Viral replication

• Viruses can only multiply on living cells because the cells provide the necessary energy and machinery for the synthesis of viral nucleic acids and proteins
• Viruses cannot multiply by binary fission like other microorganisms; instead, new viral particles are synthesized from the host cell
• Outcomes of viral replication:

  • Lytic Pathway: Viruses damage most of the host's membrane, causing the contents of the cytoplasm to leak out, leading to rapid cell death and release of viruses
  • Lysogenic Pathway: Infected cells are used as factories for the production of viral genes, with DNA being replicated many times before mass lysis occurs
• General Steps in Viral Replication Cycle:

  1. Attachment: Viruses attach to the host cell through their ligands, aiding them to attach to the receptors of the host cell

  2. Penetration: Viruses can penetrate the host cell through membrane fusion, hemifusion, or endocytosis

  3. Uncoating: Viral capsid is degraded by enzymes, releasing the viral nucleic acid into the cytoplasm for further processes
• During replication, some viruses may utilize the following enzymes:

  • DdDp: DNA-dependent DNA polymerase
  • DdRp: DNA-dependent RNA polymerase
  • RdRp: RNA-dependent RNA polymerase
  • RdDp: RNA-dependent DNA polymerase
• Some things to remember:

  • + sense and - sense strands are always complementary to each other
  • Only + sense strands have the right codons for translation
  • - sense strands serve as templates to create + sense strands
  • Messenger RNAs (mRNA) are always + sense RNAs
• Biosynthesis in DNA Viruses:

  • Nuclear ds DNA viruses utilize DdRp present in the host cell to transcribe mRNA
  • Cytoplasmic ds DNA viruses use cell DdDp to produce ds DNA which is then transcribed by the cell DdRp to produce mRNA
• Biosynthesis in RNA Viruses:

  • ds RNA viruses use their - sense strands to create mRNA
  • + sense RNA viruses bind directly to ribosomes without needing to be transcribed first
  • - sense RNA viruses use RdRp to create mRNA
  • + sense RNA viruses with reverse transcriptase (RT) convert - sense RNA strand into - sense DNA strand, which integrates into the host genome
• Assembly:

  Once everything is translated into amino acids/proteins, the viruses are assembled and packaged into a whole virion
• Release:

  • Non-enveloped viruses are released through cell lysis
  • Enveloped viruses can be released through budding/exocytosis