Virology Quiz on Viral Genetics and Life Cycle

Viruses, Their Life Cycle, and Cancer

• Viruses are microscopic infectious agents containing DNA or RNA as genetic material.
• Viral genomes are enclosed within a protein coat called a capsid.
• Some viruses have an outer envelope composed of a lipid membrane and glycoproteins.
• Viruses replicate solely inside a host cell, utilizing host cell machinery for their own functions.
• The human population has recently experienced a global pandemic caused by a virus (COVID-19).
• As of April 2024, approximately 7 million people worldwide had died from COVID-19.
• An estimated 10³¹ viruses exist on Earth, which is significantly more than the number of stars in the universe.
• Each virus typically infects only a few particular species.
• The host specificity is called host range while the cell type a virus infects is called tropism.

Viruses as Agents of Cancer

• Approximately 20% of human cancers are linked to viral infections.
• Cancer can arise from a host’s response to a viral infection or from viral genes impacting the host cell.
• Some viruses integrate their genetic material into host chromosomes, introducing oncogenes—cancer-causing genes.
• These oncogenes can disrupt host cell functions related to cell division and lead to unregulated cell growth, resulting in tumors.

Viral Classification

• Viruses are classified based on their nucleic acid type (DNA or RNA), capsid symmetry, and the presence or absence of an envelope.
• Virus classification systems include Baltimore and others.
• Viral classification factors such as genome type and nucleic acid symmetry.
• An example of such classifications: groups I through VII for different kinds of viral genome structures.

Virus Life Cycles

• Bacteriophages: Bacteriophages, viruses that infect bacteria, undergo either a lytic or lysogenic cycle, with lytic phages leading to host cell lysis.
• Non-enveloped viruses (e.g., poliovirus): Viruses such as poliovirus adhere to cell receptors for entry; then, capsids are destroyed releasing viral RNA for replication.
• Enveloped viruses (e.g., HIV, Zika virus): Entry can involve fusion with the host cell membrane or endocytosis.

Cancer-causing viruses and their associated tumors

• Certain viruses, such as Epstein-Barr Virus (EBV), Hepatitis B/C virus (HBV/HCV), human herpes virus-8, and Human Immunodeficiency Virus (HIV), are known to contribute to various cancers.

Cellular Basis of Cancer

• Cancer is a genetic disease arising from mutations in genes that control cell growth and division.
• Oncogenesis produces tumors, clumps of rapidly-dividing cells that can invade surrounding tissues.
• Cancer development may be caused by inherited mutations, DNA damage caused by environmental factors, and viral infections.
• The cell cycle includes phases such as G0, G1, S, G2, and M.
• Checkpoints during the cell cycle ensure precise progression through these phases.
• Viruses like HPV can disrupt cell cycle regulation, leading to cancer development.
• p53 is a tumor suppressor protein that detects DNA damage and stops cell division.
• HPV E6/7 proteins block p53 activation, leading to uncontrolled cell division.

The Role of Proto-oncogenes and Tumor Suppressors

• Proto-oncogenes are normal cellular genes that promote cell growth and division.
• However, mutations in proto-oncogenes can convert them into oncogenes, which promote uncontrolled cell growth.
• Tumor suppressor genes encode proteins that normally suppress cell division.
• Mutations in tumor suppressor genes can lead to the loss of tumor suppression, resulting in uncontrolled cell growth.
• Multiple mutations in both proto-oncogenes and tumor suppressor genes are frequently implicated in cancer development.

Mechanisms of Oncogenesis

• The multiple-mutation model proposes that cancer development originates from multiple gene mutations.
• This model highlights the accumulation of mutations (in proto-oncogenes and tumor suppressors) and their roles in driving cancer.
• The various phases of the cell cycle, each with unique control points, are crucial for regulated cell division.