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Viruses and cellular defence

Transmission of viruses

Viruses can be transmitted by travelling within a fluid, such as mucus droplets from a sneeze.

Once they have entered a living host they begin the process of replication [replication: Production of an identical copy.. This replication process always follows a particular pattern.


AttachmentVirus binds to a specific receptor on the host cell surface.Stage 1 of virus replication. A virus attaches itself to a host cell.
PenetrationThe viral nucleic acid enters the cell.Stage 2 of virus replication. The virus releases its nucleic acid, containing its genetic instructions, into the host cell.
Synthesis of new components.

Viral nucleic acid takes over control of the cell metabolism stopping the cell's normal nucleic acid and protein synthesis.

Viral nucleic acid is replicated using nucleotides from the host cell.

Protein coats are manufactured using the amino acids of the host cell.

Stages 3 and 4 of virus replication. The genetic material takes over the host cell, using the host cell's enzymes, nucleotides, amino acids and ATP, to replicate more copies of the genetic information. Using this genetic information, the cell creates new DNA or RNA coats, depending on the type of virus.
AssemblyWhole virus particles are made when the nucleic acids are surrounded by the protein coats.Stage 5 of virus replication. The synthesised DNA or RNA is formed into new viruses.
ReleaseMany viral particles are released when the cell bursts open (lysis) or by slow leakage.Stage 5 of virus replication. The newly formed viruses break out of the host cell. This is known as lysis, or budding.

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