receives the single strand of DNA, DNA polymerase will bind to the primer and replicate the DNA, so it will become double stranded

episomes

plasmids that are integrated into the chromosome

plasmid transfer

entire plasmid is transfererd

no chromosomal DNA is transferred

episome transfer

some or all of the plasmid and some or all of the chromosome can be transferred

genes can be inserted into the host chromosome via recombination

Hfr cells

high frequency of recombination

donor cells

conjugation steps for Hfr

1) Hfr and F- cells achieve cell to cell contact (via pilus)

2) single strand of integrated F plasmid is cut

3) donor genetic info is replicated while transferring a single DNA strand to F- cell (F plasmid followed by chromosome is transferred to recipient)

4) recipient replicates donor partial F plasmid and chromosome

5) cells spearate

6) recipient contains: orginal chromosome, part of F plasmid, part of donor chromosome

7) some of transferred DNA is incorporated into recipient chromosome (recombination)

8) recipient cell is referred to as a recombinant F- cell

9) unincorporated DNA is broken down

What is the fate of the DNA transferred from the donor (Hfr) to the recipient (F-)?

1) degraded and lost form the cell

2) recombination with host chromosome

plasmid complexity

- Plasmids take energy to maintain

- Plasmids are subject to loss during cell division

importance of gene transfer in evolution

mutations and recombination create cell diversity

diversity is the raw material for evolution

natural selection acts on populations of organisms to ensure survival of organisms fit for a particular environment

virus

-unable to replicate viral components by itself

- when a virus infects a cell, it can avquire metabolic properties (viruses have no metabolic capacities)

- viruses have genomes but no ribosomes (depend on machinery of other cells for protein synthesis)

- contains either DNA or RNA

what fo viruses rely on the host cell for?

protein synthesis

what are viruses

obligate intracellular parasites that rely on a living cell for survival

virus suffix

viridae

bacteriophages

bacterial viruses

animal viruses

viruses that infect animals and humans

nucleocapsid

nucleic acid (genetic information) and protein shell (capsid)

capsid

protein shell surrounding nucleic acid

capsomere

protein subunits

rod

helical symmetry

spherical

icosahedral symmetry

bacteriophages life cycles

lytic

lysogenic

lytic cycle

phage causes lysis and death of the host cell

lysogenic cycle

phage DNA is incorporated in the host DNA

bacterial virus life cycle - lytic phase

5 steps:

1) attachment

2) penetration/entry

3) synthesis

4) assembly

5) release

attachment

virion is adsorbed to a susceptible host (via surface receptors)

penetraiton

penetrate bacterial cell with sheath using lysozyme and inject nucleic acid into the host (host DNA is broken down)

biosynthesis

viral nucleic acid and proteins are replicated by the host cell metabolism (redirected by virus)

assembly

viral capsids are assembled, adn the viral genome is packaged (form new viruses)

release

virus exits the host cell by releasing lysozyme (becomes a virion)

lysogenic cycle big picture

- phage DNA is incorporated into the DNA of the bacterial cell (referred to as a prophage)

- as the host cell replicated its chromosome, prophage DNA is also replicated

- can result in "phage conversion"

- host cell exhibits new properties (ex production of toxins)

lytic cycle steps

1) phage attaches to host cell receptors and injects DNA

2) phage DNA circularizes and enters the lytic cycle

3) new phage DNA and proteins are synthesized and assembled

4) cell lyses, releasing mature phages

lysogenic cycle steps

1) phage attaches to host cell receptors and injects DNA

2) phage DNA circularizes and enters the lytic cycle

3) phage DNA integrates within the bacterial chromosome by recombination, becoming a prophage

4) lysogenic bacterium reproduces normally which results in a phage conversion

5) occasionally, the prophage may excise from the bacterial chromosome by another recombination event, initiating a lytic cycle

closer look at bacteriophages

- only infect bacterial cells to ensure their own survival

- production of the capsid (and other structures)

- replication of genetic information

- can also increase bacterial genetic diversity

how can a virus be involved genetic transfer between two different bacteria?

transduction

transduction

mediated by bacteriophages

random bacterial DNA is packaged inside a phage and transferred to a recipient cell

transduction steps

1) phage DNA enters bacterial host #1

2) following the phage infection, host DNA is degraded into small fragments

3) phage capsid proteins are produced & phage DNA is replicated

4) some phage capsids envelop bacteria DNA instead of phage DNA

5) phages are released and infect a second bacterial host. some of these virions contain bacterial DNA only

6) phages carrying bacterial DNA from host #1 enters bacterial host #2

7) bacterial DNA from host #1 can be integrated into the chromosome of bacterial host #2

8) bacteria undergos binary fission transferring the genetic material from bacterial host #1 to offspring

bacteria virus vs animal virus

bacteria virus: contains tail fibers

animal virus contains glycoproteins

two types of animal viruses

naked and enveloped

naked animal virus

contains capsomers, nucleocapsid (nucleic acid and capsid), and glycoproteins

enveloped animal virus

contains envelope, nucleocapsid (capsid and nucleic acid)

viral DNA

DNA -> RNA -> protein

viral RNA

all generate mRNA -> protein

positive-strand RNA

simliar to mRNA (5' to 3')

can be immediately translated by host cell

minus-strand RNA

complement to mRNA (3' to 5')

must be transcribed to plus-strand RNA (RNA replicase) prior to translation

animal virus entry strategies

1) endocytosis

2) fusion

endocytosis

viral glycoproteins binds to host cell receptors and the virus is completely brought into the cell

BOTH enveloped and naked viral entry

fusion

viral glycoproteins binds to host cell receptors and the virus releases its genome into the host cell

ONLY enveloped viral entry

naked animal virus exit

exit by lysis (pressure)

enveloped animal virus exit

exit by budding out

exiting steps for animal virus (budding out)

1) virus locates viral glycoproteins on the animal cell membrane

2) viral glycoproteins bind to the host cell membrane receptors

3) when the virus leaves the cell, it contains part of the host cell membrane (most) and viral glycoproteins (i.e. it is enveloped)

+ strand RNA animal viruses (3 examples)

poliovirus, rhinovirus, hand foot mouth virus

what cell does a poliovirus attach to?

nerve cells

poliovirus

most cases have minor symptoms (sore throat, fever)

can cause paralysis (

life cycle of a poliovirus

1) poliovirus circulates in the bloodstream and binds to nerve cell receptors

2) poliovirus RNA enters the cell via endocytosis

3) poliovirus RNA takes over the host ribosome and forces the host to only make viral capsids and copies of viral RNA

4) + strands of RNA will fill viral capsids (mature virus)

5) host cell swells and bursts (lysis) releasing the newly formed viruses to the bloodstream (to infect other cells)

goal of virus

make capsid

fill capsid

poliovirus enters the host cell with: (2 things)

plus strand RNA (starting genetic information)

VPg protein (primer specific to picornoviruses)

inside host cell, poliovirus produces: (4 things)

- copies of + strand RNA (starting genetic information)

- copies of RNA replicase (synthesis of RNA strand)

- protease (protein cleavage)

- proteins (produce capsids)

role of RNA replicase

synthesize RNA strands (from + to - or - to +)

needed and produced by - strand RNA viruses

enters cell with: viral RNA replicase

once in the cell, the virus produces:

- copies of RNA replicase

- copies - strand RNA

- proteins (to make capsids)

- strand RNA animal viruses

rhabdovirus family

> 30,000 cases of rabies/yr worldwide

HIV genus

Lentivirus

HIV classification

retrovirus

HIV genome

+ strand RNA

retrovirus viral family

retroviridae

retrovirus starting genetic information

RNA

retrovirus uniting factor

use of viral reverse transcriptase enzyme

how many retrovirus do we know of?

4

retrovirus exmaple:

HIV (human immunodeficiency virus)

HIV infection means

attack of the body's immune system

can lead to AIDS (acquired immunodeficiency syndrome)

HIV transmission

sexual contact, breast milk, infection of a fetus, blood-contaminated needles, organ transplants, artificial insemination, and blood transfusion

HIV can (survivability)

survive 6 hours outside a cell

survive >1.5 days inside a host cell

HIV-1

99% of all HIV cases

originated from viruses that infect chimpanzees and gorillas

HIV-2

typically occurs with Weset AFrica

less pathogenic than HIV-1

longer asymptomatic period with lower viral load and mortality rate than HIV-1

HIV most common mode of transmission

heterosexual

response to HIV exposure varies

impact of age on survival with HIV infection

- older adults and young children are more susceptible (weakened or underdeveloped immune systems)

Long-term survives: maintain low viral load—> no/low symptoms —> no detectable in body… done through antiviral drugs…

cell receptor mutation for HIV

CCR5 mutation

if mutation occurs, HIV can't bind to cell because receptors aren't present

What cells do HIV target?

CD4+ T cells

3 phases of HIV

1. acute infection - during this time, large amount of the virus are being produced in your body, many people develop flu-like symptoms often described as the "worst flu ever"

2. chronic latency - HIV reproduces at very low levels, still active, may not have symptoms

3. AIDS - Start to feel sick again… opportunistic infections occur, patient is immunocompromised (CD4 cells fall below 200 cell/mm^3) —> Immune system can no longer fight infection

HIV characteristics

contains + strand RNA

enveloped virus

HIV enters with what? (3 things)

viral reverse transcriptase

viral integrase

viral protease

genome (+ strand RNA)

HIV attachment mechanism

viral gp120 (glycoprotein) attaches to a CD4+ receptor on the host T helper cells

viral gp120 fuses with the host cell (leaves envelope behind) and genetic information (HIV) released and enters the cell (fusion)