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📘 A crystal that multiplies only inside a cell

Picture⠀​a⠀​drop⠀​containing⠀​millions⠀​of⠀​tobacco⠀​mosaic⠀​virus⠀​particles.⠀​Outside⠀​a⠀​host,⠀​they⠀​do⠀​not⠀​eat,⠀​grow,⠀​or⠀​generate⠀​energy;⠀​some⠀​viruses⠀​can⠀​even⠀​be⠀​crystallized.⠀​Yet⠀​when⠀​the⠀​right⠀​particle⠀​reaches⠀​a⠀​

4
lessons
~20 min
to learn
Adults
level
Start the course →

What you’ll learn

  1. A particle at life's edgeDefine a virus and virion, identify the genome, capsid, and optional envelope, and explain why viral structures and genome types differ.A virus is an acellular infectious entity that packages DNA or RNA in protein, sometimes adds a lipid envelope, and depends on a host cell to reproduce.
  2. A cell becomes the factoryTrace a general viral cycle from receptor-specific attachment through entry, gene expression, genome copying, assembly, and release.Compatible receptors and cell machinery determine whether entry succeeds; viral instructions then redirect the cell to build and release new virions.
  3. Infection meets a hostDistinguish infection from disease and explain how viral evolution and layered immune defenses shape an infection's outcome.Replication may damage cells without always causing illness, mutations supply variation for selection, and barriers, innate responses, antibodies, T cells, and memory oppose infection.
  4. Viruses in a larger worldDistinguish antibiotics from antivirals, describe the ecological importance of bacteriophages, and evaluate why viruses challenge simple definitions of life.Viruses require virus-specific control strategies, influence microbial ecosystems far beyond human disease, and combine evolution with complete dependence on cellular hosts.

Questions this course answers

Which feature is found in every virion?

Every virion packages one type of nucleic-acid genome in a protein capsid. Envelopes occur only in some viruses, and virions lack ribosomes.

All viruses use the same kind of genome and the same path to make messenger RNA.

Viral genomes include several forms of DNA and RNA. Each type requires a particular route and sometimes virus-carried enzymes to produce messenger RNA.

Put these general stages of a productive viral infection in order.

Attachment and entry make the genome accessible; gene expression and copying provide components that can then assemble and leave the cell.

Why does a viral mutation become common in a population?

Mutation creates variation without foresight. Selection can increase a variant when its traits improve success in a particular environment.

Why can a person be infected with a virus without developing noticeable disease?

Infection means a virus entered and replicated; disease depends on the amount, location, and kind of damage and on the host response.

Why do antibiotics not treat viral infections?

Antibiotics act on bacterial machinery such as cell-wall synthesis or bacterial ribosomes. Antivirals must target virus-specific stages instead.

Grounded in trusted sources

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