Viruses: Structure, Function, and Facts
What are Viruses
o Viruses are tiny infectious agents, much smaller than bacteria.
o They are comparable in size to large proteins.
o In its most basic form, a virus consists of a protein coat, called a capsid, and from one to several hundred genes in the form of DNA or RNA inside the capsid.
o No virus contains both DNA and RNA.
o Most animal viruses, some plant viruses, and very few bacterial viruses surround themselves with a lipid-rich envelope either borrowed from the membrane of their host cell or synthesized in the host cell cytoplasm.
o The envelope typically contains some virus-specific proteins.
o The capsid usually accounts for most of the weight of the virus.
o A mature virus outside the host cell is called a virion.
Viruses Types and Structures
o Viruses are not currently classified as living organisms; they do not belong to any of the taxonomical kingdoms of organisms.
o Viruses differ from living organisms in the following ways.
o They ALWAYS require the host cell’s reproductive machinery in order to replicate.
o Viruses don’t metabolize organic nutrients.
o Instead they use the ATP made available by the host cell.
o Viruses possess either RNA or DNA, but never both.
o Thus, there are viruses with the familiar double-stranded DNA, with single-stranded DNA, with double-stranded RNA, and with single-stranded RNA.
o Also the nucleic acid could be linear or circular.
o Viruses can be crystallized without losing their ability to infect.
o A viral infection begins when a virus adsorbs to a specific chemical receptor site on the host.
o The host is the cell being infected.
o The chemical receptor is usually a specific glycoprotein on the host cell membrane.
Life Cycle of Viruses
o The virus can’t infect the cell if the specific receptor isn’t available/there.
o Next, the nucleic acid of the virus penetrates into the cell.
o In a bacteriophage, a virus that infects bacteria, the nucleic acid is normally injected through the tail after viral enzymes have digested a hole in the cell wall.
o Most viruses that infect eukaryotes are engulfed by an endocytotic process.
o They could also be engulfed by a process called membrane fusion, in which the envelope of the virus is actually incorporated into the plasma membrane of the host cell releasing the capsid into the cytoplasm.
Types of Viruses Infection
Once inside the cell, there are two possible paths;
I. Lytic infection
o The virus commandeers the cell’s reproductive machinery and begins reproducing new viruses.
o There is a brief period before the first fully formed virion appears.
o This period is called the eclipse period.
o The cell may fill with viruses until it lyses or burst, or it may release the new viruses one at a time in a reverse endocytotic process.
o The period of infection to the lysis is called the latent period.
o The latent period encompasses the eclipse period.
o A virus following the lytic cycle is called a virulent virus.
II. Lysogenic infection
o The viral DNA is incorporated into the host genome, or, if the virus is an RNA virus and it possesses the enzyme reverse transcriptase.
o DNA is actually reverse-transcribed from RNA and then incorporated into the host cell genome.
o When a host replicates its DNA, the viral DNA is replicated as well.
o A virus in a lysogenic cycle is called a temperate virus.
"The chemical receptor is usually a specific glycoprotein on the host cell membrane"
o While the viral DNA remains incorporated in the host cell, the virus is said to be dormant or latent, and is called a provirus (a prophage if the host cell is a bacterium).
o There are 2 important results from the lysogenic cycle:
1) The cell infected is immune to reinfection by the same phage.
2) The host cell may exhibit new properties, this is known as phage conversion.
o The dormant virus may become active when the host cell is under some type of stress.
o Examples of stress include UV light or carcinogens.
o When the virus becomes active, it becomes virulent.
o Exhibits exponential growth b/c each new cell will create more viruses.
o It is a longer cycle than the lytic cycle.
Type of Viruses
o There are many types of viruses.
o One way to classify them is by the type of nucleic acid that they posses.
o A virus with unenveloped plus-strand RNA is responsible for the common cold.
o The “plus-strand” indicates the proteins can be directly translated from the RNA.
o Enveloped plus-strand RNA viruses include retroviruses such as the virus that causes AIDS.
o A retrovirus carries the enzyme reverse transcriptase in order to create DNA from its RNA.
o The DNA is then incorporated into the genome of the host cell.
o Minus-strand RNA viruses include measles, rabies, and the flu.
o Minus- strand RNA is the complement to mRNA and must be transcribed to plus-RNA before being translated.
o There are even double stranded RNA viruses, and single and double stranded DNA viruses.
o Reassortment – is a method with which virus’ can alter their genetic makeup it occurs If a virus has a segmented genome and if two variants of that virus infect a single cell, progeny virions can result with some segments from one parent, some from the other.
o Viroids are a related form of infectious agent.
o Viroids are small rings of naked RNA without capsids.
o Viroids only infect plants.
o There also exist naked proteins called prions that cause infections in animals.
o Prions are capable of reproducing themselves apparently without DNA or RNA.
Defense Against Viral Infection
o Although the lipid rich envelope is borrowed from the host cell, spike proteins encoded from the viral nucleic acids protrude from the envelope.
o These proteins bind to receptors on a new host cell causing the virus to be infectious.
o However, it is also the spike proteins that human antibodies recognize when fighting the infection.
o Since RNA polymerase doesn’t contain a proofreading mechanism, changes in the spike proteins are common in RNA viruses.
o When the spike proteins change, the antibodies fail to recognize them, and the virus may avoid detection until new antibodies are formed.
o A vaccine can be either an injection of antibodies or an injection of a nonpathogenic virus with the same capsid or envelope.
o The later allows the host immune system to create its own antibodies.
o Vaccines against rapidly mutating viruses are generally not very effective.
o Another difficulty of fighting viral infections is that more than one animal may act as a carrier population.
o Even if all viral infections of a certain type were eliminated in humans, the virus may continue to thrive in another animal, thus maintaining the ability to reinfect the human population.
o For instance, ducks carry the flu virus, apparently without any adverse symptoms.
o One of the reasons that the fight against smallpox was so successful was because the virus can only infect humans.
"The structure of a virus: capsid, nucleic acid, and lipid-rich protein envelope for some viruses: tail, base plate, and tail fibers for most bacteriophages"
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