Virus Replication: A Summary of Viral Replication Steps

VIRUS REPLICATION Steps of Adsorption viral replication Penetration and uncoating Eclipse synthesis of new viral components 1) Adsorption HIV → gp120 binds to CD4 Influenza → hemagglutinin binds to glycoprotein receptors...

VIRUS REPLICATION Steps of Adsorption viral replication Penetration and uncoating Eclipse synthesis of new viral components 1) Adsorption HIV → gp120 binds to CD4 Influenza → hemagglutinin binds to glycoprotein receptors present on respiratory epithelium 2) Penetration and Uncoating After viral attachment to the host cell, the virus enters the cell through one of the following mechanisms: 1. Viropexis (Endocytosis): The virus is engulfed by the host cell membrane. It is enclosed within endosomes, then released into the cytoplasm. 2. Fusion: The viral envelope fuses with the host cell membrane. The viral nucleic acid enters directly into the cytoplasm. Common in enveloped viruses (e.g., Herpes virus). 3. Direct Penetration: The viral genome is directly injected into the host cell. Seen in non-enveloped viruses (e.g., bacteriophage). Uncoating: The viral nucleic acid is released from the capsid. Often aided by lysosomal enzymes inside the host cell. 3) Eclipse Phase Refers to the period after viral entry and before the appearance of new viral particles. During this phase, the virus is actively replicating inside the cell, but no complete virions are detectable. Duration: Usually ranges from 1 to 20 hours in most human viruses. 4) Synthesis DNA replication occurs in the nucleus. of New Viral Components DNA Viruses: Protein synthesis occurs in the cytoplasm Exception: Poxviruses After the eclipse phase, the virus replicate entirely in starts synthesizing its nucleic acids the cytoplasm. and proteins using the host cell machinery. Both RNA replication and protein synthesis RNA Viruses: occur in the cytoplasm. DNA viruses Early proteins replication DNA progeny Late proteins 1) Early Viral DNA is transcribed into early mRNA using the host’s DNA-dependent RNA polymerase. Proteins Early mRNA is translated into non-structural proteins in the cytoplasm. These early proteins include: Other enzymes needed Thymidine kinase DNA polymerase for DNA replication 2) DNA Progeny DNA replication usually starts 2–4 hours after infection. Single-stranded DNA viruses must first convert to double-stranded DNA to allow transcription by DNA dependent RNA polymerase. 3) Late After DNA replication, late Proteins genes are transcribed into late mRNA. Late mRNA is translated into structural proteins, such as the capsid.