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

K12 Standards

Louisiana Student Standards for Science

High School

Life Sciences

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

From Molecules To Organisms: Structures And Processes

HS-LS1-8

What are Viruses?

Ein Virus ist eine Struktur, die ihr eigenes genomisches Material in Form von RNA oder DNA trägt, um in eine Wirtszelle einzudringen und sich dort zu vermehren.
 Nachdem sich ein Virus an Oberflächenrezeptoren der Wirtszelle gebunden hat, dringt es ein und zerlegt sich schnell, wobei es sein genetisches Material entschlüsselt.
 Im Fall von DNA-Viren lenkt die virale DNA die Replikationsproteine der Wirtszelle, um neue Kopien des viralen Genoms zu synthetisieren, die dann transkribiert und in virale Proteine übersetzt werden.
 Schließlich setzt der Wirt diese viralen Komponenten wieder zu Nachkommen zusammen, so dass ein einzelnes Viruspartikel Tausende mehr produziert, was oft zum Tod der Wirtszelle führt.

Yamauchi, Yohei und Ari Helenius. "Viruseintritt auf einen Blick." J Cell Sci 126, Nr. 6 (15. März 2013): 1289–95. [<a href="https://doi.org/10.1242/jcs.119685" target="__blank">Quelle</a>]

Lin, Derek M, Britt Koscella und Henry C Lin. "Phagentherapie: Eine Alternative zu Antibiotika im Zeitalter der Multiresistenz." World Journal of Gastrointestinal Pharmacology and Therapeutics 8, Nr. 3 (6. August 2017): 162–73. [<a href="https://doi.org/10.4292/wjgpt.v8.i3.162" target="__blank">Quelle</a>]

Nicaise, Valérie. "Immunität von Nutzpflanzen gegen Viren: Ergebnisse und zukünftige Herausforderungen." Frontiers in Plant Science 5 (21. November 2014). [<a href="https://doi.org/10.3389/fpls.2014.00660" target="__blank">Quelle</a>]

was sind viren?

<h4>Überblick</h4>
 
 Ein Virus ist ein mikroskopisch kleiner, infektiöser Partikel, der aus einem in einer Proteinhülle eingehüllten RNA- oder DNA-Genom besteht. Er ist nicht in der Lage, sich selbst zu vermehren. Der Virus kann sich nur durch Eindringen in eine Zelle vermehren. Er nutzt die zellulären Bestandteile der befallenen Zelle, um weitere Viren herzustellen. Wenn ein Virus eine Wirtszelle infiziert, entfernt der Virus seine Proteinhülle und benutzt die Maschinerie des Wirts um sein genetisches Material zu transkribieren und zu übersetzten. Die befallene Zelle baut die replizierten Komponenten zu Tausenden von Virusnachkommen zusammen, welche die Wirtszelle aufbrechen und töten können. Die neuen Viren infizieren dann weitere Wirtszellen.
 
 <h4>Warum sollte man Viren untersuchen?</h4>
 
 Viren können verschiedene Arten von Zellen infizieren: bakterielle, pflanzliche und tierische Zellen. Viren, die Bakterien befallen, werden Bakteriophagen (oder Phagen) genannt, welche weit verbreitet sind. Die aktuelle Forschung konzentriert sich auf die Behandlung von multiresistenten bakteriellen Infektionen beim Menschen mit Hilfe der Phagentherapie. Viren, die Kulturpflanzen infizieren, werden auch viel erforscht, da Epidemien zu enormen Ernte -und Wirtschaftsverlusten führen können.
 
 Viren wurden zuerst im 19. Jahrhundert entdeckt, als eine Kulturpflanze befallen wurde, die von wirtschaftlicher Bedeutung war. Tabakpflanzen wurden von einer mysteriösen Krankheit befallen. Später wurde diese als Tabakmosaikvirus identifiziert. Tierische Viren sind sowohl in der veterinärmedizinischen Forschung als auch in der medizinischen Forschung von großer Bedeutung. Darüber hinaus verursachen Viren viele menschlichen Krankheiten, wie zum Beispiel die Erkältung, Windpocken und Herpes bis hin zu gefährlicheren Infektionen wie Gelbfieber, Hepatitis und Pocken.
 
 <h4>Die Struktur eines Virus</h4>
 
 Viren existieren in vielen Formen, die darauf spezialisiert sind die Ziel-Zelle zu attackieren. Die beiden Hauptbestandteile aller Viren sind das virale Genom und die schützende Proteinhülle, welches als Kapsid bezeichnet wird. Das virale Genom besteht aus ein -oder doppelsträngiger RNA oder DNA und kodiert die Proteine, die das Kapsid bilden. Zusammen werden das virale Genom und das Kapsid als Nukleokapsid bezeichnet.
 
 Ein außergewöhnliches Merkmal vieler eukaryotischer Viren ist das Vorhandensein einer Phospholipidmembran, welches eine Hülle um das Kapsid bildet. Diese Hülle stammt typischerweise von den Membranen zuvor infizierter Wirtszellen, kann aber auch gebundene virale Proteine (sogenannte Hüllproteine) enthalten. Einige tierische Viren besitzen auch ein Cluster von viruskodierten Proteinen in dem Raum zwischen Hülle und Kapsid, das sogenannte virale Tegument.
 
 <h4>Virusinfektion</h4>
 
 Der virale Lebenszyklus kann in die folgenden fünf Schritte unterteilt werden: Bindung, Eintritt, Replikation, Zusammenbau und Freisetzung. Die Proteine auf der Oberfläche des Virus helfen ihm dabei, spezifische Wirtszellen zu identifizieren. Einige Viren nutzen diese Oberflächenproteine, um Wirtszellrezeptoren zu binden und die Internalisierung durch Endozytose einzuleiten. Dabei können behüllte Viren direkt mit der Membran der Wirtszelle fusionieren.
 
 Einige Bakteriophagen gelangen gar nicht erst in die Zelle. Sie injizieren ihr Genom (und die viralen Enzyme) in die Wirtszelle. Der Virus hat keine Hülle mehr wenn er dann im Inneren der Zelle ist und übernimmt die Maschinerie der Wirtszelle zur Transkription und Übersetzung ihres Genoms. Die Wirtszelle verpackt die neuen Kopien des Virusgenoms in Viruspartikel, um Nachkommen zu zeugen. Die viralen Nachkommen können vor der Freisetzung in der Wirtszelle gespeichert werden oder kontinuierlich durch das Abknospen von der Zellmembran aus der Zelle abgegeben werden. Virale Infektionszyklen stuft man als lytisch oder lysogen ein. Im lytischen Zyklus brechen die neuen Viren aus der Wirtszelle aus und töten sie ab. Im lysogenen Zyklus wird die virale DNA in das Wirtsgenom eingebaut, wo sie ruht und bei jeder Replikation der Wirtszelle kopiert wird.

Was sind Viren?

Überblick

Ein Virus ist ein mikroskopisch kleines infektiöses Partikel, das aus einer RNA oder einem DNA-Genom besteht, das in einer Proteinhülle eingeschlossen ist. Es ist nicht in der Lage, sich auf ...

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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-LS1-1

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HS-LS1-2

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HS-LS1-3

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HS-LS1-4

How Viruses Infect Cells and Multiply

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