Pg:- 22,23

• Viruses are microscopic infectious agents that can only replicate inside the living cells of an organism.

• They are composed of genetic material (either DNA or RNA) enclosed in a protein coat called a capsid.

• Some viruses also have an outer lipid envelope derived from the host cell membrane.

Structure of Viruses:

1. Genetic Material: The core of a virus contains its genetic material, which can be either DNA or RNA, but not both. This genetic material carries the information necessary for the virus to reproduce.

2. Capsid: The genetic material is encased in a protein shell known as a capsid. The capsid protects the viral genome and aids in the delivery of the viral genetic material into host cells.

3. Envelope: Some viruses have an outer lipid envelope, which is derived from the host cell membrane during viral replication or release. The envelope may contain viral proteins that are crucial for attaching to and entering host cells.

4. Proteins: The surface of a virus often has proteins that allow it to recognize and bind to specific receptors on the host cell, facilitating entry into the cell.

Life Cycle of a Virus:

1. Attachment: The virus attaches to a specific receptor on the surface of the host cell.

2. Entry: The virus or its genetic material enters the host cell. This can occur through direct penetration, endocytosis, or membrane fusion.

3. Replication and Transcription: Once inside, the viral genome is replicated and transcribed. For RNA viruses, replication and transcription occur in the cytoplasm. For DNA viruses, these processes generally occur in the nucleus.

4. Protein Synthesis: The host cell's machinery is used to synthesize viral proteins.

5. Assembly: Newly synthesized viral genomes and proteins are assembled into new viral particles.

6. Release: New viral particles are released from the host cell. This can occur through cell lysis, which kills the host cell, or through budding, which allows the host cell to survive for some time while releasing viruses.

Types of viruses:

1. DNA Viruses: These viruses have DNA as their genetic material. Examples include the Herpesviridae family (herpes simplex virus) and the Poxviridae family (smallpox virus).

2. RNA Viruses: These viruses have RNA as their genetic material. Examples include the Flaviviridae family (hepatitis C virus) and the Coronaviridae family (SARS-CoV-2).

3. Retroviruses: These are a type of RNA virus that can reverse transcribe their RNA into DNA using the enzyme reverse transcriptase. The DNA is then integrated into the host genome. An example is the Human Immunodeficiency Virus (HIV).

Viral Infections and Diseases:

Viruses can infect a wide range of hosts, including animals, plants, fungi, and bacteria (bacteriophages). Viral infections can cause a variety of diseases, ranging from mild to severe, including the common cold, influenza, HIV/AIDS, Ebola, and COVID-19.

Immune Response to Viruses:

The immune system responds to viral infections through innate and adaptive immune responses. The innate response involves general mechanisms such as the activation of interferons and natural killer cells. The adaptive response involves the activation of specific lymphocytes (B cells and T cells) that target the virus for destruction.

Antiviral Therapies:

Antiviral therapies aim to inhibit the replication of viruses. These include:

1. Antiviral Drugs: These drugs target specific stages of the viral life cycle. Examples include acyclovir for herpesvirus infections and antiretrovirals for HIV.

2. Vaccines: Vaccines stimulate the immune system to recognize and combat viruses. They can be live-attenuated, inactivated, or subunit vaccines.