1. Chapter 24 Objectives Summarize the discovery of viruses.
Section 1 Viral Structure and Replication
Chapter 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.

2. Chapter 24 Discovery of Viruses
Section 1 Viral Structure and Replication
Chapter 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.

3. Characteristics of Viruses
Section 1 Viral Structure and Replication
Chapter 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.

4. Characteristics of Viruses, continued
Section 1 Viral Structure and Replication
Chapter 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.

5. Characteristics of Viruses, continued
Section 1 Viral Structure and Replication
Chapter 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

6. Viral Replication, continued
Section 1 Viral Structure and Replication
Chapter 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

7. Viral Replication, continued
Section 1 Viral Structure and Replication
Chapter 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.

8. Viral Replication, continued
Section 1 Viral Structure and Replication
Chapter 24
Viral Replication, continued
Replication in Viruses That Infect Prokaryotes
Bacteriophages are viruses that infect bacteria.

9. Viral Replication, continued
Section 1 Viral Structure and Replication
Chapter 24
Viral Replication, continued
Lytic Cycle
Viruses can follow a lytic cycle, making new viral particles immediately.

10. Viral Replication, continued
Section 1 Viral Structure and Replication
Chapter 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

11. The Lytic and Lysogenic Cycles
Section 1 Viral Structure and Replication
Chapter 24
The Lytic and Lysogenic Cycles

12. Viral Replication, continued
Section 1 Viral Structure and Replication
Chapter 24
Viral Replication, continued
Viruses: Tools for Biotechnology
Viruses are important tools for biotechnology.

13. Believed to have appeared on Earth after living cells. Lysogeny
Section 1 Viral Structure and Replication
Chapter 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