Nature of viruses: nucleic acid genome (dna or rna- never both), a protein coat (capsid) that encloses the genome and in some cases, a lipid membrane called the envelope (not always present)- may be glycosylated. Infectious virus particles = virion = complete virus. Viruses are obligatory intracellular parasites , they depend on another cell for the replication , can replicate only within living cells. Missing piecies on viruses that are required for replication: enzymes to make building blocks (aa,carbo, nucleotides), enzymes that provide chemical energy (atp), enzymes that make proteins (ribosomes) , internal membrane bound structure (organelles) Envelope protein enable it to replicate and transmit from one cell to another. All viruses contain nucleic acid genome and capsid. Virus can transfer genes between organisms, play important role on the earth"s ecology. Virus particles break down are release their genomes inside the cell. Virus are not the only unicellular organisms that are obligatory intreacellular parasites.