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Viruses lurking in our genes: The surprising story of endogenous retroviruses!
Retroviruses are viruses that alter the genome of a host cell by inserting a DNA copy of its RNA genome into the cell's DNA. After occupying the cytoplasm of the host cell, this type of virus uses its own reverse transcriptase to convert RNA into DNA. This process is the reverse of the normal pattern, so it is called "retro". Once the new DNA is integrated into the host cell genome, the retroviral DNA is called a provirus. The host cell then treats this piece of viral DNA as its own genome, transcribes and translates it, and produces the various proteins needed to assemble a new virus.
Many retroviruses cause serious disease in humans and other mammals and birds.
These retroviruses can be divided into different subfamilies and categories, including cancer-causing tumor viruses such as human T-lymphotropic virus HTLV, and lentiviruses causing human immunodeficiency viruses (HIV-1 and HIV-2). Although some retroviruses have no obvious disease associations in animals or humans, they can still become important tools in molecular biology research, especially for applications in gene delivery systems. Endogenous retroviruses (viral DNA inherited in the genomes of our small animals) provide evidence that retroviruses may have been infecting vertebrates for more than 450 million years.
Structure of retrovirus
The diameter of retroviral particles is about 100 nanometers, and the outer layer has a lipid envelope. The glycoprotein in the envelope is encoded by the virus itself. These particles contain two identical single-stranded RNA molecules, 7 to 10 kilobases in length. The two molecules exist as dimers, formed by pairing of complementary sequences. Among the particles of different retroviruses, the structural and biological differences are significant, yet their major components are very similar.
The envelope acts as a protective barrier to ensure that retroviruses can enter host cells and can enter cells directly through fusion with the host cell membrane.
The structure of the RNA genome of retroviruses is also relatively complex, usually organized in a 5’–gag–pro–pol–env–3’ arrangement. These genes are each responsible for producing different polypeptides that manage the virus's envelope and its replication. Even more interestingly, in some viruses, these genes may overlap or fuse into larger polypeptide chains, forming additional genes.
Reproduction of retroviruses
When a retrovirus' genome becomes embedded in the reproductive system, the genome is passed on to the next generation. These endogenous retrovirus (ERVs) insertions occupy 5-8% of the human genome. Although most insertions are functionally relatively minor and are sometimes referred to as "junk DNA," many endogenous retroviruses play important roles in host biology, such as controlling gene transcription and cellular regulation during embryonic development. Fusion.
Reverse transcriptase not only transcribes RNA into DNA, but this process follows the central dogma of molecular biology.
During the translation process, the proteins encoded by the gag and pol genes are synthesized in the form of polypeptides, including the main viral capsid protein and enzymes related to viral replication. With the synthesis of these proteins, the virus can effectively complete its reproduction and spread process.
Genetic variation and adaptation of retroviruses
The genome of retroviruses undergoes recombination during reproduction. The template strand switching during this process helps to perform genetic recombination during the viral reverse transcription process, allowing retroviruses to effectively maintain the integrity of the genome and function as A mechanism to repair damaged genomes.
This genetic recombination may result in 5 to 14 recombination events per reproduction cycle, demonstrating the flexibility of retroviruses in their evolution.
Furthermore, the rapidly mutating nature of retroviruses allows them to rapidly adapt and develop resistance, which poses challenges in developing targeted therapies and vaccines. Likewise, the lack of normal proofreading mechanisms results in a high mutation rate in retrovirus genomes.
The role of retroviruses in disease
Many retroviruses are known to be directly associated with diseases such as cancer. For example, Rous oncovirus is thought to be involved in tumor formation. These viruses are able to trigger cellular transformation by integrating proto-oncogenes into host DNA. Specific retroviruses such as HTLV-1 are thought to be closely related to human T-cell leukemia.
The reason why retroviruses can cause disease is often because they interact with the host's genome, which may lead to damage to the normal functions of cells.
It is for these reasons that the study of retroviruses is not only about discovering the viruses themselves, but also about understanding how these viruses affect the functioning of the host organism. This begs the question: How many secrets are there to uncover about the potential threats buried in our genome?
