Lytic and lysogenic cycles: how viruses copy themselves

0:00A virus cannot reproduce on its own. So how does it copy itself?

0:06A bacteriophage, or phage, is a virus that infects bacteria. This one has DNA inside a protein coat, and a tail.

0:16First, the phage attaches to its host cell, a bacterium.

0:21Second, it injects its DNA into the cell. The empty coat stays outside.

0:27Then the virus takes one of two paths: the lytic cycle or the lysogenic cycle.

0:34In the lytic cycle, the viral DNA takes over, and the cell makes viral DNA and proteins.

0:41These parts assemble into many new phages.

0:45Then a viral enzyme breaks down the cell wall, so the cell bursts and releases the phages.

0:52This bursting is called lysis, which gives the lytic cycle its name. The new phages infect more cells.

1:00In the lysogenic cycle, the viral DNA joins the bacterium's own DNA, its chromosome.

1:07This is now called a prophage. It stays mostly inactive, and the cell lives normally.

1:14Each time the cell divides, it copies its chromosome, so both new cells get the prophage.

1:21This repeats with every division, so many cells carry the prophage.

1:26A trigger, such as ultraviolet light or stress, can make the prophage leave the chromosome.

1:33The virus then switches to the lytic cycle: it makes parts, assembles phages and bursts the cell.

1:40So the lytic cycle bursts the cell right away, while the lysogenic cycle can hide until a trigger.