Picture by medicalnewstoday.com

Simply continuing the definition of viruses characteristics with the intent of refreshing the basic knowledge. In this chapter are analyzed way they replicate and invade the host cells, life cycle, their genome and classification related, the cytopathic effects, latent and dormant status and host range, to better understand the pathogenicity of the most dangerous as of the present coronavirus.

Replication Cycle

Viruses are acellular units and for this reason do not grow through cell division, instead they use the systems and metabolism of a host cell to produce multiple copies of themselves, then they assemble inside the cell. When infected, the host cell is forced to produce thousands of identical copies of the original virus.

Their life cycle differs between species, but there are six basic stages in their life cycle:

Attachment which is a specific binding between viral capsid proteins and specific receptors on the host cellular surface. This specificity determines the host range and type of host cell of a virus. Attachment to the receptor can induce the viral envelope protein to undergo changes that result in the fusion of viral and cellular membranes.

The attachment is followed by viral entry, virions enter the host cell through receptor-mediated endocytosis or membrane fusion. Plants have a rigid cell wall made of cellulose, and fungi one of chitin, so most viruses can get inside these cells only after breaking the cell wall. Bacteria, like plants, have strong cell walls that a virus must breach to infect the cell; bacterial cell walls are much thinner than plant cell walls due to their smaller size, some viruses have evolved mechanisms that inject their genome into the bacterial cell through the cell wall, while the viral capsid remains outside.

Subsequently to the attachment comes the uncoating. Uncoating is a process in which the viral capsid is removed: this can happen by degradation performed by viral enzymes or host enzymes or by simple dissociation; the result is the releasing of the viral genomic nucleic acid.

Replication of viruses involves primarily multiplication of the genome as synthesis of viral messenger RNA, mRNA, from initial genes, with exceptions for positive sense RNA viruses, viral protein synthesis, possible assembly of viral proteins, then viral genome replication mediated by initial or regulatory protein expression. This may be followed, for complex viruses with larger genomes, by one or more series of mRNA synthesis: late gene expression usually happens with structural or virion proteins.

Then comes the assembly phase where proteins go through modifications. In viruses like HIV, this modification, called maturation, occurs after the virus has been released from the host cell.

Genome replication

During the final step viruses can be released from the host cell by lysis, a process that kills the cell by breaking its membrane and cell wall if present. Some viruses undergo a lysogenic cycle where the viral genome is incorporated by genetic recombination into a specific place in the host’s chromosome. The viral genome is then known as a provirus or, in the case of bacteriophages a prophage. Whenever the host divides, the viral genome is also replicated. The viral genome is mostly silent within the host. At some point, the provirus or prophage may generate active virus, which may lyse the host cells. 

The genetic material of virus particles, and the method of replication of this material varies significantly among different types of viruses.

DNA viruses

The genome replication of most DNA viruses takes place in the cell’s host nucleus. If the cell has the appropriate receptor on its surface, these viruses enter the cell either by direct fusion with the cell membrane or by receptor-mediated endocytosis. Most DNA viruses are totally dependent on the host cell’s DNA and RNA synthesizing and processing devices. Viruses with larger genomes may encode much of these mechanisms themselves. In eukaryotes the viral genome must cross the cell’s nuclear membrane to access this equipment, while in bacteria it is necessary only to enter the cell.

RNA viruses

Replication of RNA viruses usually takes place in the cytoplasm. RNA viruses can be placed into four different groups depending on their modes of replication. The polarity of single-stranded RNA viruses largely determines the replicative mechanism; the other major criteria is whether the genetic material is single-stranded or double-stranded. All RNA viruses use their own RNA replicase enzymes to create copies of their genomes.

Reverse transcribing viruses

Reverse transcribing viruses have ssRNA as RetroviridaeMetaviridaePseudoviridae or dsDNA as Caulimodviridae, and Hepadnaviridae in their particles. Reverse transcribing viruses with RNA genomes as Retroviruses use a DNA intermediate to replicate, while those with DNA genomes, Paretroviruses, use an RNA intermediate during genome replication. Both types use a reverse transcriptase, or RNA-dependent DNA polymerase, to carry out the nucleic acid conversion. Retroviruses integrate the DNA produced by reverse transcription into the host genome as a provirus as a part of the replication process. They are susceptible to antiviral drugs that inhibit the reverse transcriptase enzyme as zidovudine and lamivudine. An example of the first type is HIV, which is a retrovirus, examples of the second type are the Hepadnviridae, which include Hepatitis B virus.

Cytopathic effects on the host cell

These are the range of structural and biochemical effects that viruses have on the host cell. Most virus infections eventually result in the death of the host cell. The causes of death include cell lysis, alterations to the cell’s surface membrane and apoptosis. Often cell death is caused by cessation of its normal activities because of suppression by virus-specific proteins, not all of which are components of the virus particle. Some viruses, such as Epstein-Barr virus, can cause cells to proliferate without causing malignancy, while others, such as Papillomavirus, are established causes of cancer.

Dormant and latent infections

Some viruses cause no apparent changes to the infected cell. Cells in which the virus is latent and inactive show few signs of infection and often function normally. This causes persistent infections, and the virus is often dormant for many months or years; this is what typically happens with the Herpes viruses.

Host range

Viruses are the most abundant biological entities on earth. They infect all types of cellular life including animals, plants, bacteria, and fungi. Different types of viruses can infect only a limited range of hosts and many are species-specific. Some as smallpox virus for example, can infect only humans, and are said to have a narrow host range. Other viruses, such as rabies virus, can infect different species of mammals and are said to have a broad range. The viruses that infect plants are harmless to animals, and most viruses that infect other animals are harmless to humans with the exception of those cause of zoonosis as the rabies virus or the coronavirus to mention some. The host range of certain bacteriophages is limited to a single strain of bacteria and they can be used to trace the source of outbreaks of infections by a method called phage typing. The complete set of viruses in an organism or habitat is called the virome; for example, all human viruses constitute the human virome.

To be continued

Thanks for Reading

Mariarosaria M.




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