What is the Difference Between Provirus and Retrovirus

The main difference between provirus and retrovirus is that provirus is an integrated form of a retroviral genome within the host cell’s genome, whereas retrovirus is a type of virus that replicates by reverse transcription.

Virology is a scientific field of study that involves the study of viruses, their structure, replication, interaction with the host organisms, and the diseases they cause. Viruses are microscopic infectious agents that can only replicate inside living cells. They are composed of genetic material, either DNA or RNA, surrounded by a capsid protein coat. Provirus and retrovirus are two important terms in virology.

Key Areas Covered

1. What is a Provirus 
     – Definition, Transmission, Formation
2. What is a Retrovirus
     – Definition, Transmission, Life Cycle
3. Similarities Between Provirus and Retrovirus
     – Outline of Common Features
4. Difference Between Provirus and Retrovirus
     – Comparison of Key Differences

Key Terms

Provirus, Retrovirus

What is a Provirus

Provirus is an integrated form of a retroviral genome within the host cell’s genome. The formation of a provirus involves many steps, like entry and reverse transcription, integration, and provirus formation.

1. Entry and Reverse Transcription

Retroviruses enter host cells and release their RNA genome. Within the host cell cytoplasm, reverse transcriptase synthesizes a complementary DNA strand using the viral RNA as a template. This reverse transcription process generates a double-stranded DNA molecule.

2. Integration

The newly synthesized complementary DNA Molecule that represents the retroviral genome is transported into the nucleus of the host cell. Once inside the nucleus, an enzyme called integrase facilitates the integration process. Integrase catalyzes the insertion of viral DNA into the host cell’s chromosomal DNA  at specific target sites.

3. Provirus Formation

The viral DNA becomes a permanent part of the host cell’s genome. The integrated viral DNA is referred to as a provirus. It is replicated along with the host cell’s DNA during cell division, ensuring its transmission to daughter cells.

Proviruses have no direct involvement in transmission. Once a retrovirus infects a host cell and integrates its DNA into the host cell’s genome, the provirus can transmit to daughter cells during cell division.

What is a Retrovirus

A retrovirus is a type of virus that uses a special enzyme called reverse transcriptase to translate its genetic information into DNA. Retroviruses have a simple structure compared to other viruses. They consist of an outer envelope derived from the host cell’s membrane, surrounding a protein coat called the capsid. Inside of the capsid lies the viral genome, which is a single-stranded RNA molecule. The RNA  genome of retroviruses is unique among RNA viruses because it carries the information required to synthesize DNA.

Retroviruses can be classified into different groups based on their genetic similarities and disease associations. Moreover, the two primary genera of retroviruses that infect humans are Orthoretroviruses and Spumaretroviruses.

Life Cycle of Retrovirus

Retroviruses replicate through a complex life cycle. Several stages are involved in the life cycle of retrovirus.

Attachment and Entry

Retroviruses attach to specific receptors on the surface of the host cells, initiating the infection process. Once attached, the virus enters the host cell by fusing its envelope with the cell membrane or through receptor-mediated endocytosis.

Reverse Transcription

After entry, the retroviral RNA genome is converted into DNA by the action of reverse transcriptase. Reverse transcriptase synthesizes a complementary DNA molecule using the viral RNA as a template. This step is unique to retroviruses and distinguishes them from other RNA viruses.

Integration

The synthesized complementary DNA is transported into the host cell nucleus, where it integrates into the host cell’s genome. This integration is catalyzed by the enzyme integrase. The integrated viral DNA is then referred to as a provirus. Then it becomes a permanent part of the host cell’s genome.

Transcription and Translation

The proviral DNA is transcribed into RNA by the host cell’s transcription machinery. Some of the transcribed RNA molecules serve as viral genomes for the production of new virus particles, while others function as mRNA for the synthesis of viral proteins. The host cell’s ribosomes translate the viral proteins.

Assembly and Release

The newly synthesized viral components, including RNA genomes, proteins, and enzymes, assemble to form new virus particles. The mature virions bud from the host cell, acquiring their outer envelope as they exit. The released virus particles can then go on to infect new cells and repeat the replication cycle.

Retroviruses can be transmitted through various routes, including sexual contact, blood-to-blood contact, vertical transmission from mother to child during childbirth or breastfeeding and contaminated needles or syringes.

Similarities Between Provirus and Retrovirus

• Proviruses and retroviruses are types of genetic material that can integrate into the host cell’s DNA.
• Both proviruses and retroviruses are transmitted to daughter cells during cell division.

Difference Between Provirus and Retrovirus

Definition

Provirus is an integrated form of a retroviral genome within the host cell’s genome, whereas retrovirus is a type of virus that replicates by reverse transcription.

Transmission

Proviruses are not directly involved in the transmission, whereas retroviruses can be transmitted through various routes.

Reverse Transcriptase Enzyme

While provirus does not encode a reverse transcriptase enzyme, retrovirus encodes a reverse transcriptase enzyme.

Conclusion

In brief, provirus and retrovirus are two important terms in virology. The main difference between provirus and retrovirus is that provirus is an integrated form of a retroviral genome within the host cell’s genome, whereas retrovirus is a type of virus that replicates by reverse transcription.

Reference:

1. “Retrovirus.” Encyclopedia Britannica. 
2. “Provirus.” Bionity.

Image Courtesy:

1. “Life Cycle of a Retrovirus” By Mrdavis21 – Own work (CC BY-SA 3.0) via Commons Wikimedia