Section 1

Viral Structure and Replication

• Researchers in the late 1800s discovered that something smaller than bacteria could cause disease.

• Wendell Stanley demonstrated that viruses were not cells when he crystallized TMV, the virus that causes tobacco mosaic disease in tobacco and tomato plants.

• Viruses are nonliving particles containing DNA or RNA and are surrounded by a protein coat called a capsid. Some viruses also have an envelope that is derived from a host cell's nuclear membrane or cell membrane.

• Viruses do not have all of the characteristics of life and are therefore not considered to be living.

• Viruses can be classified based on whether they have RNA or DNA, whether the RNA or DNA is single or double stranded and circular or linear, by capsid shape, and whether or not they have an envelope.

Vocabulary virus (p. 483) capsid (p. 484) envelope (p. 484) provirus (p. 485)

retrovirus (p. 486) reverse transcriptase

• DNA viruses can enter host cells and directly produce RNA, or they can insert into a host's chromosome, where they are transcribed to RNA along with the host's DNA.

• The RNA genome of some RNA viruses can be directly translated to make viral proteins. Others use reverse transcriptase and RNA as a template to make DNA, which is then used to produce viral RNA and proteins.

• Bacteriophages can follow a lytic cycle (making new viral particles immediately) or a lysogenic cycle (becoming part of the host genome and making new particles later).

• Viruses are important tools for biotechnology.

• Most scientists think viruses originated from fragments of host-cell nucleic-acid material.

lytic cycle (p. 486) virulent (p. 486) lysis (p. 486) lysogenic cycle (p. 487)

temperate virus (p. 487) prophage (p. 487)

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