Viruses are infectious agents that are not assigned to any kingdom

So what are they?


Bacteriophage (attacks bacteria)

Ebola virus





Viral reproduction involves both lytic and temperate (or lysogenic cycles)

Temperate viruses can integrate their DNA into the host DNA

Lytic vs. Lysogenic cycles:


In the lytic cycle:

  • Attachment (or absorption). The virus attaches to receptors on the host cell wall.
  • Penetration. The nucleic acid of the virus moves through the plasma membrane and into the cytoplasm of the host cell. The capsid of a bacteriophage remains on the outside. (The entire HIV virus particle "melts" into and through the plasma membrane of its host cell.) In contrast, many viruses that infect animal cells enter the host cell.
  • Replication. The viral genome contains all the information necessary to produce new viruses. Once inside, the virus induces the host cell to synthesize the necessary components for its replication.
  • Release. Assembled viruses are released from the cell. Generally, lytic enzymes destroy the host cell. With the HIV virus, release does not necessarily lyse the cell. There may be a constant "stream" of particles, consisting of thousands of particles, emerging from a single infected cell.


In the lysogenic cycle:

·        Viral genome gets incorporated into specific sites of host cell chromosome.

·        Viral genome replicates every time host nucleus divides.

·        Host cell remains viable as long as the viral genome remains in this “temperate” state.

Viral taxonomy notes:

Contrast the lytic cycles of rubella and HIV viruses

Lytic cycle of rubella virus

1.     Virus attaches to host cell.

2.     Plasma membrane surrounds the virus and an endosomal vesicle forms.

3.     The envelope of the virus fuses with the membrane and the vesicle.

4.     The virus is released into the host's cytoplasm

5.     Viral nucleic acid separates from its capsid

6.     Nucleic acid enters host nucleus and replicates

7.     Transcription into mRNA

8.     Host ER directed by mRNA to synthesize glycoproteins and capsid proteins plus proteins of the viral envelope.

9.     Vesicles transport glycoproteins to plasma membrane of cell.

10. New viruses assemble and are released from host cell after being enveloped by host's modified plasma membrane.

Lytic cycle of HIV virus



1.   The HIV virus attaches to a host cell plasma membrane

2.   The virus enters the host cytoplasm and the capsid is removed.

3.   Reverse transcriptase catalyzes the synthesis of single-stranded (ss) DNA that is complementary to the viral RNA.

4.   The DNA strand then serves as a template for the synthesis of a complementary DNA strand resulting in double-stranded (ds) DNA.

5.   The ds DNA is transferred to the host nucleus and the enzyme integrase integrates the DNA into the host chromosome.

6.   Host enzymes transcribe viral RNA.

7.   Viral RNA leaves the nucleus, viral proteins are synthesized at host ribosomes and the virus is assembled.

8.   The virus buds from the cell, picking up the viral envelope in the process.