Question

In very basic terms describe the structure of a retroviral virion.

Short answer

A retroviral virion consists of an outer lipid bilayer envelope with embedded glycoprotein spikes, an inner protein capsid protecting the viral RNA genome, and essential viral enzymes such as reverse transcriptase, integrase, and protease. This structure allows the retrovirus to enter host cells, reverse transcribe its RNA genome into DNA, and propagate efficiently.

Step-by-step solution

Define a retroviral virion

A retroviral virion is an infectious particle produced by a retrovirus, which contains RNA as its genetic material. It is responsible for transferring the viral genome from one host cell to another to replicate and propagate the virus.

Describe the outer envelope

The outer envelope of a retroviral virion is a lipid bilayer membrane, derived from the host cell's membrane. It serves to protect the viral contents within and aids in the attachment and entry into a new host cell. Embedded in this envelope are viral glycoprotein spikes that help the virus bind to specific receptors on the new host cell.

Identify the capsid

Inside the outer envelope lies the capsid, which is a protein shell that encases and protects the viral RNA genome and other essential viral components. The capsid is composed of repeating protein subunits called capsomeres. The shape and arrangement of these capsomeres may vary, but their primary function is to protect the genetic material of the virus.

Describe the viral RNA genome

The retroviral virion contains two copies of single-stranded RNA as its genome. The RNA contains all the genetic information required for the virus to replicate and synthesize its proteins. Retroviruses are unique because their RNA can be reverse transcribed, a process by which the RNA is converted into DNA in the host cell. This DNA can then integrate into the host cell's genome, allowing the virus to replicate alongside the host's cells.

Detail the viral enzymes

Retroviral virions enclose several viral enzymes necessary for their replication cycle, including reverse transcriptase, integrase, and protease. Reverse transcriptase catalyzes the conversion of viral RNA into DNA, integrase facilitates the integration of this newly synthesized DNA into the host cell's genome, and protease processes viral protein precursors into functional proteins required for the formation of new virions. To summarize, a retroviral virion consists of an outer envelope, a capsid, viral RNA genome, and viral enzymes. These components play essential roles in the replication and propagation of the retrovirus.

Key concepts

RNA genome

The RNA genome in a retroviral virion is a fascinating component. Retroviruses, unlike many other viruses, contain RNA as their genetic material rather than DNA. This is a key characteristic of retroviruses and is critical for their life cycle.
The genome of a retrovirus consists of two identical strands of single-stranded RNA. These RNA strands are packed within the capsid of the virion.
The unique aspect of the retroviral RNA is its ability to undergo reverse transcription. This process converts the viral RNA into DNA inside the host cell. Once converted, this viral DNA can integrate into the host cell's genome, establishing a stable infection.

• This integration allows the virus to replicate each time the host cell divides.
• The host cell machinery will unknowingly replicate the virus along with its own DNA.

This approach of integration is why certain retroviruses, such as HIV, are known for creating persistent and difficult-to-eradicate infections.

Viral enzymes

A retroviral virion possesses a set of specialized enzymes that are crucial for its replication within a host cell. These enzymes enable the virus to effectively invade host cells and reproduce itself.
The most notable enzymes found within a retroviral virion include reverse transcriptase, integrase, and protease:

• Reverse Transcriptase: This enzyme is responsible for converting the single-stranded viral RNA into double-stranded DNA. This step is unique to retroviruses and necessary for the next stages of replication.
• Integrase: Following reverse transcription, integrase facilitates the integration of the newly formed viral DNA into the host's genome. This allows the genetic material of the virus to become a permanent part of the host's DNA.
• Protease: Once new viral particles are produced, protease helps process viral protein precursors into mature, functional proteins. These proteins are essential for assembling new virions.

These enzymes work in harmony to ensure the survival and proliferation of the virus within the host organism.

Capsid structure

The capsid of a retroviral virion is a protective protein shell that plays a vital role in safeguarding the viral genetic material. Inside this capsid resides the viral RNA genome along with the necessary viral enzymes.
Composed of protein subunits called capsomeres, the capsid forms a sturdy barrier against external threats. The arrangement and design of the capsid can vary among different retroviruses, but its primary function remains consistent: to protect and package the viral contents.

• The capsid maintains the integrity of the viral RNA genome during transmission from one cell to another.
• It also aids in delivering the viral genome into the host cell after the virion successfully attaches and enters.

The capsid is a crucial component of the virion because, without it, the viral RNA and enzymes would be vulnerable to degradation, and the virus would be unable to replicate effectively.