Virus genome replication and gene expression are critical steps in the viral life cycle that enable the production of new viral particles. These processes vary depending on the type of virus, particularly whether the virus has a DNA or RNA genome, and whether it is single-stranded or double-stranded.
1. Virus Genome Replication
DNA Viruses:
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Double-Stranded DNA (dsDNA) Viruses:
- These viruses, such as herpesviruses, typically replicate in the host cell nucleus.
- The viral DNA is transcribed into mRNA by the host’s RNA polymerase.
- Host or viral DNA polymerase then replicates the viral DNA to produce new copies.
- Example: Herpes Simplex Virus (HSV).
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Single-Stranded DNA (ssDNA) Viruses:
- ssDNA viruses, like parvoviruses, also replicate in the nucleus.
- The ssDNA is first converted into a double-stranded form by the host’s DNA polymerase.
- This dsDNA is then used as a template for transcription and replication.
- Example: Adeno-associated virus (AAV).
RNA Viruses:
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Positive-Sense Single-Stranded RNA (+ssRNA) Viruses:
- These viruses, such as poliovirus, have genomes that can be directly translated into proteins by the host’s ribosomes.
- Viral RNA-dependent RNA polymerase (RdRp) synthesizes a complementary negative-sense RNA, which serves as a template for producing more +ssRNA.
- Example: SARS-CoV-2 (causing COVID-19).
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Negative-Sense Single-Stranded RNA (-ssRNA) Viruses:
- These viruses, like influenza virus, carry an RNA genome that is complementary to the mRNA.
- The viral RdRp transcribes the -ssRNA into mRNA, which is then translated into proteins.
- The RdRp also replicates the viral genome by synthesizing a complementary +ssRNA strand that serves as a template for new -ssRNA genomes.
- Example: Influenza virus.
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Double-Stranded RNA (dsRNA) Viruses:
- These viruses, such as rotaviruses, have a genome that consists of two RNA strands.
- The viral RdRp transcribes the dsRNA into mRNA, which is then translated.
- The viral RdRp also replicates the dsRNA genome within a viral replication complex.
- Example: Rotavirus.
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Retroviruses:
- Retroviruses like HIV have an RNA genome that is reverse-transcribed into DNA by the viral enzyme reverse transcriptase.
- The viral DNA is integrated into the host cell’s genome, where it can be transcribed into mRNA and translated into proteins.
- The host cell’s machinery replicates the integrated viral DNA as part of the cell’s own DNA.
- Example: HIV.
2. Virus Gene Expression
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Transcription:
- Viral mRNA is synthesized either directly from the viral genome (in RNA viruses) or from the transcribed DNA (in DNA viruses).
- Transcription often occurs in stages, with early genes expressed first to produce proteins necessary for genome replication and late genes expressed to produce structural proteins for new virions.
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Translation:
- Viral mRNA is translated by the host cell’s ribosomes to produce viral proteins.
- These proteins include enzymes required for viral replication, structural proteins for assembling new virions, and regulatory proteins that modulate host cell functions.
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Post-Translational Modifications:
- Many viral proteins undergo post-translational modifications, such as glycosylation or cleavage, to become fully functional.
- For example, some viral glycoproteins are processed in the host’s endoplasmic reticulum and Golgi apparatus before being transported to the site of virion assembly.
Key Points:
- The replication and gene expression strategies of viruses are tightly integrated with the host cell’s machinery.
- Viruses often manipulate host cell processes to prioritize the synthesis of viral components over host cell components.
- The complexity and specific mechanisms of replication and expression vary widely among different types of viruses.
This detailed orchestration allows viruses to efficiently produce progeny and propagate infection within the host.