9.1: Overview of Viruses - Biology
Viruses lack a cell membrane and are obligate parasitic agents that lack the ability to replicate away from their host cell. A virus consists of either DNA and/or RNA encapsulated within a protective protein coat. Many animal viruses also contain an envelope surrounding the protein coat consisting of host membrane-derived lipids, etc. Viruses vary greatly in size, ranging from a few nanometers to roughly one micrometer. Most viruses appear either polyhedral or helical (rod-like in appearance).
A parasite that causes disease is called a pathogen. Since viruses are parasites of cells, they have the potential to be pathogens. All living things on this planet are hosts to viral parasites. Throughout history, humans have been plagued with a multitude of diseases caused by viruses, including influenza, encephalitis, rabies, polio, mumps, measles, small pox, AIDS (caused by HIV) and hemorrhagic fever (caused by EBOLA and Marburg virus).
The Immune System
The body’s main defense against viral and bacterial pathogens is the immune system. Proteins and glycoproteins (sugar-protein surface markers) on the surface of pathogens stimulate the production of antibodies in the host. Any substance that stimulates the immune response is called an antigen. Each antigen the body is exposed to results in the production of a specific antibody that binds to only that antigen.
Antibodies in Medical Diagnosis
By developing specific antibodies to surface antigens found on a pathogen, a diagnostic procedure known as Enzyme-Linked Immunosorbent Assay (ELISA) can be used to detect the presence of the pathogen.
Overview of Viruses
Viruses are the smallest parasites, typically ranging from 0.02 to 0.3 micrometer, although several very large viruses up to 1 micrometer long (megavirus, pandoravirus) have recently been discovered. Viruses depend completely on cells (bacterial, plant, or animal) to reproduce. Viruses have an outer cover of protein and sometimes lipid, an RNA or DNA core, and sometimes enzymes needed for the first steps of viral replication.
Viruses are classified principally according to the nature and structure of their genome and their method of replication, not according to the diseases they cause. Thus, there are DNA viruses and RNA viruses each type may have single or double strands of genetic material. Single-strand RNA viruses are further divided into those with (+) sense and (-) sense RNA. DNA viruses typically replicate in the host cell nucleus, and RNA viruses typically replicate in the cytoplasm. However, certain single-strand, (+) sense RNA viruses termed retroviruses use a very different method of replication.
Retroviruses use reverse transcription to create a double-stranded DNA copy (a provirus) of their RNA genome, which is inserted into the genome of their host cell. Reverse transcription is accomplished using the enzyme reverse transcriptase, which the virus carries with it inside its shell. Examples of retroviruses are the human immunodeficiency viruses and the human T-cell leukemia viruses. Once the provirus is integrated into the host cell DNA, it is transcribed using typical cellular mechanisms to produce viral proteins and genetic material. If the infected cell belongs to the germline, the integrated provirus can become established as an endogenous retrovirus that is transmitted to offspring.
The sequencing of the human genome revealed that at least 1% of the human genome consists of endogenous retroviral sequences, representing past encounters with retroviruses during the course of human evolution. A few endogenous human retroviruses have remained transcriptionally active and produce functional proteins (eg, the syncytins that contribute to the structure of the human placenta). Some experts speculate that some disorders of uncertain etiology, such as multiple sclerosis, certain autoimmune disorders, and various cancers, may be caused by endogenous retroviruses.
Because RNA transcription does not involve the same error-checking mechanisms as DNA transcription, RNA viruses, particularly retroviruses, are particularly prone to mutation.
For infection to occur, the virus first attaches to the host cell at one or one of several receptor molecules on the cell surface. The viral DNA or RNA then enters the host cell and separates from the outer cover (uncoating) and replicates inside the host cell in a process that requires specific enzymes. The newly synthesized viral components then assemble into a complete virus particle. The host cell typically dies, releasing new viruses that infect other host cells. Each step of viral replication involves different enzymes and substrates and offers an opportunity to interfere with the process of infection.
The consequences of viral infection vary considerably. Many infections cause acute illness after a brief incubation period, but some are asymptomatic or cause minor symptoms that may not be recognized except in retrospect. Many viral infections are cleared by the body’s defenses, but some remain in a latent state, and some cause chronic disease.
In latent infection, viral RNA or DNA remains in host cells but does not replicate or cause disease for a long time, sometimes for many years. Latent viral infections may be transmissible during the asymptomatic period, facilitating person-to-person spread. Sometimes a trigger (particularly immunosuppression) causes reactivation.
9.1: Overview of Viruses - Biology
Viruses of all shapes and sizes consist of a nucleic acid core, an outer protein coating or capsid, and sometimes an outer envelope.
Describe the relationship between the viral genome, capsid, and envelope
- Viruses are classified into four groups based on shape: filamentous, isometric (or icosahedral), enveloped, and head and tail.
- Many viruses attach to their host cells to facilitate penetration of the cell membrane, allowing their replication inside the cell.
- Non-enveloped viruses can be more resistant to changes in temperature, pH, and some disinfectants than are enveloped viruses.
- The virus core contains the small single- or double-stranded genome that encodes the proteins that the virus cannot get from the host cell.
- capsid: the outer protein shell of a virus
- envelope: an enclosing structure or cover, such as a membrane
- filamentous: Having the form of threads or filaments
- isometric: of, or being a geometric system of three equal axes lying at right angles to each other (especially in crystallography)
Viruses are acellular, meaning they are biological entities that do not have a cellular structure. Therefore, they lack most of the components of cells, such as organelles, ribosomes, and the plasma membrane. A virion consists of a nucleic acid core, an outer protein coating or capsid, and sometimes an outer envelope made of protein and phospholipid membranes derived from the host cell. The capsid is made up of protein subunits called capsomeres. Viruses may also contain additional proteins, such as enzymes. The most obvious difference between members of viral families is their morphology, which is quite diverse. An interesting feature of viral complexity is that host and virion complexity are uncorrelated. Some of the most intricate virion structures are observed in bacteriophages, viruses that infect the simplest living organisms: bacteria.
Example of a virus attaching to its host cell: The KSHV virus binds the xCT receptor on the surface of human cells. This attachment allows for later penetration of the cell membrane and replication inside the cell.
Viruses come in many shapes and sizes, but these are consistent and distinct for each viral family. In general, the shapes of viruses are classified into four groups: filamentous, isometric (or icosahedral), enveloped, and head and tail. Filamentous viruses are long and cylindrical. Many plant viruses are filamentous, including TMV (tobacco mosaic virus). Isometric viruses have shapes that are roughly spherical, such as poliovirus or herpesviruses. Enveloped viruses have membranes surrounding capsids. Animal viruses, such as HIV, are frequently enveloped. Head and tail viruses infect bacteria. They have a head that is similar to icosahedral viruses and a tail shape like filamentous viruses.
Many viruses use some sort of glycoprotein to attach to their host cells via molecules on the cell called viral receptors. For these viruses, attachment is a requirement for later penetration of the cell membrane, allowing them to complete their replication inside the cell. The receptors that viruses use are molecules that are normally found on cell surfaces and have their own physiological functions. Viruses have simply evolved to make use of these molecules for their own replication.
Overall, the shape of the virion and the presence or absence of an envelope tell us little about what disease the virus may cause or what species it might infect, but they are still useful means to begin viral classification. Among the most complex virions known, the T4 bacteriophage, which infects the Escherichia coli bacterium, has a tail structure that the virus uses to attach to host cells and a head structure that houses its DNA. Adenovirus, a non-enveloped animal virus that causes respiratory illnesses in humans, uses glycoprotein spikes protruding from its capsomeres to attach to host cells. Non-enveloped viruses also include those that cause polio (poliovirus), plantar warts (papillomavirus), and hepatitis A (hepatitis A virus).
Examples of virus shapes: Viruses can be either complex in shape or relatively simple. This figure shows three relatively-complex virions: the bacteriophage T4, with its DNA-containing head group and tail fibers that attach to host cells adenovirus, which uses spikes from its capsid to bind to host cells and HIV, which uses glycoproteins embedded in its envelope to bind to host cells.
Enveloped virions like HIV consist of nucleic acid and capsid proteins surrounded by a phospholipid bilayer envelope and its associated proteins. Glycoproteins embedded in the viral envelope are used to attach to host cells. Other envelope proteins include the matrix proteins that stabilize the envelope and often play a role in the assembly of progeny virions. Chicken pox, influenza, and mumps are examples of diseases caused by viruses with envelopes. Because of the fragility of the envelope, non-enveloped viruses are more resistant to changes in temperature, pH, and some disinfectants than are enveloped viruses.
Types of Nucleic Acid
Unlike nearly all living organisms that use DNA as their genetic material, viruses may use either DNA or RNA. The virus core contains the genome or total genetic content of the virus. Viral genomes tend to be small, containing only those genes that encode proteins that the virus cannot obtain from the host cell. This genetic material may be single- or double-stranded. It may also be linear or circular. While most viruses contain a single nucleic acid, others have genomes that have several, called segments.
In DNA viruses, the viral DNA directs the host cell’s replication proteins to synthesize new copies of the viral genome and to transcribe and translate that genome into viral proteins. DNA viruses cause human diseases, such as chickenpox, hepatitis B, and some venereal diseases, like herpes and genital warts.
RNA viruses contain only RNA as their genetic material. To replicate their genomes in the host cell, the RNA viruses encode enzymes that can replicate RNA into DNA, which cannot be done by the host cell. These RNA polymerase enzymes are more likely to make copying errors than DNA polymerases and, therefore, often make mistakes during transcription. For this reason, mutations in RNA viruses occur more frequently than in DNA viruses. This causes them to change and adapt more rapidly to their host. Human diseases caused by RNA viruses include hepatitis C, measles, and rabies.
Evolution of Viruses
Although biologists have accumulated a significant amount of knowledge about how present-day viruses evolve, much less is known about how viruses originated in the first place. When exploring the evolutionary history of most organisms, scientists can look at fossil records and similar historic evidence. However, viruses do not fossilize, so researchers can only hypothesize about viruses’ evolutionary history by investigating how today’s viruses evolve and by using biochemical and genetic information to create speculative virus histories.
While most findings agree that viruses don’t have a single common ancestor, scholars have yet to find a single hypothesis about virus origins that is fully accepted in the field—and that fully explains viruses and their characteristics. There are, however, three hypotheses that have risen as the most accepted:
- Devolution or regressive hypothesis. This hypothesis proposes to explain the origin of viruses by suggesting that viruses evolved from free-living cells. However, many components of how this process might have occurred are a mystery.
- Escapist or progressive hypothesis. This hypothesis accounts for viruses having either an RNA or a DNA genome and suggests that viruses originated from RNA and DNA molecules that escaped from a host cell. However, this hypothesis doesn’t explain the complex capsids and other structures on virus particles.
- Self-replication hypothesis. This hypothesis posits a system of self-replication similar to that of other self-replicating molecules, likely evolving alongside the cells they rely on as hosts studies of some plant pathogens support this hypothesis.
Another problem for those studying viral origins and evolution is their high rate of mutation, particularly the case in RNA retroviruses like HIV/AIDS.
As technology advances, scientists will develop and refine further hypotheses to explain the origin of viruses—or create new hypotheses. The emerging field called virus molecular systematics attempts to do just that through comparisons of sequenced genetic material. These researchers hope to one day better understand the origin of viruses, a discovery that could lead to advances in the treatments for the ailments they produce.