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The life cycle of HIV consists of following steps. They are:- 1. Entry 2. Reverse Transcription and Integration 3. Transcription and Translation 4. Assembly, Budding and Maturation.
Step # 1. Entry:
Virus binds to the host cells by attaching its Gp 120 to the host cell receptor. CD4 molecule present on T lymphocytes is a predominant receptor. Entry of virus into host cell requires certain co-receptors e.g. CCR-5, CXCR-4, CCR-2 and CCR-3 etc. Once the gp 41 of the virus fuses with the host cell membrane, the capsid is uncoated. The contents of the HIV particle are then released into the cell, leaving the envelope behind.
Step # 2. Reverse Transcription and Integration:
A ribonucleo protien complex capable of reverse transcription is formed and forms cDNA under the effect of viral enzyme, the reverse transcriptase. The nucleoprotein complex formed after transcription comprises of linear double stranded DNA, the gag matrix protein, the accessory vpr protein and the viral integrase. This is called pre- integration complex and is transported into the host cell nucleus. Viral integrase leads to a complex series of enzymatic steps and integration of virus occurs with the host cell genetic material forming the provirus.
Step # 3.. Transcription and Translation:
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HIV provirus may lie dormant within a cell for a long time. But when the cell becomes activated, it treats HIV genes in much the same way as human genes. First it converts them into messenger RNA (using human enzymes). Then the messenger RNA is transported outside the nucleus, and is used as a blueprint for producing new HIV proteins and enzymes.
Step # 4. Assembly, Budding and Maturation:
The strands of messenger RNA produced by the cell containing HIV genetic material gather together with newly made HIV proteins and enzymes to form new viral particles, which are then released from the cell. The enzyme protease plays a vital role at this stage of the HIV life cycle by chopping up long strands of protein into smaller pieces, which are used to construct mature viral cores.
The newly matured HIV particles are ready to infect another cell and begin the replication process all over again. In this way the virus quickly spreads through the human body. HIV replicates rapidly with several billion new viruses made every day in a person infected with HIV. What makes HIV so difficult to stop, is its ability to mutate and evolve.
