General Features of Virus Structure Flashcards
How do viruses differ from cells?
- they have a relatively simple structure,
- their genetic information is encoded by either DNA or RNA,
- and they are dependent on a living cell in order to replicate themselves.
All living cells have double-stranded DNA as their genetic material and follow the Central Dogma of molecular biology. In cells, the DNA is first transcribed to make messenger RNA (mRNA) and the mRNA is translated on a ribosome to synthesize protein.
What is the central dogma of molecular biology?
All cells have DNA as their genetic material.
Many viruses have RNA genomes.
Retroviruses reverse transcribe their RNA genome back to DNA.
Other RNA viruses simply replicate and transcribe their RNA genomes.
Which group of RNA viruses does have a DNA phase of replication?
Retroviruses
What are viruses?
Chemical complexes of RNA or DNA protected by protein
Viruses are obligate intracellular parasites of living cells. They are a diverse group of infectious agents.
Viruses exist in either an extracellular or an intracellular form.
A virus is an independent nucleic acid genome that replicates inside a host cell by hijacking the host’s biosynthetic machinery to propagate itself and project itself into new host cells.
The basic structure of a virus includes a genome (DNA or RNA), a protein coat (capsid), and sometimes a lipid bilayer (envelope).
Little bags of bad biochemistry
What is the extracellular form of the virus known as?
A virus particle or virion.
The extracellular form of the virus particle serves to transfer the viral genome from one cell to another.
How does an extracellular virus replicate?
In order to replicate, the virus particle bind to a host cell and the genome (and any needed proteins) must enter a cell.
What is a host cell?
A cell that supports the virus replication cycle of a virus is called a host cell.
What is an infection?
When the viral genome is introduced into a host cell and replicated, the process is called an infection.
How do viruses exist in the intracellular state?
Primarily as replicating nucleic acids.
How does an intracellular virus exploit the host?
They use the host cell to provide energy, enzymes, metabolic components (e.g., amino acids), and structural machinery (e.g., ribosomes) to replicate the genome and synthesize the viral proteins that make up the virus coat.
Once the virus particles have been assembled they are eventually released from the host cell.
How big are viruses?
Viruses are very diverse in size, genome size and organization, and overall structure, but the features are consistent for each virus family.
Viruses are small compared to cells.
The size range for eukaryotic cells is typically around 10,000–100,000 nm (10–100 μm) in diameter.
In comparison, most virus particles are around 20 – 250 nm in diameter
Describe the limited genome size of the virus.
The genome can be DNA or RNA (never both), and the nucleic acid can be single stranded or double stranded.
The genes they carry allow for replication of the viral genome, production of viral structural proteins and any enzymes required for the replication of the genome that they cannot adapt or access from their host cell.
What do viruses cover their nucleic acid genome with?
All viruses have their nucleic acid genome covered by a protective capsid.
The proteins of the capsid are encoded in the viral genome.
True or false?
True
How may a virus capsid be described?
As being helical, icosahedral (sometimes referred to as spherical) or complex in morphology.
Some viruses also have an additional layer consisting of a lipid bilayer (called the envelope).
Some viruses also have enzymes needed for the replication of their genomes packaged inside the capsid.
What is the individual protein subunit of a capsid called?
A capsomere; multiple copies of the capsomere proteins assemble to form the capsid.
What does a capsomere protein do?
Multiple copies of the capsomere proteins assemble to form the capsid.
Describe a helical capsid.
In helical capsids, the capsid protein has affinity for the viral genome and for other copies of the capsid protein.
It assembles by attaching to the virus genome as the genome is being synthesized.
The length of the helical capsid is as long as the genome itself.
Describe a icosahedron capsid.
One way to construct a symmetrical capsid with maximal internal volume with non-symmetric protein molecules is to arrange the protein molecules in identical equilateral triangular structures that can be joined to form icosahedrons.
Icosahedrons are structures formed by 20 equilateral triangles: 5 at the top, 5 at the bottom and 10 in the middle.
Depending of the specific example, the triangular faces can be made of single polypeptide or several polypeptide subunits (the subunits could be the same or different polypeptides).
How are icosahedron structures formed?
Icosahedrons are structures formed by 20 equilateral triangles: 5 at the top, 5 at the bottom and 10 in the middle.
What is a complex capsid?
Some viruses have capsid structures that cannot be placed in the helical or symmetrical categories.
Instead, they form complex capsid structures that may consist of multiple layers of proteins and other structures.
Describe a naked vs. enveloped virus particle.
Many animal viruses are surrounded by a lipid bilayer that was acquired as the virus particles egresses from the host cell.
The envelope could be derived from the nuclear, endoplasmic reticulum or plasma membranes.
The envelope is often modified with viral proteins that are required for the virus to infect the next host cell.
Viruses that do not contain envelopes are referred to as naked viruses.
Give examples of diseases caused by viruses.
Viruses cause many diseases such as AIDS, poliomyelitis, measles, mumps, the common cold, and some types of cancer.
Describe factors leading to the pathological effects of viral diseases.
The pathological effects of viral diseases may result from several factors. For example:
- The viral gene products (e.g., proteins) may have toxic effects on host cell metabolism.
- The host’s immune system may react and destroy infected cells expressing viral genes.
- The expression of host genes may be modified by structural or functional interactions with the genetic material of the virus.
- Host cells may be destroyed directly by the infecting virus as a consequence of the virus replication cycle.
Give examples of viral enzymes used as molecular biology tools.
DNA ligase, Reverse transcriptase, RNA polymerases, and RNase H are a few examples.
Give an example of virus use in gene therapy.
Viruses can potentially be used for gene therapy – they can be used to transfer genes into cells to correct genetic defects.
Viruses are living microorganisms.
True or false?
False.
Viruses are cells.
True or false?
False.
Viruses are not cells.
They cannot synthesize their own ATP or amino acids or nucleotides.
Cannot synthesize proteins on their own.
Reiterate the central dogma of molecular biology and state whether it applies to viruses.
Central dogma of molecular biology – the flow of a genetic information within a biological system.
Applies to all living cells.
Applies to viruses too – sometimes with a twist.
Describe the viral genome.
Genetic material may be ssDNA or dsDNA or ssRNA or dsRNA, linear or circular.
*Note: no viral circular double-stranded RNA
What are the two general categories of virus.
Enveloped or non-enveloped (naked)
What do enveloped viruses have that naked viruses do not?
Enveloped viruses have a lipid bilayer surrounding their capsid - the lipid bilayer can be derived from the plasma membrane or any other membrane structure.
Some envelope viruses have a matrix layer of protein just below the envelope.
Envelope glycoproteins encoded by the virus (proteins modified with carbohydrate) and embedded in the lipid bilayer - these glycoproteins are required for attachment to the host cell.
What do pharmaceutical companies use enzymes for?
Reverse transcriptase and RNase H – enables pharmaceutical companies to manufacture human products in bacteria or yeast cells.
Examples: Insulin, Factor VIII, Factor IX, human growth hormone, erythropoietin, interferons, interleukins, parathyroid hormone and others.
