LESSON #45 VIRUS REPRODUCTION
1. Read pages 288-290
in the textbook.
2. Record the following terms in your Biology Journal: bacteriophage, receptor sites, prophage, transduction.
3. Read over the Lecture Notes below.
4. Finish the Section Questions on page 290 1-5.
5. Diagram the Lytic Cucle.
Viruses can be considered as the smallest organized, infective structures capable of replicating themselves in a living cell system. They are not cells, but packages of genetic information that insert themselves into a host cell and direct its metabolic machinery to make more viruses. They are composed of either DNA or RNA, containing necessary genetic information for the replication and assembly of identical progeny within the host cell. In order to protect the genetic material, the virus possesses a coat of protein or lipoprotein molecules assembled according to precise geometrical or morphological designs. Viral nucleic acid can infect cells on its own and produce complete viruses possessing protein coats.
Entering the Host Cell
Because viruses are acellular and do not use ATP, they must utilize the machinery and metabolism of a host cell to reproduce. They are obligate intracellular parasites. Before a virus has entered a host cell, it is called a virion, a package of viral genetic material. Virions can be passed from host to host through direct contact or through a vector (an animal carrier who is not affected by the virus--often insects). Inside the organism, the virus can enter the cell in various ways. Bacteriophages have capsoids with tails that attach to the cell wall at specific protein markers on the wall surface. Once attached, enzymes on the tail core act like lysozome enzymes and make a small hole in the cell wall. The virus injects its DNA into the cell like a syringe and leaves its empty ghost behind, attached to the outside of the cell wall. Other viruses (like HIV) have capsoids enclosed in a lipid bilayer envelope and enter the host via endocytosis. Glycoproteins stud the outer leaflet of the membrane envelope and bind to receptor sites along the host cell's plasma membrane. After entering the cell, the capsoid is broken down and the virus's genetic material begins the destructive process of taking over the cell and forcing it to produce new viruses
Replication of the Viral Genome
There are three different ways genetic information can be reproduced, and they are governed by the kind of nucleic acid molecules and enzymes the virus carries in its capsoid. Some viruses have DNA, which once inside the host cell is replicated by the host along with its own DNA. Bacteriophages are DNA-containing viruses. There are two kinds of reproduction by viruses containing RNA. One kind copies the RNA directly with an enzyme (RNA replicase) that uses the copy as a template to make hundreds of duplicates of the original RNA from the host cell's nucleotide material. The second group of RNA-containing viruses is called retroviruses. Instead of copying the RNA directly, they use an enzyme (reverse transcriptase) which assembles a complimentary strand, so that the virus's genetic information is contained in a molecule of DNA rather than RNA. This is very different from normal transcription, because DNA is made from RNA rather than vice versa, hence the name "reverse" transcription. HIV is an example of a retrovirus.
Proviruses, Prophages, and the Lytic Cycle
Once the virus's genetic material has entered the cell, two things can happen. Some viruses destroy the host's DNA, breaking it down into smaller nucleic aid chains that can be used for replicating the virus's own genome. Other viruses leave the host DNA intact and incorporate their own into the host's chromosomes. Once spliced into the host genome, the virus's genetic material is copied along with the host's during regular mitosis or meiosis. Several generations can go by where the offspring of the infected host, each containing viral genetic material, show no signs of viral infection. When viral DNA is incorporated into the host's chromosome as a latent form, this is called a provirus in the case of animal and plant viruses and a prophage in the case of bacterial viruses. The herpesvirus can remain latent as a provirus in a host indefinitely only to be excised from the host's genome in times of stress. This is why someone can have recurrent infections long after he has seemingly been cured. Bacteriophages can remain latent as prophages until a disturbance such as ultraviolet radiation or chemicals remove them from the host genome. An outwardly healthy colony of bacteria which contain latent prophages that can be awakened by such a disturbance is called lysogenic bacteria.
When the virus has taken over the cell, it immediately directs the host to begin manufacturing the proteins necessary for virus reproduction. The host produces three kinds of proteins: early proteins--enzymes used in nucleic acid replication, late proteins--proteins used to construct the virus coat, and lytic proteins--used to break open the cell for viral exit. The capsoid material is made from the host's resources. The final viral product is assembled spontaneously, that is, the viruses do not grow their parts as most organisms do, their protein parts are made separately by the host and come together by chance. This self-assembly is often aided by "molecular chaperones", proteins made by the slaving host that help the capsoid parts come together but are not a part of the virus itself.
The lytic cycle, although slightly different for various viruses generally follow these steps:
1. The phage attaches itself to the cell
2. It injects the DNA into the host
3. DNA is replicated by the host cell.
4. Instructed by the viral DNA, the host manufactures viral proteins.
5. Self-assembly occurs and the new viruses burst out of the cell-destroying the cell.
6. After lysis, the cycle begins again.