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.
Paralysis caused by polio occurs when the virus replicates in and attacks the nervous system. Two polio vaccines have been available since 1955. The inactivated polio vaccine (IPV) was available first, given as a shot in 1955. A more convenient form, given as liquid drops via the mouth, was developed in 1961.aralysis caused by polio occurs when the virus replicates in and attacks the nervous system. Two polio vaccines have been available since 1955. The inactivated polio vaccine (IPV) was available first, given as a shot in 1955. A more convenient form, given as liquid drops via the mouth, was developed in 1961.