**Bio: Microbiology (Bacteria, Viruses) Flashcards
Obligate intracellular parasite
Only able (obligated) to reproduce within (intra) cells Virus
Are viruses alive?
Not alive
Describe viral genome
A given type of virus only contains one type of nucleic acid (in contrast to a true cell which contains not only its genome but mRNA, rRNA, tRNA)
Varies a lot!
DNA or RNA, single or double stranded, linear or circular
Viruses carry very few genes and so it has adaptations to take this into account:
How can virus encode more than 1 protein in a given length of genome?
Rely on host-encoded proteins for transcription, translation, and replication
Virus can encode more than 1 protein by having multiple reading frames
*Where is virus genome? What is the genome made of?
In capsid (protein) head The genome is made of DNA or RNA (nucleic acid)
What is an envelope? Naked virus
What can enveloped viruses infect?
If antibodies to a viral capsid protein are ineffective in blocking infection, what might this indicate about the virus?
Some animal viruses also possess an envelope that surrounds the capsid.
Enveloped viruses can ONLY infect animals not humans
This is a membrane on the exterior of the virus derived from the membrane of the host cell. It contains phospholipids, proteins, and carbs from host membrane and proteins encoded by the viral genome
Enveloped viruses acquire this through budding through the host cell membrane
Naked virus = Virus without envelope
This may indicate the virus is enveloped, so the Ab cannot reach its epitope on the capsid surface in infectious virus
Is viral infection specific?
Yes! A virus binds to a specific receptor on the cell surface as the first in infection
Attachment or adsorption
Eclipse
Attachment or adsorption - binding to exterior of a bacterial cell wall
Eclipse or penetration - injection of the viral genome into the host cell in a process
*Lytic cycle
What genes transcribed and translated (early vs late genes)? Disadvantage?
- Transcribe and translate the viral genome
a) all cellular energy goes toward replication of viral genome
b) Free pool of dNTPs
Early genes -> hydrolase (destroy host cell genome), capsid proteins - Replicate the viral genome
a) assembly with capsid - Lysis of host and release of new viral particles
a) Late gene: lysozyme
enzyme poke holes in the bacterial cell wall
releasing 100 progeny viruses, which can begin another round of the cycle
Disadvantage -> kill host that you need to divide
Tissue tropism
Viruses have specific affinities for certain issues
Attachment/adsorption is tissue specific
Lysogenic Cycle
Prophage? Lysogen?
What is a consequence that happens during excision?
What is transduction?
- Integrate viral genome and host genome and is now referred to as prophage and the host is now called a lysogen
- The prophage is silent and phage genes blocked by phage protein due to phage-encoded repressor protein (repressor gene from virus) that binds to specific DNA elements in phage promoters (operators) - Normal host activity, including reproduction - every time the host cell reproduces itself, the prophage is reproduced too
- Excision and lytic cycle - eventually the prophage becomes activated (stressed by heat, pH, salt, detergent, lack of sleep, upcoming exams) and it now removes itself from the host genome (called excision) and enters the lytic cycle
*When viral genome activates, excising itself from the host genome, it may take part of the host genome along with it so in subsequent infections, the virus will integrate “stolen” host DNA -The presence of new DNA will become evident if it codes for a trait that the newly infected host did not previously have, such as the ability to metabolize glucose
This process = transduction
What are two ways virus can enter cell?
Bacteria have cell wall so virus inject genetic material inside
Other cells endocytose virus
Productive Cycle
Productive Cycle similar to lytic cycle but does not destroy the host cell bc the enveloped viruses exit cell by budding through the host’s cell membrane, becoming coated with this membrane
Positive sense RNA
What does it code/carry/both? How does that protein work?
Viruses (dont memorize, just understand what enzymes it makes and needs)
What does it code for - 2 things ?
Only RNA goes into host
Codes for RNA dep - RNA polymerase but doesn’t need to carry it
Can make proteins directly from (+) RNA unlike (-) RNA
A (+) RNA virus, with a single-stranded RNA genome -> A piece of single stranded viral RNA which serves as mRNA
The viral genome acts like mRNA and host ribosomes begin to translate it
*Makes viral proteins (enzymes, caspids) including RNA replicase = RNA dependent - RNA polymerase (which makes complementary RNA strand by using a (-) strand intermediate as a template)
This RNA replicase replicates the RNA and makes complementary (-) RNA strand then RNA replicase can be used again to make more (+) RNA which can be packaged up as new virus
Negative sense mRNA
What does is carry/code/both?
Must encode AND carry RNA dep RNA polymerase (RNA replicase)
Must have both: RNA (which codes for RNA dep RNA polymerase) and the enzyme RNA dep RNA pol (RNA replicase) itself so that it can pass on genetic material for RNA dep RNA polymerase
Retroviruses
What does it encode?
examples
Why this virus is problematic for us
Do retroviruses have to carry, code or both for reverse transcriptase?
What happens to viral genome once integrated into host DNA?
Positive sense (+) RNA lysogenic Must encode reverse transcriptase
RNA dep DNA polymerase/reverse transcriptase to turn pos sense RNA into ssDNA, then replicate using host DNA pol to make dsDNA, then integrase to insert into host genome (make provirus) then two things happen
a) transcribe and translate using host enzymes to make viral proteins
b) transcribe only using host RNA polymerases to make more (+) RNA ]
Problems with this virus:
- permanently inside host genome
- Rapid mutation (error rate in RNA synthesis is much higher than DNA synthesis bc mxn to proofread and correct errors in DNA synthesis
AIDS, HTLV
There are (+) viruses that undergo lysogeny, so they integrate into host genome as proviruses
In order to integrate into our double stranded DNA genome, a viral genome must also be composed of double-stranded DNA
Since these viral genomes enter the cell in an RNA form, they must undergo reverse transcription to make DNA from an RNA template
(lower yield) Double-Stranded DNA viruses
What do they encode? (more in textbook biochem pg 144)
often encode enzymes required for dNTP synthesis and DNA replication
This is because host cells will only make dNTPs in preparation for replication. If virus wants to reproduce without waiting for the host to do so, it must encode its own enzymes for the synthesis of DNA building blocks
Prions Characteristics, what are the two categories?
Misfolded version of protein that already exists
No DNA or RNA, no membranes, no organelles, very small, extremely stable
Two categories = Normal prions (good) and mutant prions (bad like made cow disease - Spongiform encephalopathies)
Bad prions can also be inherited if mutation in sperm or egg (genetically linked), bad prions lead to degeneration of nervous system
Infectious proteins -> when normally folded protein comes into contact with prion, the prion acts as a template; the shape of the normal protein is altered and it too becomes infectious
How do mutant “Bad” prions come about?
What do they do to proteins?
Mutant prions accumulate and lead to cell death
They have high affinity for central nervous system and change good prions into bad prions, leads to cell death, no cure
a) spontaneous mutation in gene which ends up coding for mutant “bad” prion
b) can be inherited
c) transmission by consumption
What are viroids? What does it lack? How does it spread? What does it do?
virus-like, circular single stranded positive sense RNA, lack capsids
Spread through mouths of bugs and most diseases caused by viroids are found in plants, The only human disease linked to viroids is Hepatitis D but it can only enter hepatocytes if it is contained in a capsid with a binding protein and since viroids do not have capsids, successful Hepatitis D infection required coinfection with Hepatitis B, from with derives its capsid.
*Can hijack cell’s DNA dep RNA polymerase and direct it to read RNA templates
What is the main difference between eukaryotes and prokaryotes?
Who has ribosomes?
Prokaryotes lack membrane bound organelles (since ribosomes don’t have membranes, prokaryotes do have these)
