Viruses must infect a host cell to hijack their reproductive machinery to replicate; they cannot replicate themselves.
Viruses cannot replicate by themselves – they are essentially just protein envelopes containing genetic material. Viruses depend on their host’s cellular resources to replicate their genetic material and make more protein envelopes. In comparison to bacteria, fungi and animal/plant cells viruses are much smaller and require magnification using an electron microscope.
The replication mechanism depends on the viral genome. DNA viruses usually use host cell proteins and enzymes to make additional DNA that is transcribed to messenger RNA (mRNA), which is then used to direct protein synthesis of the ribosomes into making copies of the virus. RNA viruses usually use the RNA core as a template for synthesis of viral genomic RNA and mRNA. The viral mRNA directs the host cell to synthesize viral enzymes and capsid proteins and to assemble new virions. Retroviruses, such as HIV, have an RNA genome that must be reverse transcribed into DNA, which then is incorporated into the host cell genome.
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Key Points
• Viruses must infect a host cell to use their reproductive machinery to replicate.
• The method of replication depends on the type of virus.
• Viruses are much smaller than most eukaryotic and prokaryotic cells in size.
Key Terms
retrovirus: a type of RNA virus that integrates its genome into the DNA of its host cell
HIV: a retrovirus that causes acquired immune deficiency syndrome (AIDS)
genome: the whole of an organisms hereditary information encoded in its DNA
ribosomes: organelles that carry out protein synthesis
mRNA: is a single-stranded RNA molecule that is read by the ribosome in the process of producing a protein
capsid: the protein shell of a virus