A chart listing the difference between Virus and Cell.

FIGURE 20–6 Comparing Viruses and Cells The differences between viruses and cells are listed in this chart. Form an Opinion Based on this information, would you classify viruses as living or nonliving? Explain.

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Viruses and Cells Viruses must infect living cells in order to grow and reproduce, taking advantage of the nutrients and cellular machinery of their hosts. This means that all viruses are parasites. Parasites depend entirely upon other living organisms for their existence, harming these organisms in the process.

Despite the fact that they are not alive, viruses have many of the characteristics of living things. After infecting living cells, viruses can reproduce, regulate gene expression, and even evolve. Some of the main differences between cells and viruses are summarized in Figure 20–6.

Although viruses are smaller and simpler than the smallest cells, it is unlikely that they were the first organisms. Because viruses are dependent upon living organisms, it seems more likely that viruses developed after living cells. In fact, the first viruses may have evolved from the genetic material of living cells. Viruses have continued to evolve, along with the cells they infect, for billions of years.

MYSTERY CLUE

Face of a cow.

When scientists injected extracts from BSE-infected cow brain tissue into mice, the mice developed BSE. The extracts induced BSE even when they contained no nucleic acids (RNA or DNA). Could a virus be the cause of BSE?

20.1 Assessment

Review Key Concepts

    1. Review What do viruses depend on for their reproduction?

    2. Compare and Contrast How is viral reproduction different from that of cell-based organisms?

    1. Review Describe each of the two paths viruses may follow once they have entered a cell.

    2. Compare and Contrast How are lytic and lysogenic infections similar? How are they different?

Apply the Big idea

Structure and Function

  1. Compare the structure of a virus to the structure of both a prokaryotic cell and a eukaryotic cell. Use a graphic organizer of your choice to organize the information. You may wish to refer to Chapter 7, which discusses the structures of cells in detail.

End ofPage 579

Table of Contents

Miller & Levine Biology
About the Authors
Changing the World of Biology Education
Contents
Diversity of Life: A Visual Guide DOL•1–DOL•64
Labs and Activities
Features
UNIT 1 The Nature of Life
1 The Science of Biology
2 The Chemistry of Life
Unit Project
UNIT 2 Ecology
3 The Biosphere
4 Ecosystems and Communities
5 Populations
6 Humans in the Biosphere
Unit Project
UNIT 3 Cells
7 Cell Structure and Function
8 Photosynthesis
9 Cellular Respiration and Fermentation
10 Cell Growth and Division
Unit Project
UNIT 4 Genetics
11 Introduction to Genetics
12 DNA
13 RNA and Protein Synthesis
14 Human Heredity
15 Genetic Engineering
Unit Project
UNIT 5 Evolution
16 Darwin's Theory of Evolution
17 Evolution of Populations
18 Classification
19 History of Life
Unit Project
UNIT 6 From Microorganisms to Plants
20 Viruses and Prokaryotes
21 Protists and Fungi
22 Introduction to Plants
23 Plant Structure and Function
24 Plant Reproduction and Response
Unit Project
UNIT 7 Animals
25 Introduction to Animals
26 Animal Evolution and Diversity
27 Animal Systems I
28 Animal Systems II
29 Animal Behavior
Unit Project
UNIT 8 The Human Body
30 Digestive and Excretory Systems
31 Nervous System
32 Skeletal, Muscular, and Integumentary Systems
33 Circulatory and Respiratory Systems
34 Endocrine and Reproductive Systems
35 Immune System and Disease
Unit Project
A Visual Guide to The Diversity of Life
HOW TO USE THIS GUIDE
THE TREE OF LIFE
Bacteria
Protists
Fungi
Plants
Animals
Appendices
Appendix A Science Skills
Appendix B Lab Skills
Appendix C Technology & Design
Appendix D Math Skills
Appendix E Periodic Table
Glossary
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q, R
S
T
U
V
W
X, Y, Z
Index
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
Credits
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