The P22 virus is a lysogenic bacteriophage

Bacteriophage

A bacteriophage is any one of a number of viruses that infect bacteria. They do this by injecting genetic material, which they carry enclosed in an outer protein capsid...

 that infects Salmonella

Salmonella

Salmonella is a genus of rod-shaped, Gram-negative, non-spore-forming, predominantly motile enterobacteria with diameters around 0.7 to 1.5 µm, lengths from 2 to 5 µm, and flagella which grade in all directions . They are chemoorganotrophs, obtaining their energy from oxidation and reduction...

. Its linear, double-stranded DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 genome

Genome

In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

 is 44 kilobases long. Like many phage viruses, it has been used in molecular biology

Molecular biology

Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...

 to induce mutations in cultured bacteria and to introduce foreign genetic material

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

. The viral capsid

Capsid

A capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The capsid encloses the genetic...

 has been the subject of studies in virus assembly. Its "tailspike" protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

, which is anchored in the viral coat and used to aid in penetrating the membranes of host cells, has an unusual beta helix

Beta helix

A beta helix is a protein structure formed by the association of parallel beta strands in a helical pattern with either two or three faces. The structure is stabilized by inter-strand hydrogen bonds, protein-protein interactions, and sometimes bound metal ions...

fold.

When it is ready to leave the cell, it is packaged into its capsid by the "headful" mechanism, meaning that DNA is packaged into the capsid until it is full. This usually encompasses 48Kb so part of the host DNA is transferred along with the phage. The cell is then lysed, releasing the phage into the environment to infect other cells.