Lambda Phage Replication Cycle

Why do some viruses have two different reproductive cycles? A virus is a cellular parasite. A virus has a very different structure and life cycle than a more complex parasite, such as the single-celled organism that causes malaria. Both of these entities live inside their host and require the host in order to reproduce. Both also need to move between hosts at some point or the parasite will die when the host does. Viruses replicate themselves using the host cell’s organelles and other machinery to manufacture new viral components. If only the viruses DNA (or RNA) is copied then it will be passed on to any daughter cells of the host cell. This is an effective way for a virus to reproduce although it does not allow the virus to spread to other cells. If entire new viruses are created they can infect new cells. However they need to destroy the original host cell in order to escape into the environment.

Viruses, like other parasites experience a trade-off between the two ways of reproducing. A virus that remains dormant in the cell and allows itself to be passively copied does not destroy the host that is driving the reproductive process. On the other hand the virus is not exploiting all the hosts that are available. Viruses that reproduce rapidly and destroy their host are capable of spreading more rapidly but if they destroy their hosts too quickly then they risk exterminating the cells that are needed for reproduction.

View the animation below, then complete the quiz to test your knowledge of the concept.


1	In the lytic cycle of phage replication

		A)	phage genes integrate into the host cell genome and are not expressed
		B)	phage genes exist as prophage
		C)	phage DNA remains inside the protein coat
		D)	phage nucleic acid is replicated and phage genes are expressed, making new phage protein and lysing the host cell

2	A prophage is

		A)	phage DNA integrated into host cell genome
		B)	phage DNA being actively expressed
		C)	phage protein left outside the bacterium
		D)	toxic to the host bacterial cell

3	Which of the following can cause a phage in the lysogenic stage to revert to the lytic stage?

		A)	lack of nutrients
		B)	ultraviolet light
		C)	a competing phage
		D)	darkness
		E)	an electrical charge

4	In the lysogenic stage of phage replication, the host cell is lysed.

		A)	True
		B)	False

5	Once a phage becomes lysogenic, it will remain lysogenic and never be lytic again.

		A)	True
		B)	False

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