The Human Immunodeficiency Virus (HIV) is an enveloped virus that targets immune system cells, leading to Acquired Immunodeficiency Syndrome (AIDS). HIV is unequivocally a single-stranded RNA (ssRNA) virus, not double-stranded (dsRNA). This distinction is significant because the single-stranded nature of its genetic material dictates how the virus hijacks a host cell to replicate. HIV belongs to a unique class of viruses known as retroviruses, requiring a complex conversion process before it can integrate into the host’s DNA.
The Genetic Material of HIV
The genetic material contained within the core of an HIV particle is single-stranded ribonucleic acid (ssRNA). This RNA is described as “positive-sense,” meaning its sequence is directly readable by the host cell’s machinery, functioning similarly to a messenger RNA (mRNA) molecule. The virus does not use this RNA immediately for protein synthesis upon infection.
A distinct feature of the HIV genome is that it is diploid, meaning two identical copies of the positive-sense ssRNA are packaged within each viral particle. The RNA sequence is approximately 9.2 to 9.75 kilobases long and contains nine genes. These two strands are tightly bound to nucleocapsid proteins and are enclosed within the capsid, a cone-shaped protein shell. The capsid also contains the enzymes necessary for the initial stages of infection, including reverse transcriptase and integrase.
What Makes HIV a Retrovirus
HIV is classified as a retrovirus, a term derived from the unique way it reverses the standard flow of genetic information. The central dogma of molecular biology describes information flowing from DNA to RNA to protein. Retroviruses, however, use a specialized enzyme to convert their RNA genome back into DNA.
This process is carried out by the viral enzyme Reverse Transcriptase (RT), which is packaged inside the viral core. RT allows the virus to create a DNA copy from its RNA template, a process known as reverse transcription. The ability to carry out this RNA-to-DNA conversion is the defining characteristic that places HIV in the retrovirus family.
The Reverse Transcriptase enzyme is notoriously error-prone because it lacks the “proofreading” function found in host cell DNA polymerases. This high error rate leads to frequent mutations during the copying process, which generates immense genetic diversity within the viral population. This rapid mutation is a primary reason why the virus can quickly develop resistance to antiviral drugs and evade the host’s immune system.
The Replication Process
The single-stranded RNA structure dictates the functional life cycle of HIV, beginning after the viral particle enters a host cell, typically a CD4+ T cell. Once inside the host cell’s cytoplasm, the two ssRNA molecules are released from the viral core. The Reverse Transcriptase enzyme then begins converting the viral ssRNA into a DNA copy.
Reverse transcription occurs in a step-wise manner, first using the ssRNA template to synthesize a complementary DNA strand, forming a hybrid RNA/DNA molecule. The RT enzyme then degrades the original RNA strand and synthesizes a second, complementary DNA strand. This results in a stable double-stranded DNA (dsDNA) molecule, which is often referred to as the provirus.
This newly formed dsDNA is then transported to the host cell’s nucleus. Another viral enzyme, Integrase, catalyzes the joining of the viral dsDNA into the host cell’s own chromosomal DNA. Once integrated, the viral genome is permanently embedded within the host’s genetic material, a state known as the provirus. This integrated provirus may lie dormant for long periods, which is the basis for the persistent nature of the HIV infection.