Bio Review Notes

Bio Review Notes #33
VIRUSES Performance Objectives:

After completing this lesson, students will be able to:

Explain the general characteristics of viruses and their life cycles

Explain that viruses are classified according to the character of their nucleic acids

Viruses are fragments of nucleic acid, often surrounded by protein that can replicate only with the help of intact cells. All viruses have a lytic cycle, in which they invade a cell, replicate inside, then rupture the cell and release their progeny. Some viruses also have a lysogenic cycle, in which they lie dormant and replicate as part of the host DNA. Viruses are classified by the the type of nucleic acid (either DNA or RNA).

General characteristics:
Viruses are fragments of nucleic acids (DNA or RNA, never both), often surrounded by protein. A few viruses also have capsules derived from host cell membranes.
Viral shapes can be helical, icosahedral (20-sided), or complex (with head and tail).
Viruses have very few of the characteristics of life; they cannot reproduce without the gene-replicating machinery of the host cell.

Lytic cycle (in all viruses):

Virus first attaches to host cells and injects nucleic acid only.
Viral DNA or RNA is replicated using the host cell's enzymes.
Host cell ruptures, releasing thousands of new virus particles.

Lysogenic cycle (in some viruses only):

Viral DNA inserts into the host cell chromosome.
Virus then hides (lysogenic stage), replicating as part of host DNA.
Upon "activation," the virus takes over the cell's reproductive machinery and resumes the lytic cycle.

Virus classification:

DNA viruses:
- With single-stranded DNA:
  - Parvoviruses
  - ΦX174
- With double-stranded DNA:
  - T2, T4, T6 bacteriophages
  - Lambda (λ) bacteriophage
  - Adenoviruses
  - Herpes viruses, pox viruses
RNA viruses:
- With single-stranded RNA:
  - PicoRNA viruses (rhinovirus, polio, etc.)
  - Togaviruses (rubella, etc.)
  - Rhabdoviruses (rabies, etc.)
  - Measles and mumps viruses
  - Influenza viruses
  - Retroviruses (HIV/AIDS, etc.)
  - Tobacco mosaic virus (TMV)
- With double-stranded RNA:
  - Reoviruses

Index Syllabus

rev. July 2010