Cell Biology
Retroviruses, like the human immunodeficiency virus (HIV), have a distinct life cycle. HIV belongs to the family Retroviridae, a group of envelope viruses with single-stranded positive-sense RNA genomes. The virus primarily targets helper T-cells which have a CD4 receptor and a CCR5 co-receptor on their surfaces.
The life cycle of HIV involves three essential enzymes: reverse transcriptase, integrase, and protease. Reverse transcriptase transcribes the single-stranded RNA genome into double-stranded DNA, which integrase incorporates into the host cell's genome. The host cell machinery then transcribes the viral genome, converting viral DNA back into RNA. The viral RNA serves two functions: it can either be used as genomic RNA for the next generation of viruses or translated into long inactive polyproteins by host cell ribosomes. The genomic RNA and polyproteins exit the host cell, taking a piece of the cell's plasma membrane for the viral envelope. Protease cleaves the inactive polyprotein chain into functional viral proteins, resulting in a mature virion that can infect new host cells and continue the life cycle.
Lesson Outline
<ul> <li>HIV's Genome</li> <ul> <li>Single-stranded positive-sense RNA genome</li> <li>Envelope viruses with a phospholipid bilayer</li> <li>Diploid genome: two identical copies of positive-sense single-stranded RNA</li> </ul> <li>HIV's Life Cycle</li> <ul> <li>HIV uses CD4 receptor and CCR5 co-receptor to enter host cells</li> <li>Viral fusion with host cell membrane</li> <li>Introduction of viral contents into host cell cytoplasm</li> <li>Three essential enzymes: reverse transcriptase, integrase, and protease</li> <ul> <li>Reverse transcriptase: converts single-stranded RNA genome into double-stranded DNA</li> <li>Integrase: integrates viral DNA into host cell genome</li> <li>Protease: cleaves inactive polyproteins into functional viral proteins</li> </ul> <li>Host cell machinery transcribes viral genome, synthesizing viral RNA</li> <li>Viral RNA serves two functions:</li> <ul> <li>Messenger RNA: creates chains of viral polyproteins</li> <li>Genomic RNA: becomes genome of the next generation of viruses</li> </ul> <li>Viral RNA and polyproteins bud from host cell, forming viral envelope</li> <li>Protease cleaves inactive polyproteins, creating infectious HIV virion</li> </ul> </ul>
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FAQs
Retroviruses are a family of RNA viruses, Retroviridae, characterized by their ability to reverse transcribe their RNA genome into DNA and integrate it into the host cell's genome. A key example is HIV, which infects CD4+ cells and uses the CD4 receptor for entry. The main stages of a retrovirus life cycle include attachment to target cell receptors, fusion and entry, reverse transcription of RNA to DNA, integration of viral DNA into the host cell's genome, transcription and translation to produce new viral particles, assembly and maturation, and finally, release of infectious virions.
Retroviruses, such as HIV, are classified as envelope viruses because their viral particles are surrounded by a lipid envelope derived from the host cell membrane. This envelope contains viral glycoproteins that recognize and bind to specific receptors on the host cell, including the CD4 receptor and CCR5 co-receptor for HIV. This interaction facilitates the fusion of the viral envelope with the host membrane, facilitating entry of the viral core into the cell.
Reverse transcription is essential for retroviruses because it converts their positive-sense RNA genome into DNA, which can then be integrated into the host cell's genome. This step is necessary for the virus to hijack the host's cellular machinery to generate viral transcripts and proteins. The enzyme responsible for reverse transcription is called reverse transcriptase, which is encoded by the retroviral genome and is part of the viral core that enters the host cell.
Once the retroviral RNA genome is reverse transcribed into DNA, the viral DNA must integrate into the host cell's genome to produce new viral particles. This is achieved through the action of the enzyme integrase, which recognizes specific sequences in the viral DNA and catalyzes the insertion of the viral DNA into the host genome. This integration ensures that the viral genes are transcribed and translated by the host's cellular machinery, enabling the production of new viral particles.
Proteases are enzymes that cleave or break down proteins. In the context of the retrovirus life cycle, proteases, such as HIV protease, play an essential role in the maturation of new viral particles. After assembly and budding from the host cell, the viral polyproteins are cleaved by proteases into individual functional proteins, leading to the structural rearrangement and proper folding of the viral core. This maturation process is crucial for the infectivity of the newly released virions, allowing them to infect other cells and continue the viral life cycle.
