🔬General Biology I Unit 21 – Viruses

What Are Viruses?

• Viruses are microscopic infectious agents that can only replicate inside the living cells of other organisms
• Consist of genetic material (DNA or RNA) encased in a protein coat called a capsid
• Viruses are not considered living organisms as they lack the ability to reproduce independently and do not have their own metabolism
• Viruses are obligate intracellular parasites meaning they require a host cell to survive and multiply
• Viruses infect all forms of life including animals, plants, bacteria, and archaea
• Viral infections can lead to various diseases in humans (influenza, HIV, COVID-19)
• Some viruses can cause latent infections where they remain dormant in the host cell for extended periods before reactivating (herpes simplex virus)
• Viruses are highly specific to their host species and cell types due to the unique receptors on their surface that bind to complementary receptors on the host cell

Viral Structure and Components

• Viruses are typically much smaller than bacteria and can only be visualized using electron microscopes
• The basic structure of a virus consists of genetic material (DNA or RNA) surrounded by a protein coat called a capsid
  • The capsid protects the viral genome and facilitates attachment to host cells
• Some viruses also have an additional lipid envelope derived from the host cell membrane that surrounds the capsid (enveloped viruses)
  • The envelope contains viral glycoproteins that aid in host cell recognition and entry
• The combination of the capsid and the genetic material is called the nucleocapsid
• Viral genomes can be single-stranded or double-stranded, and can be DNA or RNA
  • The genome encodes the essential proteins required for viral replication and assembly
• Viruses may also contain enzymes necessary for replication (reverse transcriptase in retroviruses)
• The shape of viruses can vary greatly, including icosahedral, helical, and complex structures
  • The shape is determined by the arrangement of the capsid proteins

Viral Life Cycle

• The viral life cycle consists of several stages: attachment, entry, replication, assembly, and release
• Attachment: Viruses bind to specific receptors on the host cell surface using proteins in their capsid or envelope
• Entry: Viruses enter the host cell through endocytosis or membrane fusion, releasing their genetic material into the cytoplasm
• Replication: The viral genome is replicated using the host cell's machinery and viral enzymes
  • DNA viruses typically replicate in the nucleus, while RNA viruses replicate in the cytoplasm
• Assembly: Newly synthesized viral components are assembled into complete virions
  • The capsid proteins self-assemble around the viral genome
• Release: Mature virions are released from the host cell through lysis (cell rupture) or budding (enveloped viruses)
  • Lysis leads to the death of the host cell, while budding allows the cell to continue functioning
• Some viruses integrate their genetic material into the host genome (provirus) and can remain latent for extended periods before reactivating

Types of Viruses

• Viruses can be classified based on their genetic material, capsid symmetry, presence of an envelope, and host specificity
• DNA viruses: Contain DNA as their genetic material (adenoviruses, herpesviruses, poxviruses)
  • Most DNA viruses have double-stranded DNA genomes, but some have single-stranded DNA (parvoviruses)
• RNA viruses: Contain RNA as their genetic material (influenza viruses, coronaviruses, retroviruses)
  • RNA viruses can have single-stranded or double-stranded RNA genomes
  • Retroviruses (HIV) use reverse transcriptase to convert their RNA genome into DNA, which integrates into the host genome
• Enveloped viruses: Possess a lipid envelope derived from the host cell membrane (influenza viruses, herpesviruses, HIV)
• Non-enveloped viruses: Lack a lipid envelope and only have a protein capsid (adenoviruses, rotaviruses)
• Bacteriophages: Viruses that specifically infect bacteria (T4 phage, lambda phage)
• Plant viruses: Viruses that infect plants (tobacco mosaic virus, cauliflower mosaic virus)
• Animal viruses: Viruses that infect animals, including humans (influenza viruses, rabies virus, measles virus)

Viral Infections and Diseases

• Viruses are responsible for numerous diseases in humans, animals, and plants
• Viral infections can be acute (short-lived) or chronic (long-lasting)
  • Acute infections are typically resolved by the immune system (common cold, influenza)
  • Chronic infections persist for extended periods and can lead to ongoing health issues (HIV, hepatitis B)
• Viral diseases in humans include:
  • Respiratory infections (influenza, common cold, COVID-19)
  • Sexually transmitted infections (HIV, human papillomavirus, herpes simplex virus)
  • Gastrointestinal infections (rotavirus, norovirus)
  • Neurological infections (rabies, polio, measles)
  • Hemorrhagic fevers (Ebola, dengue)
• Viral infections in animals can have significant economic impacts (foot-and-mouth disease in livestock)
• Plant viral diseases can lead to substantial crop losses (tobacco mosaic virus, potato virus Y)
• Some viruses are oncogenic, meaning they can cause cancer (human papillomavirus, Epstein-Barr virus)

Immune Response to Viruses

• The immune system is responsible for defending the body against viral infections
• Innate immune response: The first line of defense against viruses
  • Includes physical barriers (skin, mucous membranes), antimicrobial peptides, and immune cells (natural killer cells, macrophages)
  • Innate immune cells recognize viral components and initiate an inflammatory response
• Adaptive immune response: A specific response tailored to the invading virus
  • Involves B lymphocytes (produce antibodies) and T lymphocytes (directly kill infected cells)
  • Antibodies neutralize viruses by binding to their surface proteins, preventing attachment and entry into host cells
  • Cytotoxic T cells recognize and destroy virus-infected cells
• Interferons: Signaling proteins produced by infected cells that help limit viral replication and spread
  • Interferons stimulate nearby cells to enter an antiviral state and activate immune cells
• Memory B and T cells: Long-lived cells that provide rapid and enhanced response upon re-exposure to the same virus
• Some viruses have evolved mechanisms to evade or suppress the immune response (HIV, herpes simplex virus)

Antiviral Treatments and Vaccines

• Antiviral drugs are medications used to treat viral infections by inhibiting specific stages of the viral life cycle
  • Nucleoside analogues: Interfere with viral DNA or RNA synthesis (acyclovir for herpesviruses, zidovudine for HIV)
  • Protease inhibitors: Block viral proteases required for maturation of viral proteins (ritonavir for HIV)
  • Entry inhibitors: Prevent viral attachment or fusion with host cells (maraviroc for HIV)
• Antiviral drugs are most effective when administered early in the course of infection
• Vaccines are biological preparations that stimulate the immune system to provide protection against viral diseases
  • Vaccines contain inactivated viruses, attenuated (weakened) viruses, or viral components (proteins or peptides)
  • Vaccines induce the production of specific antibodies and the development of memory B and T cells
  • Successful vaccines have been developed for many viral diseases (measles, polio, influenza, HPV)
• Herd immunity: When a significant portion of a population is vaccinated, reducing the spread of the virus and protecting unvaccinated individuals
• Challenges in vaccine development include rapidly mutating viruses (influenza) and viruses with complex immune evasion strategies (HIV)

Viruses in Research and Medicine

• Viruses are valuable tools in scientific research and have contributed to numerous advances in biology and medicine
• Viral vectors: Modified viruses used to deliver genetic material into cells for gene therapy or vaccine development
  • Adenoviruses and retroviruses are commonly used as viral vectors
  • Viral vectors have been used in the development of gene therapies for genetic disorders and cancer
• Phage therapy: The use of bacteriophages to treat bacterial infections
  • Phages are highly specific to their bacterial hosts and can be used as an alternative to antibiotics
  • Phage therapy is particularly useful against antibiotic-resistant bacteria
• Viruses as model systems: Studying viruses has provided insights into fundamental biological processes
  • The study of bacteriophages led to the discovery of DNA as the genetic material and the understanding of gene regulation
  • Retroviruses (Rous sarcoma virus) have been used to study the molecular basis of cancer
• Oncolytic viruses: Genetically engineered viruses that specifically target and kill cancer cells
  • These viruses exploit the altered metabolism and immune evasion strategies of cancer cells
  • Examples include modified herpes simplex virus and adenovirus strains
• Viruses have also been used in the production of vaccines and recombinant proteins
  • Insect viruses (baculoviruses) are used to produce recombinant proteins in insect cell cultures
  • Plant viruses (tobacco mosaic virus) have been engineered to produce vaccines and other biopharmaceuticals in plants