What do viruses consist of

The core is covered with a capsid, a protective coat made of protein. Around the capsid, there may be a spiky covering known as the envelope. These spikes are proteins that enable viruses to bind to and enter host cells. There, if the conditions are right, they can multiply. There is some dispute about whether viruses meet the criteria for living organisms. They can grow and reproduce, but they do not produce adenosine triphosphate, a compound that drives many processes in living cells.

They also do not contain ribosomes, so they cannot make proteins. This makes them unable to reproduce independently and totally dependent on their host. After entering a host cell, a virus hijacks the cell by releasing its own genetic material and proteins into the host. Next, the virus continues to reproduce, but it produces more viral protein and genetic material instead of the usual products that the cell would produce.

Viruses have different shapes and sizes. Scientists categorize viruses according to various factors, including:. Examples of viruses with an envelope include the influenza virus and HIV. Within these categories are different types of viruses. A coronavirus, for example, has a sphere-like shape and a helical capsid containing RNA. It also has an envelope with crown-like spikes on its surface.

Seven coronaviruses can affect humans, but each one can change or mutate, producing many variants. Learn more about coronaviruses here. Just as there are friendly bacteria in the intestines that are essential to gut health , humans may also carry friendly viruses that help protect against dangerous bacteria, including Escherichia coli. Viruses do not leave fossil remains, so they are difficult to trace through time. Scientists use molecular techniques to compare the DNA and RNA of viruses and find out more about where they come from.

Three competing theories try to explain the origin of viruses. In reality, viruses may have evolved in any of these ways. The regressive, or reduction, hypothesis suggests that viruses started as independent biological entities that became parasites. Over time, they shed genes that did not help them parasitize, and became entirely dependent on the cells they inhabit.

In this way, they gained the ability to become independent and move between cells. The virus-first hypothesis suggests that viruses evolved from complex molecules of nucleic acid and proteins either before or at the same time as the first cells on Earth appeared, billions of years ago.

When a viral disease emerges, it is not always clear where it comes from. A virus exists only to reproduce. Viruses are so small that they are best viewed using an electron microscope , which is how they were first visualized in the s. Viruses generally come in two forms: rods or spheres. However, bacteriophages viruses that infect bacteria have a unique shape, with a geometric head and filamentous tail fibers. No matter the shape, all viruses consist of genetic material DNA or RNA and have an outer protein shell, known as a capsid.

There are two processes used by viruses to replicate: the lytic cycle and lysogenic cycle. Some viruses reproduce using both methods, while others only use the lytic cycle. In the lytic cycle, the virus attaches to the host cell and injects its DNA. Then fully formed viruses assemble. These viruses break, or lyse, the cell and spread to other cells to continue the cycle. Like the lytic cycle, in the lysogenic cycle the virus attaches to the host cell and injects its DNA.

In humans, viruses can cause many diseases. For example, the flu is caused by the influenza virus. Typically, viruses cause an immune response in the host, and this kills the virus. However, some viruses are not successfully treated by the immune system, such as human immunodeficiency virus, or HIV.

This leads to a more chronic infection that is difficult or impossible to cure; often only the symptoms can be treated. Unlike bacterial infections, antibiotics are ineffective at treating viral infections. Viral infections are best prevented by vaccines, though antiviral drugs can treat some viral infections. Most antiviral drugs work by interfering with viral replication. Most of the linear DNAs from viruses of other families have characteristics which enable them to adopt a circular configuration temporarily, presumably during replication.

The two strands of poxvirus DNA are covalently cross-linked at each end, so that on denaturation, the molecule becomes a large single-stranded circle Fig. The linear dsDNA of some herpesviruses and the linear ssRNA of retroviruses contains repeat sequences at the ends of the molecule. In the case of the adenoviruses, these terminal repeats are inverted; hence, even without enzymatic digestion, denatured molecules self-anneal to form single-stranded circles Fig.

Specialized arrangements at the termini of linear DNA viral genomes. A Adenovirus DNA has inverted terminal repeats, with a covalently linked protein located at each end of the molecule. B Herpes simplex virus DNA consists of two covalently linked components, long L and short S , each of which consists of a large unique sequence U L and U S , respectively flanked by inverted repeats.

In a viral population, four isomeric forms differing in the orientation of the unique regions relative to each other occur in equimolar amounts. Intact single strands anneal as shown on the right. C Vaccinia virus DNA has inverted terminal repeats and each end is covalently closed, so that on denaturation it forms a large, single-stranded circular molecule. This has an essential function in replication of the genome. The DNA of certain iridoviruses genus Ranavirus contains a high proportion of 5-methylcytosine instead of cytosine.

The size of viral DNA genomes ranges from 4. As 1 kb or 1 kbp contains enough genetic information to code for about one average-sized protein, we recognize as an approximation that viral DNAs contain from about 4 to genes and code for 4 to proteins.

However, the relationship between any particular nucleotide sequence and its protein product is not as straightforward as this. First, the DNA of most of the larger viruses—like that of cells—contains what appears to be redundant information, in the form of 1 repeat reiterated sequences and 2 introns, i.

Furthermore, a given mRNA sequence may be read in two different reading frames theoretically, up to three, because each codon is a triplet , giving rise to two or three proteins with different amino acid sequences. These fascinating examples of genetic economy are well illustrated by the papovaviruses see Fig. Viral DNAs contain several kinds of noncoding sequences, in addition to introns and various types of terminal repeat sequences, described above.

Consensus sequences, which tend to be conserved through evolution because they serve vital functions, include those of RNA splice sites, polyadenylation sites, RNA polymerase recognition sites and promoters, initiation codons for translation, and termination codons. The genome of RNA viruses may also be single-stranded or double-stranded.

Furthermore, while some occur as a single molecule, others are segmented. Arenavirus and birnavirus RNAs consist of 2 segments, bunyavirus RNA of 3, orthomyxovirus RNA of 7 or 8 in different genera , and reovirus 10, 11, or 12 in different genera. All viral RNAs are linear; none is a covalently closed circle. However, the ssRNAs of arenaviruses and bunyaviruses have sticky ends, hence these molecules occur as circles. Single-stranded viral nucleic acid, which is generally RNA, can also be defined according to its sense also known as polarity.

This is the case with picornaviruses, caliciviruses, togaviruses, flaviviruses, coronaviruses, and retroviruses. If, on the other hand, its nucleotide sequence is complementary to that of mRNA, it is said to have negative — sense.

Such is the case with the paramyxoviruses, orthomyxoviruses, rhabdoviruses, arenaviruses, and bunyaviruses, all of which have an RNA-dependent RNA polymerase transcriptase in the virion, in order that mRNA can be transcribed.

With the arenaviruses and at least one genus of bunyaviruses one of the RNA segments is ambisense, i. The size of ssRNA viral genomes varies from 7. Accordingly they code, in general, for fewer than a dozen proteins. In the case of the segmented RNA genomes of orthomyxoviruses and reoviruses, one can consider most of the segments to be individual genes, each coding for one unique protein.

No such simple relationship applies to the other RNA viruses. The essential features of the genomes of viruses of vertebrates are summarized in Table Their remarkable variety is reflected in the diverse ways in which the information encoded in the viral genome is transcribed to RNA, then translated into proteins, and the ways in which the viral nucleic acid is replicated see Chapter 4.

Viral preparations often contain some particles with an atypical content of nucleic acid see Chapter 5. Host cell DNA is found in some papovavirus particles, and cellular ribosomes are incorporated in arenaviruses. Several copies of the complete viral genome may be enclosed within a single particle, or viral particles may be formed that contain no nucleic acid empty particles or that have an incomplete genome defective interfering particles.

Some virus-coded proteins are structural, i. A major role of structural proteins is to provide the viral nucleic acid with a protective coat. The virions of all viruses of vertebrates contain several different proteins, the number ranging from 3 in the case of the simplest viruses to over in the case of the complex poxviruses.

In isometric viruses, the structural proteins form an icosahedral capsid which sometimes encloses a polypeptide core that is intimately associated with the nucleic acid. Some virions, e. The capsid proteins are assembled in the virion in groups, to form the capsomers visible in electron micrographs. Each capsomer is composed of one to six molecules of polypeptide, usually of the same kind homopolymers but sometimes different heteropolymers.

Capsomers from the vertices and the faces are usually composed of different polypeptides. A few viruses have a double capsid, each being composed of a different set of polypeptides. Other proteins, invariably glycoproteins, make up the peplomers projecting from the envelope; a second type of envelope protein is the nonglycosylated matrix protein that occurs as a layer at the inner surface of the lipid envelope of orthomyxoviruses, paramyxoviruses, and rhabdoviruses.

One or more of the proteins on the surface of the virion has a specific affinity for complementary receptors present on the surface of susceptible cells; the same viral protein contains the antigenic determinants against which neutralizing antibodies are made. Virions of several families carry a limited number of enzymes, transcriptases being the most important Table As a consequence, the composition of lipids of particular viruses differs according to the composition of the membrane lipids of the cells in which they have replicated.

The poxviruses, ranaviruses, and African swine fever virus contain cellular lipid in their envelopes, and other lipids in the inner part of the virion.

Lipid occurs in the outer membrane of poxviruses, and has a different composition from that of host cell lipids. In ranaviruses and African swine fever virus the additional viral lipid occurs within the icosahedral capsid. Apart from that associated with viral nucleic acid, carbohydrate occurs as a component of viral glycoproteins, which usually occur as peplomers, with their hydrophobic ends buried in the lipid bilayer of the envelope, while their glycosylated hydrophilic ends project into the medium.

Poxviruses also contain internal glycoproteins, in the membrane of the core, and one of the outer capsid proteins of rotaviruses is glycosylated. In general, viruses are more sensitive than bacteria or fungi to inactivation by physical and chemical agents. As a result, the capsid of viruses is like a coat of armor for the nucleic acid genome, and the repetitive nature of the identical protein subunits give nearly all viruses geometrical symmetry.

As mentioned above, the capsid of a virion is metastable — with the right kind and amount of perturbation, the capsid can become undone, allowing host cellular machinery to get access to the viral genome. The ability of the virion to disassemble is afforded by the fact that viral capsid subunits are NOT covalently bound, and will release from each other with the appropriate signal. Some viruses, such as the now famous coronavirus, also have a lipid membrane that surrounds the capsid.

These sugar-protein complexes are found on the surface of a virus particle, and are called glycoproteins. While glycoproteins are not specific to viruses there are many examples of glycoproteins throughout all life , they do provide a way for viruses to attach themselves to host cells. Since viral glycoproteins are one of the key ways viruses can infect cells, many scientists are working on medicines that can impact how the glycoproteins work in order to prevent viral illnesses in people, pets, and plants.

In addition to being varied in their shapes and sizes, viruses also demonstrate diversity when it comes to their nucleic acid genomes. The primary function of a viral genome is to store the instructions for building more virus particles. Regardless of which type of genome a virus has, there are two main routes for packing it: viruses can either assemble their capsid shell around their nuclear genome, or viruses can make a capsid shell, and insert their nuclear genome into it.

Viruses also need to make sure that they are packaging their genomes, and not the genomes of their host cells. Because there are millions of different viruses, there are millions of different viral genomes. So far, scientists have mapped the genomes of 75, viruses, but that is merely a fraction of what is out there.

As next generation sequencing and analysis continues to grow in its sophistication, scientists will continue building knowledge when it comes to viral genomes! Gelderblom, H. Structure and classification of viruses.