The genetic blueprint of every human contains a significant amount of DNA that is not actually human. Dormant fossil viruses have infiltrated our genome. Estimates range from 3 to 8 % of the human genome as being comprised of sections of viral DNA. These and other parasitic, self-replicating pieces of nucleic acid have evolved with us over millions of years, after being inserted into our ancestors DNA. To better understand fossil viruses, it may help to first review what viruses are and how they infect hot cells.
* Viruses Are Not Alive *
Viruses are acellular infectious particles. Although they may seem to behave like living things, viruses are not alive.
* are not made of cells
* cannot reproduce on their own
* do not grow or undergo division
* do not transform energy
* lack machinery for protein synthesis
* are so small that they can only be seen with an electron microscope
* Intracellular and Extracellular Viral Form *
Viruses exist in one of two states; extracellular and intracellular.
Extracellular State: Before it invades a host cell, a virus is in the 'extracellular state'. An extracellular virus, called a virion (vie-ree-on), consists of a protein coat (capsid) surrounding nucleic acid. In addition, some viruses have phospholipid envelope surrounding the capsid.
Intracellular State: Once the virus invades a host cell, it is in an 'intracellular state.' In this state, the capsid is removed and the virus exists as only as nucleic acid (genetic material).
* How Do Viruses Reproduce? *
Once inside their host, viruses transform the infected cell into a factory for making more viruses. Although the specifics of viral replication vary depending on the type of virus, viruses generally reproduce via four basic steps, including:
* Delivery of the viral genetic material into a host cell
* Commandeering of the host cell's transcription and translation machinery
* Use of the host cell's building blocks to copy viral genomes and synthesize viral proteins
Viral genomes and proteins then self-assemble and exit host cells as new infectious particles.
* How Does Viral DNA Become Part of the Human Genome? *
During the course of a viral infection, some viruses insert their DNA into the host's genome (provirus) and direct the host's cellular machinery to make the proteins and genetic material needed to make more viruses. If this process of gene insertion takes place in a cell that is destined to become an egg or a sperm, the host's offspring will have a copy of the virus in every single one of their cells. This is a virus that infected an ancestor-a fossil virus.
* Sources *
Glausiusz, Josie (2007) Your Body Is a Planet. Discover Magazine.
Human Genome Bears a Virus Related to HIV-1, Howard Hughes Medical Institute Research News.