THE LIFECYCLE OF HIV
From the study on the origin of hiv, HIV is a retrovirus belonging to the genus Lentivirus, and as a retrovirus carries an RNA genome that is transcribed into DNA by the use of viral reverse transcriptase after the virion enters the target cell.
The viral genome contains 9 genes encoding 16 viral proteins; three major genes (gag, pol, env) encoding structural proteins and three viral enzymes: protease, integrase and reverse transcriptase (RT); two regulatory (rev, tat) and four accessory (vif, vpu, nef, vpr) genes.
The viral surface protein gp120 of HIV-1 binds to the cluster of differentiation (CD4) receptor on the host cell, inducing a conformational change that enables binding to a_−chemokine coreceptor, either CCR5 or CXCR4.
the lifecycle of hiv, the CD4 receptor is expressed on the surface of T lymphocytes, monocytes, macrophages, microglia and dendritic cells. During the earlier part of the infection, viral strains (called R5 or M-tropic strains) use the CCR5 co-receptor, primarily expressed on activated memory CD4+ T-cells and macrophages. At later stages of the disease, about 50 % of infected individuals experience a shift in viral tropism to a predominately CXCR4-tropic (X4 or T-tropic strains) or mixed R5/X4 (dualtropic strains) viral population.
The shift to the use of CXCR4, expressed mainly on naïve T-cells, is usually accompanied by a rapid decline in CD4+ T-lymphocytes numbers and clinical progression to AIDS. After binding to the cell surface, fusion of the viral and cell membranes allows the virus to enter the cell.
By reverse transcription, the RNA genome is transcribed into a DNA intermediate (unintegrated provirus) that is subsequently transported to the nucleus and integrated into the host cell genome by viral integrase.
The process of reverse transcription is very error-prone, likely due to the lack of proof-reading capacity of RT in the study ofthe lifecycle of hiv.
As a consequence, the virus is highly mutagenic, allowing it to evade neutralizing antibodies and to develop resistance to antiretroviral agents . Following integration, production of viral proteins and assembly of new virions takes place at the cell surface.