Biology · Variety of life. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
A virus is a tiny non-cellular particle made of a nucleic acid (DNA or RNA, never both) inside a protein coat called the capsid. It has no ribosomes or enzymes for making its own food, so it can reproduce only inside a living host cell. That is why viruses are called obligate intracellular parasites.
A bacteriophage is a virus that attacks bacteria. In the lytic cycle it takes over the cell, makes new phages and bursts the cell. In the lysogenic cycle its DNA joins the bacterial chromosome as a prophage and is copied quietly each time the cell divides.
HIV and the hepatitis viruses are the human examples you must know.
Students say the whole phage enters the bacterium. Only the DNA is injected; the protein coat stays outside on the cell wall.
Answer: 1 600 phages.
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