EVERYDAY SCIENCE

Detailed explanation-1: -Viruses have several common characteristics: they are small, have DNA or RNA genomes, and are obligate intracellular parasites. The virus capsid functions to protect the nucleic acid from the environment, and some viruses surround their capsid with a membrane envelope.

Detailed explanation-2: -Usually, enveloped viruses with helical capsids are pleomorphic, forming either spherical or filamentous particles (Figs. 5 and 6). Helical capsids can package only single-stranded RNA, but not double-stranded DNA or RNA, possibly because of the rigidity of the double-stranded nucleic acids.

Detailed explanation-3: -Many mature infectious viruses with positive-sense RNA genomes are known to consist of a single copy of the genome inside a one-molecule-thick shell of protein (the capsid protein [CP]).

Detailed explanation-4: -The capsid surrounds the virus and is composed of a finite number of protein subunits known as capsomeres, which usually associate with, or are found close to, the virion nucleic acid.

Detailed explanation-5: -DNA is packaged in a “headful” mechanism whereby concatemers are cleaved at conserved sequences that define the genome ends. The DNA is tightly packed, producing a rigid capsid and the capsid is “sealed” by a PCP. DNA-containing capsids leave the nucleus and traffic through the cytoplasm.