1. Viruses

2. Learning Objective Describe the structure and properties of viruses
Describe the life cycle of viruses on an example of bacteriophage

3. Vocabulary Virus Capsid Bacteriophage Lytic infection
Lysogenic infection
Prophage
Retrovirus

4. What is a Virus?

5. What is a Virus?
Virus – particle made of proteins, nucleic acids, and sometimes lipids that can replicate only by infecting living cells.
Viruses are not “cells” and are not considered living organisms.

6. Common Viruses

7. Virus Structure
A typical virus is composed of a core of DNA or RNA surrounded by a protein coat.
Viral Components
Capsid – the protein shell of a virus
Nucleic Acids – the genetic material
Some Viruses
Envelopes – found is some viruses; an outer wrapping composed of a lipid membrane obtained from the host cell.
Glycoprotein (receptor) – proteins attached to carbohydrates. Used by the virus as an attachment method to enter the host cell.

8. Viral Envelopes
Many viruses that infect animals have a membranous envelope
Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell
Some viral envelopes are formed from the host cell’s plasma membrane as the viral capsids exit

9. Capsid and viral genome enter the cell RNA
Figure 19.7
Capsid
Capsid and viral genome enter the cell
RNA
Envelope (with glycoproteins)
HOST CELL
Viral genome (RNA)
Template
mRNA
Capsid proteins ER
Copy of genome (RNA)
Figure 19.7 The replicative cycle of an enveloped RNA virus.
Glyco- proteins
New virus

10. Viral Enzymes
Some Viruses Contain Enzymes that assist in the replication of the virus once it enters the host cell.

11. Retrovirus A retrovirus has RNA as its genetic information.
A retrovirus is a single-stranded positive-sense RNA virus with a DNA intermediate and, as an obligate parasite, targets a host cell. Once inside the host cell ...

12. Virus Structure: Many different types of Viruses

13. Virus Structure: Many types of Viruses
Membranous envelope
Capsomere
RNA
RNA
DNA
Capsid
DNA
Head
Tail sheath
Capsomere of capsid
Tail fiber
Glycoprotein
Glycoproteins
18  250 nm
70–90 nm (diameter)
80–200 nm (diameter)
80  225 nm
20 nm
50 nm
50 nm
50 nm
(a)
Tobacco mosaic virus
(b) Adenoviruses
(c) Influenza viruses
(d) Bacteriophage T4

14. Viral Infections

15. Viral Infection Viral Infection
Once the virus is inside the host cell, two different processes may occur.
Some viruses replicate immediately, killing the host cell.
Lytic Infection
Others replicate, but do not kill the host cell immediately.
Lysogenic Infection
Copyright Pearson Prentice Hall

16. Bacteriophage injects DNA into bacterium
Bacteriophage DNA forms a circle
Lytic Infection
Lysogenic Infection
Bacteriophage may infect cells in two ways: lytic infection and lysogenic infection.

17. The Lytic Cycle
The lytic cycle is a phage replicative cycle that culminates in the death of the host cell
The lytic cycle produces new phages and lyses (breaks open) the host’s cell wall, releasing the progeny viruses
Virion – a complete virus that is released from the host cell.

18. Figure 1
Attachment
Figure 19.5 The lytic cycle of phage T4, a virulent phage.

19. Entry of phage DNA and degradation of host DNA
Figure 1
Attachment 2
Entry of phage DNA and degradation of host DNA
Figure 19.5 The lytic cycle of phage T4, a virulent phage.

20. Entry of phage DNA and degradation of host DNA
Figure 1
Attachment 2
Entry of phage DNA and degradation of host DNA
Figure 19.5 The lytic cycle of phage T4, a virulent phage. 3
Synthesis of viral genomes and proteins

21. Entry of phage DNA and degradation of host DNA
Figure 1
Attachment 2
Entry of phage DNA and degradation of host DNA
Phage assembly
Figure 19.5 The lytic cycle of phage T4, a virulent phage. 4
Assembly 3
Synthesis of viral genomes and proteins
Head
Tail
Tail fibers

22. Entry of phage DNA and degradation of host DNA
Figure 1
Attachment 2
Entry of phage DNA and degradation of host DNA 5
Release
Phage assembly
Figure 19.5 The lytic cycle of phage T4, a virulent phage. 4
Assembly 3
Synthesis of viral genomes and proteins
Head
Tail
Tail fibers

23. The Lysogenic Cycle
The lysogenic cycle replicates the phage genome without destroying the host
The viral DNA molecule is incorporated into the host cell’s chromosome
This integrated viral DNA is known as a prophage
Every time the host divides, it copies the phage DNA and passes the copies to daughter cells

24. Daughter cell with prophage The phage injects its DNA.
Figure 19.6
Phage DNA
Daughter cell with prophage
The phage injects its DNA.
Cell divisions produce a population of bacteria infected with the prophage.
Phage DNA circularizes.
Phage
Bacterial chromosome
Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle.
Lytic cycle
Lysogenic cycle
Certain factors determine whether
The bacterium reproduces, copying the prophage and transmitting it to daughter cells.
The cell lyses, releasing phages.
Figure 19.6 The lytic and lysogenic cycles of phage , a temperate phage.
lytic cycle is induced or
lysogenic cycle is entered
Prophage
New phage DNA and proteins are synthesized and assembled into phages.
Phage DNA integrates into the bacterial chromosome, becoming a prophage.

25. Daughter cell with prophage
Figure 19.6b
Daughter cell with prophage
Cell divisions produce a population of bacteria infected with the prophage.
Phage DNA circularizes.
Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle.
Lysogenic cycle
Certain factors determine whether
The bacterium reproduces, copying the prophage and transmitting it to daughter cells.
Figure 19.6 The lytic and lysogenic cycles of phage , a temperate phage.
lytic cycle is induced
lysogenic cycle is entered or
Prophage
Phage DNA integrates into the bacterial chromosome, becoming a prophage.

26. An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode
Phages that use both the lytic and lysogenic cycles are called temperate phages

27. The phage injects its DNA.
Figure 19.6a
Phage DNA
The phage injects its DNA.
Phage DNA circularizes.
Phage
Bacterial chromosome
Lytic cycle
Certain factors determine whether
The cell lyses, releasing phages.
Figure 19.6 The lytic and lysogenic cycles of phage , a temperate phage.
lytic cycle is induced
lysogenic cycle is entered or
New phage DNA and proteins are synthesized and assembled into phages.