These twilight creature are responsible for causing many diseases in living things (herpes and AIDS in humans, for example), yet they consist of little more than a core of nuclei acid, either DNA or RNA, and a protective coat of protein molecules and sometimes lipids.

In isolation, viruses and bacteriophages show none of the expected signs of life. They do not respond to stimuli, they do not grow, they do not do any of the things we normally associate with life. Strictly speaking, they should not be considered as "living" organisms at all. However they are more complex than a lifeless collection of macromolecules and they do show one of the most important signs of life: the ability to reproduce.

In the example shown in the accompanying diagram a bacteriophage attacks a bacterial cell by attaching itself to the outer wall. It then uses a specialized protein to digest a small hole in the wall of the bacterial cell and inject its nucleic acid molecule into the bacterial cell's cytoplasm.

Once there, the bacteriophage's nucleic acid takes over the synthetic machinery of the bacteria and directs the full scale production of more phage nucleic acid and more phage proteins.

When these components are ready, they are assembled into new bacteriophages and then released as the host cell bursts open, or lyses. In this way a single infecting bacteriophage can give rise to over 200-300 new offspring. The Lysogenic Cycle Other bacteriophages and many of the viruses do not automatically take over the host cell and begin making new "offspring". Instead, the invading nucleic acid somehow joints up with the host cell's DNA.

A break occurs in the host chromosome and a piece of phage or viral DNA is inserted. Enzymes splice the two molecules together, like splicing recording tape, and the recombinant molecule is a hybrid of host genetic information and virus genetic information.

From this moment on, every time the host cell makes a new copy of its own DNA it also inadvertently makes a copy of the viral or phage DNA. Lying dormant like this the inserted information can continue to be replicated and passed on from generation to generation within the host cell line.

Such an arrangement may go on indefinitely, but, sooner or later, the invading DNA takes over and redirects the cellular machinery for its own ends.

Invading DNA disassociates itself from the host DNA and begins its own replication. At the same time it directs the synthesis of new phage or viral proteins. Once again, when all is ready, the components are assembled and the new bacteriophage or viral offspring are liberated by lysis.