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Viral Structure and Function

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Cell Biology

All viruses consist of a protective protein coat called a capsid that surrounds a core of genetic material, either DNA or RNA. Some viruses also have an additional wrapping of lipid bilayer around their capsids called an envelope. Viruses are classified as obligate intracellular parasites, which means they must enter a host cell to survive and replicate. They lack cellular machinery, so they hijack the machinery of host cells to carry forward the next generation of viruses.

Viral genomes can be made of DNA or RNA. RNA viruses can either be positive-sense, meaning their genomes can be directly translated by host cell ribosomes, or negative-sense, which means their genomes must be transcribed into mRNA by viral RNA replicase. The mRNA strand is then translated by host cell ribosomes. Bacteriophages are viruses that exclusively infect bacteria and archaea. They use their tail sheaths to inject their genetic material into the cytoplasm of host cells.

Lesson Outline

<ul> <li>Viral structure <ul> <li>Capsid: protective protein coat surrounding genetic material</li> <li>Genetic material: either DNA or RNA</li> <li>Envelope: additional lipid bilayer wrapping around capsids in some viruses</li> </ul> </li> <li>Obligate intracellular parasites <ul> <li>Hijack cellular machinery of host cells for replication</li> <li>Produce viral progeny</li> </ul> </li> <li>Viral genomes <ul> <li>Positive-sense RNA genomes: can be directly translated by host cell ribosomes</li> <li>Negative-sense RNA genomes: require transcription into mRNA by viral RNA replicase, then translation by host cell ribosomes</li> <li>DNA genomes</li> </ul> </li> <li>Bacteriophages <ul> <li>Infect bacteria and archaea</li> <li>Use tail sheaths to inject genetic material into host cell cytoplasm</li> <li>Lytic and lysogenic cycles for infecting host cells</li> </ul> </li> </ul>

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What is the basic structure of a virus and the role of viral capsid?

Viruses have a simple structure, which consists of genetic material (DNA or RNA) surrounded by a protein coat called a capsid. The capsid acts as a protective barrier to the genetic material, as well as being essential for viral infectivity. The capsid's shape and surface proteins play significant roles in viral attachment, entry, and release, as well as helping the virus evade the host's immune system.

What is the difference between DNA and RNA viruses, and how do they replicate?

DNA viruses contain deoxyribonucleic acid (DNA) as their genetic material, while RNA viruses contain ribonucleic acid (RNA). DNA viruses typically replicate in the nucleus of the host cell, where they use the host's DNA polymerase to synthesize new DNA. RNA viruses usually replicate in the host cell's cytoplasm using viral RNA replicase to synthesize new RNA. Furthermore, RNA viruses can be classified into positive-sense RNA and negative-sense RNA, which affects the way they are translated into functional proteins by the host cell ribosomes.

What are positive-sense RNA and negative-sense RNA viruses?

Positive-sense RNA viruses have a single-stranded RNA genome that can be directly translated by the host cell ribosomes into functional viral proteins. In contrast, negative-sense RNA viruses have RNA genomes that are complementary to the viral mRNA, requiring a viral RNA replicase to synthesize positive-sense RNA copies that can then be translated into proteins by the host cell ribosomes.

What are obligate intracellular parasites, and how do viruses exhibit this characteristic?

Obligate intracellular parasites are organisms that cannot reproduce or carry out fundamental metabolic functions outside of a host cell. Viruses are examples of obligate intracellular parasites, as they depend on the host cell machinery to replicate their genetic material, synthesize essential proteins, and assemble new viral particles. Viruses cannot replicate or perform these functions on their own, making them completely reliant on host cells for survival and reproduction.

What are bacteriophages and how do they interact with host cells?

Bacteriophages are viruses that specifically infect bacterial cells. They attach to specific receptors on the bacterial cell surface and inject their genetic material (DNA or RNA) into the cell. Once inside, the viral genomes hijack the host cell's machinery to replicate and express viral proteins, ultimately leading to the assembly and release of new bacteriophage particles, which can infect other bacterial cells. Bacteriophages can have lytic and/or lysogenic life cycles, the former resulting in the lysis and death of the host cell, while the latter results in the integration of the viral genome into the host's genetic material to be replicated and expressed along with the host's own DNA.