Question

A) What is the difference between a lytic cycle and a lysogenic cycle in bacteriophage biology (2pt)?

B) Can the same phage undergo both types of cycle and, if so, under what circumstances (2 pt)? Explain (with example).

Answer 1

A) What is the difference between a lytic cycle and a lysogenic cycle in bacteriophage biology (2pt)?

Answer) Below are the differences in lytic and lysogenic cycle of bacteriophage.

	Lytic cycle	Lysogenic cycle
1.	This cycle causes lysis of bacterial cell(host)	This type of life cycle virus replicates along with bacteria.
2.	Viral DNA does not integrate into the bacterial genome.	Viral DNA integrates with the host DNA.
3.	Viral DNA replication occurs independently of bacterial genome replication.	Viral DNA replication occurs along with the bacterial genome replication
4.	This cycle is virulent that it harms the host	This cycle is non-virulent that it does not harm the host
5.	This occurs in a short period	This occurs for a long time
6.	Symptoms of viral infection appear in bacterial cells. Ex: viral particles will be budding out of bacterial cells.	No viral symptoms appear in the host.
7.	Occurs when bacteria or the host experiencing environmental stress like nutrition deficit, chemical stress and UV damage to the genome.	Occurs when bacteria are under normal conditions.
8.	There is no prophage(integration of the viral genome into the bacterial genome) takes place in this cycle.	Prophage formation is present where the viral genome integrates into the bacterial genome.
9.	Once the lytic cycle is induced virus cannot switch to the lysogenic cycle	Once the lysogenic cycle is induced virus can switch to the lytic cycle if the host experiences any stress.
10.	Gene P, Q, O promotes the lytic cycle in the bacteriophage lambda.	Hfl or high frequency of lysogeny promotes a lysogenic cycle.

Note: Once virus infects the bacteria, decision to enter either lytic or lysogeny depends upon the situation of the host. If the host is under stress virtual proteins senses that and promotes the lytic cycle. If the bacterial cell is under normal viable conditions then the viral proteins sense that and promote the lysogenic cycle.

B) Can the same phage undergo both types of the cycle and, if so, under what circumstances (2 pt)? Explain (with example).

Answer)

yes, the same phage can undergo lytic and lysogenic cycles, such viruses are called "temperate phages".

Explanation :

It is highly possible that a virus infecting a bacterial cell under normal environmental conditions. In such cases viral proteins inside bacteria promote lysogeny.

They integrate their DNA into the host genome and undergo a lysogenic cell cycle where viral genome replicates along with the bacterial genome.

After a few generations of the bacteria replication, it is also possible that host experiencing environmental stress like nutrition deficit, chemical stress and UV damage to the genome. In such cases the integrated viral genome excises itself and starts lytic cycle killing the bacterial cell.

This mechanism of decision between lytic and lysogenic cycle in bacteriophage is called the "Genetic switch".