SINGLE-STRANDED DNA VIRUS: STRUCTURE, REPLICATION, IMPORTANCE - BIOLOGY - 2024

A single stranded DNA virus (ssDNA) or ssDNA (“single stranded”) is an infectious biological entity whose genome is made up of a single strand of genomic material, which is why they are called by that name.

These viruses are characterized by not using RNA as a classic intermediate during viral replication, being the viruses that do use RNA viruses (single or double stranded) or dsDNA viruses (double stranded).

Source: English Wikipedia user Graham Beards

According to David Baltimore's classification, these viruses belong to Group II, including very small viruses (up to 20nm) that can infect bacteria, plants and animals.

Structure

These viruses have a very simple structure. They consist of a capsid without coating or envelope that does not exceed 20 nm. Its shape is usually spherical with icosahedral symmetry (20-sided polyhedron). Each capsomer that makes up the capsid has a triangular shape.

The capsid, made up of a set of viral proteins, includes a single strand of DNA (hence ssDNA Virus), thus protecting the genome and also includes proteins specialized in recognizing host cells. Inside it there are also proteins associated with ssDNA.

Replication

Viruses only acquire a reproductive or replicative capacity by entering or infecting a cell and taking over its replication machinery. Although the latter have a genome, the genetic material is insufficient to replicate independently.

In a simplified way, the virus must induce the host cell to make all the components necessary to make more viruses. Viruses with double-stranded DNA have bands with positive and negative polarity, which will complement each other.

Meanwhile, single-stranded viruses must develop the chain they need to be able to replicate. This means that before the mRNA from these viruses can be synthesized, a complementary DNA band must be synthesized, which is often the negative band (-) because the RNA polymerase protein uses only double-stranded DNA (dsDNA).

The latter is used by these viruses in replication and is also an intermediary in transcription, where the negative strand of dsDNA is transcribed into viral mRNA. The synthesis of double-stranded DNA and subsequent transcription is mediated by cellular enzymes and in some cases by the participation of some proteins of a viral nature.

Types of single-stranded DNA viruses

Most viruses that have a single-stranded genome (ssDNA / ssDNA) are of + polarity, this means that when packaged in heritable material within the virion, the DNA strand has positive polarity (+).

However, sometimes the negative band of DNA is packaged in the virus, thus having negative polarity (-).

In the above cases, the virus appropriates the cellular replication machinery to synthesize the complementary strand and then synthesize mRNA and replicate.

In other less frequent cases, the viral genome has +/- polarity, which indicates that it is a double-sense chain, where one or another complementary chain can be synthesized.

SsDNA bacteriophage

Bacteriophages of the Ff family (or filamentous phages), including phage f1, fd, and phage M13, are very similar to bacterial pili that contain a single circular single-stranded DNA molecule.

On the other hand, the phage phi-x174 also has ssDNA as a genome. This tiny virus at just 25 nm in diameter is several times smaller than the thinnest cell membrane. It contains only 5386 nucleotides, which code for only eleven proteins that will make up part of its structure.

Single-stranded DNA viruses in plants

In the plant kingdom there are also a large number of diseases caused by this virus. In general, these ssDNA viruses belong mainly to the Geminiviridae and Nanoviridae families, which cause several diseases that are of phytosanitary importance.

Vectors are always insects that use the plant as a food source. Among these diseases we can mention the chlorosis virus in chickpeas and the corn ray, which have grasshoppers as their main vector. In both cases the virus involved is of the Mastrevirus genus.

Other examples are the Chinese tomato virus, caused as a result of an infection with Begomovirus, whose vector is whiteflies (Aleyrodidae) and the necrotic leaf in beans produced by the Nanovirus, which mainly affects leguminous plants.

Some Medically Important Single Stranded DNA Viruses

There are several viruses of medical importance to man whose genome is made up of single-stranded DNA. Some ssDNA viruses that affect man are viruses belonging to the Parvoviridae family such as parvovirus and bocavirus.

Parvovirus B19

Parvovirus is a small virus (between 18 and 26nm) that has a naked icosahedral capsule (without envelope), with a muco-chain genome that can be negative or positive sense (- / +). For its replication, the presence of growing cells or an adjuvant virus (dependovirus) is necessary.

The hypotheses about its replication indicate that, once internalized in the host cell, it transmits its genome to the nucleus in such a way that the single-stranded DNA becomes double-stranded by the host's replication factors, as well as DNA polymerases that only exist in the host cells. growing cells.

The symptoms of this viral disease are not usually dangerous, affecting mainly children and causing the so-called "fifth disease". The most characteristic symptoms are the appearance of a noticeable redness in the cheeks, nasal congestion, low fever, muscular pain and headache (typical characteristics of the cold).

However, despite not being serious, in patients with chronic hemolytic anemia it can cause episodes of aplastic crisis and in adults cause acute polyarthritis.

In those cases in which the virus affects a woman during pregnancy, it can cause a condition known as 'hydrops fetalis', which will cause the death of the fetus.

Bocavirus

Bocavirus is also a recently discovered single-stranded DNA virus. In children it can cause an acute respiratory illness that is often complicated and severe. In those patients younger than two years, the bocavirus can cause bronchiolitis with wheezing and viremia extended for several days. There is only one fatal case recorded.

Transmission of this virus has generally been associated with secretions from the respiratory tract.

Adeno-associated viruses

They are those viruses belonging to the genus Dependovirus. These usually infect humans, but are only capable of multiplying with the help of an adjuvant virus, which is usually an adenovirus. These viruses do not cause disease per se, and they do not change the effects of infection with an adenovirus.

However, by possessing these characteristics, together with the property of integrating into the host's chromosomes, they have allowed adeno-associated viruses that have been genetically modified to be excellent candidates for gene replacement therapies.

Other viruses of the Parvoviridae family with single-stranded DNA affect only particular zoological groups. Densoviruses, for example, mainly infect insects.

References

1. Erskine, W. (2009). The lentil: botany, production and uses. CABI.
2. Evans, J., & Manson, A. (2011). The essentials in cell and genetics + StudentConsult in Spanish. Elsevier Spain.
3. King, AM, Lefkowitz, E., Adams, MJ, & Carstens, EB (Eds.). (2011). Virus taxonomy: ninth report of the International Committee on Taxonomy of Viruses (Vol. 9). Elsevier.
4. Marcdante, K., & Kliegman, RM (2016). Nelson Essentials of Pediatrics-E-Book: First South Asia Edition. Elsevier Health Sciences.
5. Murray, PR, Rosenthal, KS, & Pfaller, MA (2017). Medical microbiology. Elsevier Health Sciences.
6. Oxford, JS, Collier, LH, & Kellam, P. (2016). Human virology. Oxford University Press.
7. Woodbury, CP (2003). Biochemistry: The Chemical Reactions of Living Cells, Volumes 1 and 2, By David E. Metzler and Carol M. Metzler (Iowa State University). Academic Press, New York.