Chapter 19, Problem 1IQ

In the following diagram of lytic and lysogenic cycles, describe steps numbered 1-8 and label structures a-e.

Summary Introduction

To list and describe: The following diagram is of lytic and lysogenic cycle.

Introduction: Virus is an infectious agent that cannot be seen through the naked eyes. Virology is the study of viruses and virologists are the biologists who study viruses. Viruses are obligate intracellular parasites because it needs a living host cell for replication. Obligate intracellular parasites do not have the ability to reproduce outside their host cell.

Answer to Problem 1IQ

Pictorial representation: The lytic and lysogenic cycle is shown in Fig.1.

Fig.1: Lytic and lysogenic cycle

Explanation of Solution

Phages can infect and reproduce in the host cell by either of the two cycles— lysogenic cycle and lytic cycle. The lytic cycle is also known as virulent cycle, whereas the lysogenic cycle is a nonvirulent cycle. Attachment, penetration, uncoating, replication, assembly, and release are the steps involved in the replication of virus.

1. During attachment and penetration, the virus attaches to a host cell and injects its genetic material into the host cell.
2. During uncoating, the viral DNA or RNA enters to the nucleus of host cell and takes control over it.
3. The host cell synthesizes the viral components.
4. The newly-created virus release from the host cell either by lysis of cell, waiting for the cell to die, or by budding through cell membrane.
5. In the lysogenic cycle, the phage DNA integrates to the bacterial chromosome and become prophage.
6. Prophage is the genetic material of a bacteriophage that is incorporated to the genome of a bacterium and is able to produces phage if specifically activated.
7. The bacteria reproduce by passing the prophage to daughter cells and a large population of infected bacteria forms.
8. The prophage exits from the bacterial chromosome and starts the lytic cycle.

In lytic cycle, the viral DNA does not integrate with the bacterial DNA, but it destroys the host DNA and takes over the cellular replicative machinery in order to reproduce its own genome. In the lytic cycle, after the replication process is over, the host cell wall is lysed and new virions are released, whereas in the lysogenic cycle the virus does not destroy the host cell. It remains dormant and rarely produces virions during unfavorable conditions.