Viruses

Page
3

Fig. 19-4

VIRUS

1

2

3

DNA

Capsid

4

Replication

HOST CELL

Viral DNA

mRNA

Capsid

proteins

Viral DNA

Slide 19

Reproductive Cycles of Phages

Phages are the best understood of all viruses

Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle

Slide 20

The Lytic Cycle

The lytic cycle is a phage reproductive cycle that culminates in the death of the host cell

The lytic cycle produces new phages and digests the host’s cell wall, releasing the progeny viruses

A phage that reproduces only by the lytic cycle is called a virulent phage

Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA

Animation: Phage T4 Lytic Cycle

Slide 21

Fig. 19-5-1

Attachment

1

Slide 22

Fig. 19-5-2

Entry of phage

DNA and

degradation of

host DNA

Attachment

1

2

Slide 23

Fig. 19-5-3

Synthesis of viral

genomes and

proteins

Entry of phage

DNA and

degradation of

host DNA

Attachment

1

2

3

Slide 24

Fig. 19-5-4

Phage assembly

Assembly

Synthesis of viral

genomes and

proteins

Entry of phage

DNA and

degradation of

host DNA

Attachment

1

2

4

Head

Tail

Tail fibers

3

Slide 25

Fig. 19-5-5

Phage assembly

Head

Tail

Tail fibers

Assembly

Release

Synthesis of viral

genomes and

proteins

Entry of phage

DNA and

degradation of

host DNA

Attachment

1

2

4

5

3

Slide 26

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

Animation: Phage Lambda Lysogenic and Lytic Cycles

Slide 27

An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode