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How does a virus survive outside a cell?
How do viruses work in a cell?
How does a virus enter a cell?
Mar 3, 2016 · Viral mechanisms are capable of translocating proteins and genetic material from the cell and assembling them into new virus particles. Contemporary research has revealed specific mechanisms viruses use to get inside cells and infect them. An individual viral particle, called a virion, is a far simpler structure than a bacterium.
- Fredric S. Cohen
- 10.1016/j.bpj.2016.02.006
- 2016
- Biophys J. 2016 Mar 8; 110(5): 1028-1032.
Nov 21, 2023 · After the virus binds to the cell, specific molecules on the cell’s surface or within the cell’s recycling machinery activate viral coat proteins for entry. An example is the SARS-CoV-2 spike...
- Peter Kasson
- Overview
- Key points:
- Introduction
- What is a virus?
- How are viruses different from bacteria?
- The structure of a virus
- Virus capsids
- Virus envelopes
- Virus genomes
- What is a viral infection?
What a virus is. The structure of a virus and how it infects a cell.
•A virus is an infectious particle that reproduces by "commandeering" a host cell and using its machinery to make more viruses.
•A virus is made up of a DNA or RNA genome inside a protein shell called a capsid. Some viruses have an external membrane envelope.
•Viruses are very diverse. They come in different shapes and structures, have different kinds of genomes, and infect different hosts.
•Viruses reproduce by infecting their host cells and reprogramming them to become virus-making "factories."
Scientists estimate that there are roughly 1031 viruses at any given moment1 . That’s a one with 31 zeroes after it! If you were somehow able to wrangle up all 1031 of these viruses and line them end-to-end, your virus column would extend nearly 200 light years into space. To put it another way, there are over ten million times more viruses on Earth than there are stars in the entire universe2 .
Does that mean there are 1031 viruses just waiting to infect us? Actually, most of these viruses are found in oceans, where they attack bacteria and other microbes3 . It may seem odd that bacteria can get a virus, but scientists think that every kind of living organism is probably host to at least one virus!
A virus is a tiny, infectious particle that can reproduce only by infecting a host cell. Viruses "commandeer" the host cell and use its resources to make more viruses, basically reprogramming it to become a virus factory. Because they can't reproduce by themselves (without a host), viruses are not considered living. Nor do viruses have cells: they're very small, much smaller than the cells of living things, and are basically just packages of nucleic acid and protein.
Still, viruses have some important features in common with cell-based life. For instance, they have nucleic acid genomes based on the same genetic code that's used in your cells (and the cells of all living creatures). Also, like cell-based life, viruses have genetic variation and can evolve. So, even though they don't meet the definition of life, viruses seem to be in a "questionable" zone. (Maybe viruses are actually undead, like zombies or vampires!)
Even though they can both make us sick, bacteria and viruses are very different at the biological level. Bacteria are small and single-celled, but they are living organisms that do not depend on a host cell to reproduce. Because of these differences, bacterial and viral infections are treated very differently. For instance, antibiotics are only helpful against bacteria, not viruses.
Bacteria are also much bigger than viruses. The diameter of a typical virus is about 20 - 300 nanometers (1 nm = 10-9 m )4 . This is considerably smaller than a typical E. coli bacterium, which has a diameter of roughly 1000 nm ! Tens of millions of viruses could fit on the head of a pin.
There are a lot of different viruses in the world. So, viruses vary a ton in their sizes, shapes, and life cycles. If you're curious just how much, I recommend playing around with the ViralZone website. Click on a few virus names at random, and see what bizarre shapes and features you find!
Viruses do, however, have a few key features in common. These include:
•A protective protein shell, or capsid
•A nucleic acid genome made of DNA or RNA, tucked inside of the capsid
•A layer of membrane called the envelope (some but not all viruses)
Let's take a closer look at these features.
The capsid, or protein shell, of a virus is made up of many protein molecules (not just one big, hollow one). The proteins join to make units called capsomers, which together make up the capsid. Capsid proteins are always encoded by the virus genome, meaning that it’s the virus (not the host cell) that provides instructions for making them.
[More about capsomers and capsids]
Capsids come in many forms, but they often take one of the following shapes (or a variation of these shapes):
1.Icosahedral – Icosahedral capsids have twenty faces, and are named after the twenty-sided shape called an icosahedron.
2.Filamentous – Filamentous capsids are named after their linear, thin, thread-like appearance. They may also be called rod-shaped or helical.
3.Head-tail –These capsids are kind of a hybrid between the filamentous and icosahedral shapes. They basically consist of an icosahedral head attached to a filamentous tail.
In addition to the capsid, some viruses also have an external lipid membrane known as an envelope, which surrounds the entire capsid.
Viruses with envelopes do not provide instructions for the envelope lipids. Instead, they "borrow" a patch from the host membranes on their way out of the cell. Envelopes do, however, contain proteins that are specified by the virus, which often help viral particles bind to host cells.
All viruses have genetic material (a genome) made of nucleic acid. You, like all other cell-based life, use DNA as your genetic material. Viruses, on the other hand, may use either RNA or DNA, both of which are types of nucleic acid.
We often think of DNA as double-stranded and RNA as single-stranded, since that's typically the case in our own cells. However, viruses can have all possible combos of strandedness and nucleic acid type (double-stranded DNA, double-stranded RNA, single-stranded DNA, or single-stranded RNA). Viral genomes also come in various shapes, sizes, and varieties, though they are generally much smaller than the genomes of cellular organisms.
[How small?]
Notably, DNA and RNA viruses always use the same genetic code as living cells. If they didn't, they would have no way to reprogram their host cells!
In everyday life, we tend to think of a viral infection as the nasty collection of symptoms we get when catch a virus, such as the flu or the chicken pox. But what's actually happening in your body when you have a virus?
At the microscopic scale, a viral infection means that many viruses are using your cells to make more copies of themselves. The viral lifecycle is the set of steps in which a virus recognizes and enters a host cell, "reprograms" the host by providing instructions in the form of viral DNA or RNA, and uses the host's resources to make more virus particles (the output of the viral "program").
For a typical virus, the lifecycle can be divided into five broad steps (though the details of these steps will be different for each virus):
Steps of a viral infection, illustrated generically for a virus with a + sense RNA genome.
1.Attachment. The virus recognizes and binds to a host cell via a receptor molecule on the cell surface.
[More about attachment]
Nov 7, 2020 · As obligate intracellular parasites, viruses exploit host cells to replicate, amplify, and subsequently spread from cell to cell and from host to host. Thousands of viruses have been sequenced thus far, but only a few are known to cause fatal diseases.
- Pierre-Yves Lozach
- 10.3390/cells9112431
- 2020
- Cells. 2020 Nov; 9(11): 2431.
May 6, 2016 · Steps in Virus Life Cycle. A virus encounters multiple obstacles during its journey to enter the host cells. Cellular membranes pose as barriers for the invaders. The plasma membrane represents the first barrier that all animal viruses have to penetrate.
- Wang-Shick Ryu
- 10.1016/B978-0-12-800838-6.00003-5
- 2017
- 2017
Mar 23, 2020 · A virus is a kind of infectious particle, made of genetic code in a protective shell. Most scientists don’t think of viruses as living things, because they can’t reproduce without help from a living cell. They need to invade a cell in order to make more viruses.
Mar 13, 2020 · The virions make their way out of the cell, usually destroying it in the process, and then head off to infect more cells. Does this “life cycle” make viruses alive?
