chapter 19

Viruses are intracellular parasites and can reproduce only within cells.

Virus reproductive cycle.
A virus is an intracellular parasite that needs a host cell to reproduce, using the host's enzymes, ribosomes, and other molecules to synthesize progeny viruses.
In this simplified viral cycle, the virus contains a DNA genome enclosed in a protein capsid.

Viruses are made up of nucleic acid (DNA or RNA) enclosed in a protein coat (capsid), which is made up of subunits called capsomeres. Bacterial viruses are called phages.
Tobacco Mosaic Virus has a helical capsid enclosing RNA.
Adenoviruses have a polyhedral capsid with glycoprotein spikes.
Flu viruses have 8 capsids with an RNA genome wrapped in a membranous envelope embedded with glycoproteins.
Phage T4 has a polyhedral head and a tail apparatus.
Review:
Bacterial viruses ( ) go through two alternative reproductive mechanisms.
- The lytic cycle describes a phage.
  
  Lytic cycle.
  A virulent phage such as T4 reproduces only by a lytic cycle which leads to lysis of the host.
- T4 uses its tail fibers to bind to receptors on a bacterial cell.
- Phage DNA is injected.
- Host resources are used to produce viral DNA and proteins.
- Viral DNA is packaged in the head, attached to tail apparatus.
- Progeny phages burst (lyse) the host cell.
  Review:
- The lysogenic cycle describes a phage.
The lytic and lysogenic cycles of l, a temperate phage. After entering the host cell and circularizing, the l (lambda) DNA can enter the lytic cycle or integrate into the bacterial chromosome (lysogenic cycle) as a prophage, where it may be carried in the host DNA for many generations. Review:
Animal viruses include RNA viruses, some of which are such as HIV that contain their own transcriptase.

Many animal viruses have a membranous envelope with glycoprotein that bind to specific receptor molecules on the surface of a host cell.

An RNA virus. The viral RNA genome functions as a template for synthesis of complementary RNA strands. The complementary RNA can be used to make copies of viral genome RNA, or can serve as mRNA, which is translated into capsid proteins and glycoproteins of the viral envelope.

HIV, a retrovirus.
The Human Immunodeficiency Virus (HIV) is a retrovirus.
It has the enzyme reverse transcriptase to copy the RNA genome into DNA, which is integrated into the host genome as a provirus, where it remains a permanent resident.
The host's RNA polymerase transcribes the proviral DNA into RNA molecules, which may function both as mRNA for synthesis of viral proteins and as new viral genomes.
AZT and reverse transcriptase
Review and exercise: