Figure 2. Evolutionary layers of the X and Y chromosomes. (a) Comparative mapping of genes from the human X in marsupial mammals detected an ancient region conserved on the X in all mammals and denoted XCR, as well as a recently added region denoted XAR that is autosomal in marsupials.
Poliovirus, the causative agent of poliomyelitis (commonly known as polio), is a human enterovirus and member of the family of Picornaviridae. Poliovirus is composed of an RNA genome and a protein capsid. The genome is a single-stranded positive-sense RNA genome that is about 7500 nucleotides long.
Polio is a contagious viral illness that in its most severe form causes nerve injury leading to paralysis, difficulty breathing and sometimes death. In the U.S., the last case of naturally occurring polio was in 1979. Today, despite a worldwide effort to wipe out polio, poliovirus continues to affect children and adults in parts of Asia and Africa.