Deoxyribose nucleic acid, or simply DNA, contains every genome that makes up an organism. However, out of the 3 billion nucleotides of the human genome, only 1.5% of the sequence forms the 20,000 genes that code for proteins. Retroviruses, which are a type of virus that inserts its DNA into host cells, therefore incorporating itself into the chromosomes and changing the genome of the host cell, influencing the genetic makeup. They are normally not passed on because they don’t usually affect reproductive cells, but nonetheless years of evolution have added retroviruses into the human DNA that about 8% of the human genome is the direct result of retroviruses. How, then, do retroviruses impact an organism’s genome, and what are the consequences of doing so?
Retroviral infection in reproductive germ cells has long been an important factor in matters of evolution. How exactly how mammal germ cells respond to this retroviral infection has not been known previously. However, recently scientists have discovered a retrovirus called KoRV-A that affects the koala population in Australia, and is believed to cause susceptibility to infection and cancer. Unlike most retroviruses, KoRV-A also infects the germ cells, causing most wild koalas to be born with KoRV-A as part of their genetic code. The koala germ cells recognized an essential step in the viral life cycle, and turned it against the invader to suppress genome infection.
Because of the ability of retroviruses to alter the genomes of the infected organism, KoRV-A multiplies and inserts itself into the chromosomes. This alters host genome organization and function, and the process continues until the invader is “tamed” by the host. At the end of this infection cycle, the genome of the koala has been altered. This highly implies that the genome system has the ability to differentiate between the host’s genes and the virus.
Spacer sequences called introns briefly interrupt genes, which are removed in a process called splicing. This produces mRNA, which in turn is used to create protein. Retroviruses, which also have introns, create their own RNA molecules to influence the host. However, the host’s germ cells have the ability to recognize the virus-created RNAs and distribute them in a class separate from the rest of the host’s genome, which blocks the formation of the virus.
While the exact details of retroviral effects on host organisms haven’t been fully discovered, The discovery of KoRV-A’s interactions with koalas have been a major benefit. Scientists continue to conduct experiments based on how germ cells differentiate between RNA molecules, and of how unspliced RNA transcripts are chopped up. With a better understanding of the workings of retroviruses and gene manipulation, the secret to how viruses change the genome of the host will be revealed.