What do viruses reproduce

The copying process occurs in a double-membrane compartment that keeps the virus hidden from the cell. Some of the RNA copies are packaged into the next generation of viruses. Other copies are used to tell the cell how to make viral proteins , including the ones that will encase the RNA. A full copy of the original coronavirus RNA, for insertion into the next generation of coronaviruses. Copies of specific RNA sections that tell the cell how to make viral proteins. These proteins will be used to build the next generation of coronaviruses.

These proteins are made and assembled in the part of the cell that serves as a factory, warehouse and delivery system. When the RNA-enclosed membrane pinches shut, a new virus is formed. Host cell. Not to scale. Coronaviruses are hundreds of times smaller in width than human cells. We can speculate that the acquisition of a few structural proteins could allow the element to exit a cell and enter a new cell, thereby becoming an infectious agent.

Indeed, the genetic structures of retroviruses and viral-like retrotransposons show remarkable similarities. In contrast to the progressive process just described, viruses may have originated via a regressive, or reductive, process. Microbiologists generally agree that certain bacteria that are obligate intracellular parasites, like Chlamydia and Rickettsia species , evolved from free-living ancestors. Indeed, genomic studies indicate that the mitochondria of eukaryotic cells and Rickettsia prowazekii may share a common, free-living ancestor Andersson et al.

It follows, then, that existing viruses may have evolved from more complex, possibly free-living organisms that lost genetic information over time, as they adopted a parasitic approach to replication. These viruses, which include smallpox virus and the recently discovered giant of all viruses, Mimivirus, are much bigger than most viruses La Scola et al.

A typical brick-shaped poxvirus, for instance, may be nm wide and nm long. About twice that size, Mimivirus exhibits a total diameter of roughly nm Xiao et al. Conversely, spherically shaped influenza virus particles may be only 80 nm in diameter, and poliovirus particles have a diameter of only 30 nm, roughly 10, times smaller than a grain of salt.

Again, poxvirus genomes often approach , base pairs, and Mimivirus has a genome of 1. In addition to their large size, the NCLDVs exhibit greater complexity than other viruses have and depend less on their host for replication than do other viruses.

Poxvirus particles, for instance, include a large number of viral enzymes and related factors that allow the virus to produce functional messenger RNA within the host cell cytoplasm. Because of the size and complexity of NCLDVs, some virologists have hypothesized that these viruses may be descendants of more complex ancestors. According to proponents of this hypothesis, autonomous organisms initially developed a symbiotic relationship. Over time, the relationship turned parasitic, as one organism became more and more dependent on the other.

As the once free-living parasite became more dependent on the host, it lost previously essential genes. Eventually it was unable to replicate independently, becoming an obligate intracellular parasite, a virus. Analysis of the giant Mimivirus may support this hypothesis.

This virus contains a relatively large repertoire of putative genes associated with translation — genes that may be remnants of a previously complete translation system. Interestingly, Mimivirus does not differ appreciably from parasitic bacteria, such as Rickettsia prowazekii Raoult et al. Figure 4 The progressive and regressive hypotheses both assume that cells existed before viruses.

What if viruses existed first? Recently, several investigators proposed that viruses may have been the first replicating entities. Koonin and Martin postulated that viruses existed in a precellular world as self-replicating units. Over time these units, they argue, became more organized and more complex. Eventually, enzymes for the synthesis of membranes and cell walls evolved, resulting in the formation of cells. Viruses, then, may have existed before bacteria, archaea , or eukaryotes Figure 4; Prangishvili et al.

We also know that some RNA molecules, ribozymes, exhibit enzymatic properties; they can catalyze chemical reactions. Perhaps, simple replicating RNA molecules, existing before the first cell formed, developed the ability to infect the first cells. Villarreal and DeFilippis and Bell described models explaining this proposal.

Perhaps, both groups postulate, the current nucleus in eukaryotic cells arose from an endosymbiotic-like event in which a complex, enveloped DNA virus became a permanent resident of an emerging eukaryotic cell. Where viruses came from is not a simple question to answer. One can argue quite convincingly that certain viruses, such as the retroviruses, arose through a progressive process. Mobile genetic elements gained the ability to travel between cells, becoming infectious agents.

One can also argue that large DNA viruses arose through a regressive process whereby once-independent entities lost key genes over time and adopted a parasitic replication strategy.

Finally, the idea that viruses gave rise to life as we know it presents very intriguing possibilities. Perhaps today's viruses arose multiple times, via multiple mechanisms.

Perhaps all viruses arose via a mechanism yet to be uncovered. Today's basic research in fields like microbiology, genomics , and structural biology may provide us with answers to this basic question. Contemplating the origins of life fascinates both scientists and the general public. Understanding the evolutionary history of viruses may shed some light on this interesting topic. To date, no clear explanation for the origin s of viruses exists.

Viruses may have arisen from mobile genetic elements that gained the ability to move between cells. They may be descendants of previously free-living organisms that adapted a parasitic replication strategy.

Perhaps viruses existed before, and led to the evolution of, cellular life. Continuing studies may provide us with clearer answers. Or future studies may reveal that the answer is even murkier than it now appears. Andersson, S.

The genome sequence of Rickettsia prowazekii and the origin of mitochondria. Nature , — doi Bell, P. Viral eukaryogenesis: Was the ancestor of the nucleus a complex DNA virus? Journal of Molecular Evolution 53 , — doi Koonin, E. On the origin of genomes and cells within inorganic compartments. Trends in Genetics 21 , — Lander, E. Initial sequencing and analysis of the human genome. La Scola, B. A giant virus in Amoebae. Science , doi Nelson, M. The evolution of epidemic influenza.

Prangishvili, D. This page has been archived and is no longer updated. A virus is an infectious agent that can only replicate within a host organism. Viruses can infect a variety of living organisms, including bacteria, plants, and animals. Viruses are so small that a microscope is necessary to visualize them, and they have a very simple structure.

When a virus particle is independent from its host, it consists of a viral genome, or genetic material, contained within a protein shell called a capsid. In some viruses, the protein shell is enclosed in a membrane called an envelope.

The viral replication process begins when a virus infects its host by attaching to the host cell and penetrating the cell wall or membrane.