Introduction to Viruses structure and Classification

Viruses are non-cellular infectious agents that can replicate only inside the living cells of a host organism. They are much smaller than bacteria and lack the machinery needed for metabolism or protein synthesis, making them obligate intracellular parasites.

1. PMAS Arid Agriculture University Rawalpindi Group Presentation Submitted to : MAM ANIEE Class : BS Biology Semester 5 Subject : Microbiology

2. VIRUSES Topic:

3. Definition A virus can be defined as a submicroscopic infectious agent that can only replicate inside the living cells of an organism. It consists of genetic material, either DNA or RNA, enclosed within a protein coat called a capsid. Some viruses also have an outer lipid envelope derived from the host cell membrane. Viruses are obligate intracellular parasites, meaning they cannot reproduce or carry out metabolic processes on their own and must infect a host cell to multiply. They infect all forms of life, including animals, plants, fungi, bacteria, and archaea

4. Characteristics of Viruses • Viruses are microscopic infectious agents, much smaller than bacterial cells, usually ranging from 20 to 300 nanometers. • They are obligate intracellular parasites, meaning they cannot reproduce outside living host cells. • Viruses infect a wide range of organisms including animals, plants, fungi, bacteria, and archaea. • They do not possess cellular structure, lack cytoplasm, organelles, and metabolism; they rely entirely on the host cell for energy and replication. • Viruses contain genetic material either as DNA or RNA, which directs the synthesis of viral components.

5. General Properties and Morphology of Viruses • The basic structure of a virus consists of a nucleic acid core surrounded by a protective protein coat called the capsid. • Capsids are composed of multiple protein subunits called capsomers arranged in symmetrical patterns. • Some viruses have an outer lipid envelope derived from the host cell membrane, embedded with viral glycoproteins essential for host attachment. • Morphologically, viruses can be helical (rod-shaped), icosahedral (spherical or polyhedral), or complex forms (such as bacteriophages with tail structures). • The size and shape of virus particles are important for their classification and infectivity.

6. Chemical Composition and Structure of Viruses • Viral nucleic acid may be single-stranded or double-stranded DNA or RNA, but never both in one virus. • The capsid proteins protect the nucleic acid from degradation and help in the delivery of the genome into host cells. • The lipid envelope, when present, protects the virus and contains proteins that facilitate virus binding to host cells. • Some viruses package enzymes like reverse transcriptase or RNA polymerase inside the capsid, which are critical for early phases of replication after infection. • The arrangement of capsomers and envelope proteins forms the unique morphology for each virus species.

7. Viral Replication Cycle • The viral replication cycle begins with attachment, where the virus binds to specific receptors on the surface of a susceptible host cell. • Entry occurs by fusion of the viral envelope with the host membrane or by endocytosis for non-enveloped viruses. • After entry, uncoating takes place to release the viral genome into the host cell cytoplasm or nucleus depending on the virus. • Viral genome replication and transcription utilize host or viral enzymes according to the type of nucleic acid. • Viral proteins and nucleic acids are synthesized and assembled into new virions. • Mature virions exit the host cell by budding (for enveloped viruses) or cell lysis (for non-enveloped viruses), completing the cycle.

8. Eclipse Phase of Viral Replication • The eclipse phase refers to the period right after viral entry and uncoating until new infectious virus particles appear. • During this phase, no complete virions can be detected inside the host cell. • It is characterized by the synthesis of viral RNA, proteins, and replication of viral genomes. • This phase is critical as the virus takes over host machinery for producing new virus components.

9. Abnormal Replicative Cycles • Some viruses enter dormant stages known as latency, where replication halts, and the viral genome persists silently in the host (e.g., Herpes simplex virus). • Persistent infections involve slow replication with continuous low-level production of virus (e.g., HIV). • Viruses may induce oncogenesis by integrating into host DNA, changing normal cell growth controls, leading to cancer (e.g., Human papillomavirus causing cervical cancer). • Defective viruses and satellite viruses require helper viruses for replication.

10. Basis for Viral Classification • Viruses are classified primarily based on the type of nucleic acid (DNA or RNA), which can be single or double-stranded. • Capsid symmetry is another major criterion: icosahedral (spherical) or helical (rod-shaped) structures. • The presence or absence of an envelope is important for classifying enveloped vs. non-enveloped viruses. • Other factors include genome size and organization, mode of replication, and host range. • The Baltimore classification system groups viruses according to their genome type and replication strategy.

11. Nomenclature of Viruses • Virus names often derive from the disease they cause, such as the Influenza virus (flu). • Other names come from geographic locations where they were first identified, like the Ebola virus. • Some viruses are named after the host or tissue they infect, for example, bacteriophages infect bacteria. • Viral taxonomy is regulated by the International Committee on Taxonomy of Viruses (ICTV), which assigns family, genus, and species names.

12. Prions and Viroids • Prions are infectious proteins that cause neurodegenerative diseases by inducing abnormal folding of normal proteins in the brain. They contain no nucleic acid and are resistant to usual methods of sterilization (e.g., Creutzfeldt-Jakob disease). • Viroids are the smallest infectious agents composed only of a short strand of circular RNA without any protein coat. They cause diseases in plants by interfering with normal plant RNA function. • Both prions and viroids challenge traditional concepts of infectious agents as they lack conventional genetic material.