Introduction to Medical Virology: Understanding Viruses

RNA Viruses and mRNA Synthesis

• Positive-sense RNA viruses synthesize mRNA directly for host translation.
• Negative-sense RNA viruses must first be converted to mRNA before being translated by host ribosomes.

Virus Assembly and Components

• RNA viruses typically assemble in the cytoplasm of host cells.
• Capsid and peplomers are synthesized during the maturation stage of viral replication.

Virus Release Mechanisms

• Non-enveloped viruses exit host cells via cell lysis, causing the cell to burst.
• Enveloped viruses leave the host cell through budding, acquiring a portion of the host cell membrane as their envelope.

Viral RNA and Host Interaction

• Viruses that utilize their own RNA as mRNA are classified as positive-sense RNA viruses.
• Protein building blocks of DNA viruses migrate to the nucleus for recovery and assembly.

Stages of Viral Replication

• Synthesis of replication enzymes occurs during the early phase of the viral replication cycle.
• The term "maturation" refers to the stage where viral structure components, like capsid and peplomers, are synthesized.

Virus Characteristics and Host Dependency

• Viruses are classified as obligate intracellular parasites due to their dependence on host cellular machinery for replication.
• The primary reason for viral dependence on host cells is to harness cellular resources for energy and protein synthesis.

Viral Interaction with Host Cells

• Viruses exhibit metabolic inactivity outside host cells, relying on hosts for metabolic functions.
• The diversity of hosts for viruses is largely determined by the presence of specific receptor proteins on the host cell surface.

Viral Entry and Host Relationships

• The ability of a virus to enter a host cell is dictated by the presence of specific receptors that the virus can attach to.
• The primary reason viruses replicate within host cells is to ensure efficient production and assembly of new virions.

Distinction from Cellular Microorganisms

• Viruses are distinguished from cellular microorganisms by their lack of cellular structure and independent metabolism.
• The attachment structure of naked viruses is usually a protein that interacts with host cell receptors.

Structure and Symmetry of Viruses

• A virion's main function is to deliver the viral genome into the host cell.
• Segmented genome viruses include certain influenza viruses, characterized by multiple RNA segments.
• The length of the nucleocapsid in helical viruses is determined by the length of the viral RNA and the encapsidation process.
• Large viruses, around 200-300 nm in diameter, typically exhibit icosahedral symmetry.

Envelopes in Viruses

• The envelope of enveloped viruses originates from the host cell membrane during the budding process.
• The main structure that forms the envelope of enveloped viruses is composed of lipid bilayers and viral glycoproteins.

This quiz provides an introduction to medical virology, focusing on the nature of viruses, their genetic makeup, and their dependence on host cells for metabolism and replication. It covers the basic characteristics of viruses and their classification as acellular organisms.