Section 1 Viral Structure and ReplicationChapter 24

Objectives

• Summarize the discovery of viruses.

• Describe why viruses are not considered living organisms.

• Describe the basic structure of viruses.

• Compare the lytic and lysogenic cycles of virus replication.

• Summarize the origin of viruses.

Section 1 Viral Structure and ReplicationChapter 24

Discovery of Viruses

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

• In 1935, Wendell Stanley demonstrated that viruses were not cells when he crystallized TMV, the virus that causes tobacco mosaic disease in tobacco and tomato plants.

Section 1 Viral Structure and ReplicationChapter 24

Characteristics of Viruses

• Viruses do not have all of the characteristics of life and are therefore not considered to be living.– Do not have cytoplasm or organelles, can’t carry out

cellular functions such as homeostasis or metabolism, they don’t grow as cells do by dividing in two, they can’t reproduce genetic material outside of host cell

– They depend on something else to be able to survive (HOST)

• Studied in biology because they are active in living cells and caused disease.

Section 1 Viral Structure and ReplicationChapter 24

Characteristics of Viruses, continued

• Viral Size and Structure– 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.

Section 1 Viral Structure and ReplicationChapter 24

Characteristics of Viruses, continued

• Classification of Viruses– Viruses can be classified based on:

• Having RNA or DNA, • RNA or DNA is single or double stranded• RNA or DNA circular or linear• by capsid shape• Envelope or no envelope

Section 1 Viral Structure and ReplicationChapter 24

Viral Replication, continued

• Replication in DNA Viruses– 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.• Provirus – viral DNA that has attached

to a host cell’s chromosome and that is replicated with the chromosome’s DNA

Section 1 Viral Structure and ReplicationChapter 24

Viral Replication, continued

• Replication in RNA Viruses– The RNA genome of some RNA viruses

can be directly translated to make viral proteins.

– Retroviruses use reverse transcriptase and RNA as a template to make DNA, which is then used to produce viral RNA and proteins.

Section 1 Viral Structure and ReplicationChapter 24

Viral Replication, continued

• Replication in Viruses That Infect Prokaryotes– Bacteriophages are viruses that infect

bacteria.

Section 1 Viral Structure and ReplicationChapter 24

Viral Replication, continued

• Lytic Cycle– Viruses can follow a lytic cycle, making

new viral particles immediately.

Section 1 Viral Structure and ReplicationChapter 24

Viral Replication, continued

• Lysogenic Cycle– Viruses can follow a lysogenic cycle,

becoming part of the host genome and making new particles later.

– Viral DNA becomes integrated into host DNA

Chapter 24

The Lytic and Lysogenic Cycles

Section 1 Viral Structure and Replication

Section 1 Viral Structure and ReplicationChapter 24

Viral Replication, continued

• Viruses: Tools for Biotechnology– Viruses are important tools for biotechnology.

Section 1 Viral Structure and ReplicationChapter 24

The Origin of Viruses

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

• Believed to have appeared on Earth after living cells.

• Lysogeny• lytic cycle• Viruses