Question

The human immunodeficiency virus (HIV) is a retrovirus with an RNA genome. a. Assume that each HIV contains two RNA genomes and 50 molecules of the reverse transcriptase enzyme. b. Assume that each reverse transcriptase molecule acts on each RNA genome 10 times to produce DNA. c. Assume that an integrase enzyme successfully integrates \(1 \%\) of the available reverse transcribed HIV genomes into the genome of a human host cell. d. Assume that each integrated copy of the viral genome is transcribed 500 times/day. How many HIV RNA genomes are created per day from one infected cell?

Short answer

5000 RNA genomes are created per day from one infected cell.

Step-by-step solution

Determine the number of DNA copies produced

Each reverse transcriptase acts on each of the 2 RNA genomes 10 times. With 50 molecules of reverse transcriptase, the number of DNA copies produced can be calculated as follows:\[ 2 \text{ RNA genomes} \times 10 \times 50 = 1000 \text{ DNA copies} \]

Calculate the number of DNA copies integrated

Only 1% of the reverse transcribed HIV genomes successfully integrate into the human genome. So, calculate this:\[ 1\% \times 1000 = 10 \text{ DNA copies integrated} \]

Calculate the amount of RNA transcripts produced per day

Each integrated DNA copy is transcribed 500 times a day. Therefore, the total RNA genomes produced per day is given by:\[ 10 \text{ integrated copies} \times 500 = 5000 \text{ RNA genomes produced per day} \]

Key concepts

Retrovirus

HIV, or human immunodeficiency virus, is classified as a retrovirus. Retroviruses are a unique group of viruses that use RNA instead of DNA as their genetic material. This characteristic allows them to integrate their genetic code into the DNA of the host cell, effectively hijacking the cell's machinery to replicate themselves.

Retroviruses have a peculiar life cycle that involves converting their RNA genome into DNA once inside a host cell. This process is necessary because although the host cells primarily use DNA for their genetic instructions, retroviruses need to insert their genetic information into the host's DNA to replicate. This integration step is what makes retroviruses particularly persistent and hard to eradicate once they infect a host.

Reverse Transcriptase

A key enzyme in the life cycle of HIV is reverse transcriptase. This enzyme is essential for converting the viral RNA genome into a DNA form that can integrate into the host cell's genome. This process of transformation is known as reverse transcription.

During reverse transcription, the reverse transcriptase enzyme reads the viral RNA and builds a complementary strand of DNA. HIV brings its own reverse transcriptase packed within the virus particle. As described in the exercise, each HIV virion contains two RNA genomes and about 50 molecules of reverse transcriptase, each capable of producing multiple DNA copies from the viral RNA templates due to their repetitive binding and replication action. This adaptation increases the likelihood that the viral DNA can integrate successfully into the host genome.

Viral Integration

Once the viral RNA is transcribed into DNA, the next crucial step is viral integration. This involves incorporating the newly synthesized viral DNA into the host's own DNA. The HIV virus uses another enzyme called integrase to facilitate this process.

Viral integration is a stochastic event. Not all reverse transcribed viral genomes find their way into the host's DNA. In typical scenarios, as highlighted in the exercise, only about 1% of these genomes manage successful integration. Once integrated, the viral DNA becomes a permanent part of the host's genetic material, allowing the virus to exploit the host's cellular machinery to produce viral particles.

RNA Transcription

RNA transcription is a vital process where the integrated viral DNA is converted back into RNA. This step is crucial for producing new viral genomes and proteins necessary for assembling new virus particles.

In the case of HIV, once the viral DNA is integrated, it is treated like any other part of the host's genome. The host's cellular mechanisms transcribe this integrated DNA into viral mRNA through a process catalyzed by the host's own RNA polymerases.

• Each integrated viral DNA can undergo transcription multiple times a day, leading to the production of numerous RNA transcripts.
• In the given exercise, each integrated copy undergoes transcription 500 times a day, resulting in thousands of new viral RNA genomes ready for packaging into new viral particles.

By continuously redirecting the host cell's machinery to produce viral components, HIV ensures its propagation and persistence within the host.