Name: how viruses infect cells and multiply
Uploaded: 2019-03-11T09:21:23.000Z
Duration: 50 s
Channel: JoVE

K12 Standards

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High School

Life Sciences

from-molecules-to-organisms-structures-and-processes

From Molecules To Organisms: Structures And Processes

HS-LS1-8

What are Viruses?

- [Narrateur] Un virus est une structure qui porteson propre matériel génomique sous la forme d'ARNou d'ADN pour envahir et se répliquerà l'intérieur d'une cellule hôte. 
Lorsqu'un virus se lie aux récepteurs de surfacede la cellule hôte, il entre et se désassemble rapidement,sans coder son matériel génétique. 
Dans le cas des virus à ADN, l'ADN viralcommande aux protéines de réplication de la cellule hôtede synthétiser de nouvelles copies du génome viral, qui sontensuite transcrites et traduites en protéines virales. 
Enfin, l'hôte réassemble ces composants virauxen une progéniture permettant à une seule particule de virusd'en produire des milliers d'autres entraînant souventla mort de la cellule hôte.

Yamauchi, Yohei et Ari Helenius. « Entrée du virus en un coup d’œil ». J Cell Sci 126, n° 6 (15 mars 2013) : 1289-95. [<a href="https://doi.org/10.1242/jcs.119685 » target="__blank">Source</a>]

Lin, Derek M, Britt Koskella et Henry C Lin. « Phagethérapie : une alternative aux antibiotiques à l’ère de la multirésistance aux médicaments », World Journal of Gastrointestinal Pharmacology and Therapeutics 8, n° 3 (6 août 2017) : 162-73. [<a href="https://doi.org/10.4292/wjgpt.v8.i3.162 » target="__blank">Source</a>]

Nicaise, Valérie. « L’immunité des cultures contre les virus : résultats et défis futurs », Frontiers in Plant Science 5 (21 novembre 2014). [<a href="https://doi.org/10.3389/fpls.2014.00660 » target="__blank">Source</a>]

que sont les virus ?

<h4>Aperçu</h4>
 
 Un virus est une particule infectieuse microscopique qui se compose d’un génome d’ARN ou d’ADN enfermé dans une coque protéique. Il n’est pas capable de se reproduire seul : il ne peut fabriquer plus de virus qu’en entrant dans une cellule et en utilisant sa machinerie cellulaire. Lorsqu’un virus infecte une cellule hôte, il enlève sa couche de protéines et dirige la machinerie de l’hôte pour qu’elle transcrive et traduise son matériel génétique. La cellule détournée assemble les composants répliqués en milliers de descendants viraux, qui peuvent rompre et tuer la cellule hôte. Les nouveaux virus continuent ensuite à infecter plus de cellules hôtes.
 
 <h4>Pourquoi étudier les virus ?</h4>
 
 Les virus peuvent infecter différents types de cellules : bactéries, plantes et animaux. Les virus qui ciblent les bactéries, appelées bactériophages (ou phages), sont très abondants. La recherche actuelle se concentre sur la phagothérapie pour traiter les infections bactériennes multirésistantes chez l’homme. Les virus qui infectent les plantes cultivées sont également très étudiés puisque les épidémies entraînent d’énormes pertes de récoltes et économiques.
 
 Les virus ont été découverts pour la première fois au XIXe siècle lorsqu’une culture économiquement importante, la plante de tabac, a été en proie à une mystérieuse maladie, plus tard identifiée comme le virus de la mosaïque du tabac. Les virus d’animaux sont d’une grande importance tant dans la recherche vétérinaire que dans la recherche médicale. En outre, les virus sont à la base de nombreuses maladies humaines, allant du rhume, de la varicelle et de l’herpès, à des infections plus dangereuses comme la fièvre jaune, l’hépatite et la variole.
 
 <h4>La structure d’un virus</h4>
 
 Les virus sont présents dans une variété de formes qui sont spécialisées dans l’attaque de leur cellule cible. Les deux principales composantes de tous les virus sont le génome viral et sa couche protéique protectrice, connue sous le nom de capside. Le génome viral est composé d’ARN ou d’ADN à simple ou double brin, et il code les protéines qui composent la capside. Ensemble, le génome viral et la capside sont connus sous le nom de nucléocapside.
 
 Une caractéristique unique de nombreux virus d’eucaryotes est la présence d’une membrane de phospholipides, connue sous le nom d’enveloppe qui entoure la capside. Cette enveloppe provient généralement des membranes des cellules hôtes précédemment infectées, mais elle peut également comporter des protéines virales (appelées protéines d’enveloppe) qui y sont attachées. Enfin, certains virus d’animaux comportent un groupe de protéines codées par le virus, le tégument viral, dans l’espace entre l’enveloppe et la capside.
 
 <h4>L’infection virale</h4>
 
 Le cycle de vie viral peut être divisé en cinq étapes suivantes : attachement, pénétration, réplication, assemblage et libération. Les protéines à la surface du virus l’aident à reconnaître des cellules hôtes spécifiques. Certains virus utilisent ces protéines de surface pour se lier aux récepteurs des cellules hôtes et initier l’internalisation par endocytose, tandis que les virus enrobés d’enveloppe peuvent fusionner directement avec la membrane de la cellule hôte.
 
 Certains bactériophages n’entrent pas dans la cellule ; ils injectent leur génome (et les enzymes virales) dans la cellule hôte. Une fois à l’intérieur de la cellule, le virus perd son enrobage et dirige la machinerie de la cellule hôte qui transcrit et traduit son génome. La cellule hôte emballe les nouvelles copies du génome viral en particules virales pour faire une descendance. Les virus de la descendance peuvent être stockés dans la cellule hôte avant d'être libérés ou être continuellement extrudés de la cellule par bourgeonnement hors de la membrane cellulaire. Le cycle d’infection virale est classé comme lytique ou lysogène. Dans le cycle lytique, les nouveaux virus éclatent de la cellule hôte, la tuant ainsi. Dans le cycle lysogène, l’ADN viral est incorporé dans le génome de l’hôte où il est dormant et il est copié chaque fois que la cellule hôte se reproduit.

Que sont les virus ?

Aperçu

Un virus est une particule infectieuse microscopique constituée d’un génome d’ARN ou d’ADN enfermé dans une enveloppe protéique. Il n’est pas capable de se reproduire sur ...

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how viruses infect cells and multiply

Viruses infect cells and make new copies by using a host cell’s own machinery. A virus is a tiny infectious particle with an RNA or DNA genome inside a protective protein coat. It cannot reproduce on its own, so it must enter a cell to make more viruses.

After infection, the virus sheds its protein coat and uses the host cell to transcribe and translate its genetic material. The cell then assembles many new viral particles. In many cases, these new viruses rupture the host cell and kill it. The released viruses can then infect other cells.

Viruses can attack bacteria, plants, and animals. Viruses that infect bacteria are called bacteriophages, or phages, and they are very abundant. Scientists also study phages as a possible therapy for multidrug-resistant bacterial infections in humans. Plant viruses matter because outbreaks can cause major crop and economic losses.

Animal viruses are important in both veterinary research and medical research. Viruses were first discovered in the 19th century during studies of tobacco plants with a mysterious disease. That disease was later identified as Tobacco mosaic virus. Viruses are also linked to many human illnesses, including the common cold, chickenpox, herpes, yellow fever, hepatitis, and smallpox.

Viruses vary in shape, but they share two main parts: the viral genome and the capsid. The genome can be single-stranded or double-stranded RNA or DNA, and it carries the instructions for making capsid proteins. The genome and capsid together form the nucleocapsid. Many eukaryotic viruses also have an envelope, which is a phospholipid membrane often taken from a host cell. Some animal viruses also contain a tegument, a layer of viral proteins between the envelope and the capsid.

A viral infection usually follows five stages: attachment, entry, replication, assembly, and release. Viral surface proteins help the virus recognize the right host cell. Some viruses bind to host receptors and enter by endocytosis, while enveloped viruses can fuse with the host cell membrane. Some bacteriophages do not enter the cell at all. They inject their genome and viral enzymes into the host cell instead.

Once inside, the virus is uncoated and directs the host cell to copy and read its genetic material. The host cell packages the new genomes into viral particles to form progeny viruses. These progeny may stay in the cell for a time or leave by budding from the cell membrane. Viral infection can be lytic or lysogenic. In the lytic cycle, new viruses burst out and kill the host cell. In the lysogenic cycle, viral DNA becomes part of the host genome, stays dormant, and is copied when the host cell divides.

How Viruses Infect Cells and Multiply

what are viruses?

Viruses infect cells and make new copies by using a host cell’s own machinery. A virus is a tiny infectious particle with an RNA or DNA genome inside a protective protein coat. It cannot reproduce on its own, so it must enter a cell to make more viruses. After infection, the virus sheds its protein coat and uses the host cell to transcribe and translate its genetic material. The cell then assembles many new viral particles. In many cases, these new viruses rupture the host cell and kill it.

viral structure

How Virus Shapes Help Infection

Viruses have specific structures that help them infect host cells and trigger immune responses. Each virus has a core that contains either a DNA-based or RNA-based genome. That core is surrounded by a protective protein coat called the capsid. The capsid is made of smaller subunits called capsomeres. The capsid and the genome-containing core together are called the nucleocapsid. Viruses are often grouped by capsid design. Most viruses have either icosahedral or helical capsids, but some have...

lytic cycle of bacteriophages

How Phages Infect and Burst Bacteria

Bacteriophages, or phages, are viruses that infect bacteria. They have a distinctive head-tail shape. In the lytic cycle, a phage first attaches to the outside of a bacterial cell. Tail proteins bind to specific receptor proteins on the bacterium’s surface. After attachment, the phage tail injects its DNA genome into the bacterial cytoplasm. The phage then uses the bacterium’s cellular machinery to make proteins needed for replication and dispersal. When phage replication is complete, some of...

lysogenic cycle of bacteriophages

Prophage Integration in Bacteria

The lysogenic cycle shows how bacteriophages can infect bacteria without killing the host right away. In this cycle, the phage genome joins the bacterial genome. The integrated phage DNA is called a prophage, and the bacterium copies it each time it replicates. Some prophages can switch back into the lytic cycle. This change can happen after DNA damage or even without an obvious outside trigger. When the prophage is induced, the phage begins making new virus particles again. Prophages can...

retrovirus life cycles

How Retroviruses Replicate in Host Cells

Retroviruses replicate by copying their RNA genome into DNA inside a host cell. After entry, the viral enzyme reverse transcriptase makes a double-stranded DNA copy from the single-stranded RNA genome. The viral DNA is then carried into the nucleus and inserted into the host genome by integrase. Once integrated, the retroviral DNA is transcribed whenever the host genome is transcribed. This produces new viral RNA that can be copied and used again. The new RNA moves into the cytoplasm, where it...

viral recombination

How Viruses Swap Genes and Spread

Viral gene swapping can happen when more than one virus infects the same cell. After the viruses break open and expose their genomes for replication, matching regions can pair up and exchange sequences. This process is called recombination. Viruses with segmented genomes can also swap whole segments, which is called reassortment. These changes can help create new diseases. Some diseases can infect more than one species. Pigs can be infected by some human and bird viruses, along with the...

viral mutations

How Virus Mutations Drive Outbreaks

Virus mutations can change how quickly a virus evolves and spreads. A mutation is a change in the sequence of bases in DNA or RNA. Some mutations happen when the genome is copied. They are caused by errors made by polymerase enzymes, which replicate DNA or RNA. RNA viruses often change faster than DNA viruses because RNA polymerase does not proofread its work. That means RNA-based genomes, such as HIV, can gather mutations more quickly. Mutation and recombination give viruses the raw material...

key terms and concepts

Learn how viruses enter host cells, use cellular machinery to replicate, and spread through lytic and lysogenic cycles in plants, animals, and bacteria.

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HS-LS1-4

How Viruses Infect Cells and Multiply

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