Structure and genome of HIV

Human Immunodeficiency Virus (HIV) is a retrovirus that causes HIV infection and over time Acquired Immunodeficiency Syndrome (AIDS). AIDS is a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive. Understanding the structure and genome of HIV is crucial for developing treatments and vaccines.

Structure of HIV

HIV is characterized by its complex structure, which is key to its ability to replicate and infect host cells. The virus is roughly spherical and has a diameter of about 120 nanometers. Its structure includes several layers:

• Envelope: The outermost layer is a lipid bilayer derived from the host cell membrane. Embedded in this envelope are glycoproteins, primarily gp120 and gp41, which play a crucial role in the virus's ability to attach to and enter host cells.
• Capsid: Beneath the envelope is the capsid, a conical or cylindrical structure made of the protein p24. The capsid encloses the virus's genetic material and several enzymes essential for the HIV life cycle.
• Genome: HIV's genome consists of two identical strands of RNA, along with reverse transcriptase, integrase, and protease enzymes. These enzymes are critical for converting the viral RNA into DNA, integrating it into the host's genome, and processing new viral proteins.

Genome of HIV

The HIV genome is approximately 9,800 nucleotides long and contains nine genes encoded by the two RNA strands. These genes can be grouped into three categories:

• Structural genes: gag, pol, and env encode structural proteins for the virus. Gag produces the matrix, capsid, and nucleocapsid proteins. Pol encodes the enzymes reverse transcriptase, integrase, and protease. Env codes for the envelope glycoproteins gp120 and gp41.
• Regulatory genes: tat and rev are involved in regulating the expression of the virus's genes. Tat enhances transcription of the viral RNA, while Rev regulates the transport of RNA from the nucleus to the cytoplasm.
• Accessory genes: nef, vif, vpr, and vpu encode proteins that help the virus evade the host's immune system, enhance viral replication, and assist in the release of new virions from infected cells.

Replication Cycle

The replication cycle of HIV involves several steps: 1. Attachment and Entry: HIV binds to the CD4 receptor and a co-receptor (usually CCR5 or CXCR4) on the surface of a host cell, typically a T-helper cell, macrophage, or dendritic cell. 2. Reverse Transcription: Once inside the host cell, HIV's reverse transcriptase converts its RNA genome into DNA. 3. Integration: The viral DNA is transported into the nucleus and integrated into the host's DNA by the enzyme integrase. 4. Transcription and Translation: The host cell's machinery transcribes the integrated viral DNA into mRNA, which is then translated into viral proteins. 5. Assembly and Release: New viral particles are assembled and bud from the host cell, acquiring their envelope in the process.

Impact on the Immune System

HIV primarily targets CD4+ T cells, leading to a gradual depletion of these cells and weakening of the immune system. This makes the body more susceptible to opportunistic infections and certain types of cancers, marking the progression to AIDS.

Prevention and Treatment

Prevention strategies include safe sex practices, needle exchange programs, and pre-exposure prophylaxis (PrEP). Treatment involves antiretroviral therapy (ART), which, while not a cure, can significantly prolong the lives of those infected and reduce the risk of transmission.