Microbiology Chapter 5- Exam 3
negative sense RNA virus
must make the positive RNA first to create the double-stranded RNA before completing gene expression.
viral budding
the capsid attaches to cytoplasmic tails of viral spikes
Nucleocapsid
the capsid together with the nucleic acid
Retroviridae
- A group of single-stranded positive sense RNA viruses that do not use RNA-dependent-RNA polymerase. - Instead, retroviruses such as HIV use reverse transcriptase to create DNA from their original RNA. - A double- stranded DNA molecule containing the viral genes is made and then integrates into the DNA of a host cell chromosome becoming part of the cell's genome. - The integrated viral DNA is referred to as a provirus and when the DNA is activated after this point, it is expressed just as the cell's DNA is expressed (and replicated), but in making new virions, the mRNA that was made is used instead of the DNA.
What do viruses do with a specific host?
- Infect it resulting in cell death.
What do viruses do to cells?
- Invade cells and take over the workings of the cell to promote viral reproduction
Are viruses alive?
- Many scientists consider viruses as a living organism since they can be transmitted, reproduce, and cause disease.
Life Cycle of a Positive Sense RNA Virus
- RNA viruses are unusual in that their genetic code is stored as RNA instead of DNA - there will need to be viral specific enzymes present in order to complete this process. - So in order to make new virions, capsomeres and RNA will need to be made by the virus. - The host cells can make proteins via ribosomes but have no mechanisms for making RNA without DNA.
Translation of DNA virus
- Since the typical DNA virus looks and acts genetically similar to the host cell, the translation of the RNA will be done on the cell's ribosomes using the same mechanism as the cell uses. - The difference is that the proteins will serve viral function instead of cell function
What is the size of a virus?
- Smallest organisms measuring at around 10nm to about 100nm.
capsomeres
- The capsid is made of a series of proteins which fit together to create an enclosure for the virus's genetic material. - The form of the capsid shell can be used to begin classifying viruses.
adsorption
- The first step of viral life cycles is the attachment of the virus to the host cell - This occurs as receptors on the cell will bind with viral proteins or glycoproteins either in the envelope or on the capsid. - HIV, for example, binds with human CD4 which is found only on human helper T cells so HIV will only infect helper T cells. - Influenza virus binds to sialic acid residues which are common on a large number of cell types allowing the virus to infect many different cells.
complex viruses
- Viruses with capsid shapes other than polyhedral or helical - These viruses can be shaped like hourglasses, bullets or spheres. - It is important to be sure that the shape is determined by the capsid shape since viruses containing an envelope will appear globular with an electron microscope.
So a typical positive sense RNA virus has a single stranded positive RNA which must first be used to transcribed...
- a negative strand RNA - This negative strand will then serve as the template to make more positive RNA.
icosahedral (polyhedral)
- composed in a way to have many flat triangle-like groups of capsomeres coming together at the points (called vertices). - This produces a very geometric shape which can enclose the nucleic acid with a few proteins.
Replication of DNA virus
- he DNA is replicated in the same way that the host cell's DNA is replicated using the host cell's DNA polymerase and other enzymes. - DNA replication will synthesize the DNA to be used in assembling new virions.
helical viruses
- made as the capsomeres align themselves with the helical nature of the nucleic acid producing a spiral design. - When observed with an electron microscope, the individual capsomeres are not distinguishable so the viral appears to be a strand instead of a helix since the capsomeres are so tightly packed.
What are viruses composed of?
- protein coat and nucleic acid
Hepadnavirus
- reproduce their DNA not using host cell DNA polymerase, but by using a viral specific enzyme called reverse transcriptase. - transcribes DNA to RNA using the cell's enzymes and then uses the viral specific reverse transcriptase to make viral DNA from the RNA.
envelope
- taken from the host cell's membrane and/or viral-specific enzymes. - phospholipid bilayer with embedded molecules. - The embedded proteins, glycoproteins and/or glycolipids are viral specific and typically serve as attachment receptors for the virus. - Note that enveloped viruses can have icosahedral, helical, or complex capsid shape, but will appear globular upon microscopy due to the amorphous envelope.
negative sense strand
- the complementary sequence.
positive sense strand
- the strand that contains the code which is translated into protein
Transcription of DNA virus
1. As the viral genes are transcribed into RNA, the first genes have promoter sequences which are identical to the host cell's allowing the cell's RNA polymerase to work. 2. Some of these early genes will be used to make viral enzymes which slowly begin to redirect the gene expression of the cell towards viral gene expression. 3. The last genes transcribed are typically the structural genes including capsomeres.
Structure of a DNA virus
1. Typical DNA viruses contain double stranded DNA as its only internal genetic matter. 2. The DNA is structurally identical to that of the host cell, but the genes contained within are viral specific.
All viruses contain what?
1. capsid 2. nucleic acid 3. envelope
Release
A virus is released to continue its life cycle.
There are 5 basic phases in the life cycle of viruses:
Adsorption (or attachment to host cell) Penetration and Uncoating Synthesis Assembly (or maturation) Release
assembly
After synthesis of viral components, new virions are put together creating the "offspring" of the infecting virus.
Viruses can have either DNA or RNA inside the capsid serving as its genetic material but no viruses have been found which contain what?
BOTH
Are viruses composed of cells?
NO
Other non-cellular agents of disease are the prions, which are not viruses at all, but protein fibers.
Other examples include viroids, which are extremely small lengths of proteincoated nucleic acid and satellite viruses, which require the presence of larger viruses to cause disease.
negative sense RNA
a complementary sequence to the translation template so must be used to make + sense RNA first before proteins can be made.
virion
a fully formed virus that is able to establish an infection in a host cell
Viruses use a multiplication cycle that generally involves
a. Adsorption: attaching to a host cell b. Penetration: entering the host cell c. Uncoating: exposing the viral nucleic acid d. Synthesis and Assembly: making new virus particles (virions) e. Release by lysis or budding f. These events turn the host cell into a factory solely for making and shedding new viruses. This results in the ultimate destruction of the cell.
Viruses are easily responsible for several billion infections each year
a. It is conceivable that many chronic diseases of unknown cause will eventually be connected to viral agents. b. Viral infections are difficult to treat because the drugs that attack viral replication also cause side effects in the host. c. Examples of viral diseases include influenza, rabies, measles, and AIDS. d. Examples of prion diseases include BSE, Creutzfeldt-Jakob disease, and scrapie. e. Viroid diseases are mostly plant based. f. A satellite virus called Delta agent can only cause infections in cells already infected with hepatitis B.
Viral cytopathic effects (CPE) are the manifestation of a viral infection on a cell or tissue which are seen with microscopic analysis.
a. Most host cells will undergo physical changes once infected with a virus. These could be size, shape, and/or internal changes. b. Inclusion bodies are aggregations of material or virions visible in microscopic analysis. c. Syncytia are multiple infected cells which fuse together creating a larger conglomeration of cells. These are usually recognized due to multiple nuclei in the cell mass. d. In cell culture, infected cells will often be destroyed leaving opening in the cell layer called plaques.
Animal viruses must be studied in some sort of living cell or tissue, using aseptic techniques to exclude unwanted microorganisms.
a. Virus growth in cell culture, and bacteriophage growth on bacterial lawns, is detected by the appearance of plaques. b. Viruses are often grown in live animals including mice, rabbits, and hamsters. c. Some viruses must be grown in embryos still in the eggs such as influenza is typically grown in chicken embryos.
Virus size range is from 20 to 450 nm (diameter).
a. Viruses are composed of an outer protein capsid enclosing either DNA or RNA plus a variety of enzymes. b. Some viruses also exhibit an envelope, comprised mainly of lipids, around the capsid. c. Spikes on the surface of the virus capsid or envelope are critical for their attachment to host cells.
Viruses are non-cellular entities whose properties have been identified through technological advances in microscopy and tissue culture.
a. Viruses are infectious particles that invade every known type of cell. b. They are not alive, yet they are able to redirect the metabolism of living cells to reproduce virus particles. Because of this, they are generally referred to as "active" or "inactive" instead of "live" or "dead". c. The International Committee on the Taxonomy of Viruses oversees naming and classification of viruses. Viruses are classified into orders, families, and genera. d. We will use viral structure, genetic composition, and host range for categorization.
The DNA or RNA inside the viral capsid can be
double stranded or single stranded, in one chromosome or in segments, and more than one copy may be present as well.
viral spikes
embedded in the cell membrane and push out of the cell via exocytosis
penetration
endocytosis in which the entire virion can enter the cell in a vesicle or vacuole, direct penetration of the virion, or injection of the viral genome.
Types of viruses causing infection
g. Animal viruses can cause acute infections or can persist in host tissues as chronic latent infections that can reactivate periodically throughout the host's life. h. Some persistent animal viruses can cause cancer.
Viruses can have either DNA or RNA genomes, but none are currently known which have both.
i. DNA viruses can have double or single stranded DNA molecules and have either one or multiple copies or segments. ii. RNA viruses can have either double or single stranded RNA molecules and have either one or multiple copies.
Virus culture is import in
i. Isolating and identifying the virus ii. Preparing vaccines against the virus iii. Researching the virus
The capsid shape is one factor in categorizing viruses
i. Polyhedral (or icosahedral) viruses have a geometric capsid with flat sides and vertices, often forming a mostly spherical type appearance. ii. Helical viruses have a capsid which is tightly wound around the nucleic acid forming an elongated filament-like appearance. iii. Complex capsids are those that do not fit either of the 2 other shapes listed. This includes the "bullet" shaped Rhabdoviruses and the robot-like bacteriophages.e.
reverse transcriptase
is using an RNA strand to create a strand of DNA.
enveloped viruses
leave the cell via membrane budding in which the capsid attaches to cytoplasmic tails of viral spikes embedded in the cell membrane and push out of the cell via exocytosis taking some of the membrane as its viral envelope.
inactive virus
not able to replicate in a host
synthesis
nucleic acid and protein follows as the viral genes are expressed producing viral specific enzymes used for reproduction, capsid proteins, envelope spikes (if needed) and copies of the nucleic acid to be packaged into the new virions.
Viruses are entirely...
parasitic
capsid
protein shell of a virus
uncoating
required to allow the genome to be available for synthesis.
RNA-dependent-RNA polymerase
uses RNA as a template to make RNA.
naked virus
virus without an envelope
active virus
viruses that enter cells and immediately begin to multiply- the cell dies quickly
Non-enveloped viruses
will leave the cell via cell lysis.
positive sense RNA
works as an mRNA in that protein can be made directly since the gene direction is correct for the translation template.
single-stranded DNA viruses (again atypical)
would need to create a double-stranded DNA before continuing gene expression.
