1. Ch. 19 Viruses
Objective:
EK 3.C.3: Viral replication results in genetic variation,
and viral infection can introduce genetic variation into the hosts.
L.O The student is able to construct an explanation of how viruses introduce genetic variation in host organisms.

2. 19.1 A Virus Consists of a Nucleic Acid Surrounded By a Protein Coat
1st discovered viruses – Tobacco Mosaic Virus
Filtered plant sap still infected plants.
Something smaller (viruses) was infected them.
Extracted sap from tobacco plant with tobacco mosaic disease
RESULTS
Passed sap through a porcelain filter known to trap bacteria
Healthy plants became infected
Rubbed filtered sap on healthy tobacco plants 1 2 3 4

3. Structure of Viruses
Nucleic Acid: typically 1 linear strand or circular molecule
Double stranded DNA
Single stranded DNA
Double stranded RNA
Single stranded RNA
Protein shell (capsid) encloses nucleic acid. These come in many shapes/sizes.
Some have accessory structures:
Viral envelopes
Tail fibers
Capsomere of capsid
RNA
Capsomere
DNA
Glycoprotein
Glycoproteins
Membranous envelope
Capsid
Head
Tail sheath
Tail fiber
18  250 nm
80  225 nm
70–90 nm (diameter)
80–200 nm (diameter)
20 nm
50 nm
(a)
Tobacco mosaic virus
(b) Adenoviruses
(c) Influenza viruses
(d) Bacteriophage T4

4. 19.2 Viruses Replicate Only in Host Cells
Viruses do not have organelles to build proteins, replicate nucleic acids, metabolize, etc. Needs a host cell to do it.
Viruses can only infect certain hosts (host range)
Only certain species
Only specific cells/tissues within species
Ex: flu only infects cells of the respiratory system

5. General Features of Viral Replicative Cycles
Infection begins with the insertion of the viral nucleic acid into the host cell:
Use tail fibers to inject DNA into host cell (like a needle)
Endocytosis of entire virus
Viral DNA commandeers cells to make more viruses using the cell’s own machinery.
Kills cells when done.

6. The Lytic Cycle The virus injects DNA into the host cell
New phage DNA and proteins are synthesized and assembled into phages.
The cell lyses, releasing phages.
Phage
Phage DNA
The phage injects its DNA.
Bacterial chromosome
Lytic cycle
lytic cycle is induced or
Phage DNA circularizes.
Certain factors determine whether
lysogenic cycle is entered
The virus injects DNA into the host cell
Host cells starts copying viral DNA and making capsid and accessory parts.
Once done, the cell lyses (bursts) releasing new viruses to infect other cells.

7. Animation: Phage T4 Lytic Cycle
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.

8. The Lysogenic Cycle Same beginning and end as lytic cycle
Insert DNA …… make copies and cell lyses
After the DNA is inserted, it is integrated into host cell’s DNA.
Prophage; 1 gene codes for a protein that prevents transcription of the rest of its genes.
Cell grows and replicates (binary fission) normally with viral DNA still “hiding” in host DNA.
lytic cycle is induced or
Phage DNA circularizes.
Certain factors determine whether
lysogenic cycle is entered
Lysogenic cycle
Prophage
Daughter cell with prophage
Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle.
Cell divisions produce a population of bacteria infected with the prophage.
The bacterium reproduces, copying the prophage and transmitting it to daughter cells.
Phage DNA integrates into the bacterial chromosome, becoming a prophage.

9. Animation: Phage Lambda Lysogenic and Lytic Cycles
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.

10. Replicative Cycles of Animal Viruses
Be aware of (but don’t memorize) that animal viruses are classified according to type of nucleic acid and presence of an envelope.

11. RNA as Viral Genetic Material
Class IV – viral RNA serves as mRNA to immediately make proteins.
Class V – viral RNA used to make many mRNAs.
Class VI (retroviruses) –
RNA + reverse transcriptase  DNA
Viral DNA embeds in host’s DNA
Viral proteins made with host’s

12. Figure 19.8
Membrane of white blood cell
Glycoprotein
Viral envelope
HIV
Capsid
RNA (two identical strands)
HOST CELL
Reverse transcriptase
HIV
Reverse transcriptase
Viral RNA
RNA-DNA hybrid
DNA
0.25 m
HIV entering a cell
NUCLEUS
Provirus
Chromosomal DNA
RNA genome for the next viral generation
Figure 19.8 The replicative cycle of HIV, the retrovirus that causes AIDS.
mRNA
New virus
New HIV leaving a cell

13. Animation: HIV Reproductive Cycle
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.

14. Evolution of Viruses Not living but effects all living things.
Evolved after life.
Naked DNA infected injured cells at first, then evolved to have an envelope and could infect healthy cells.
Viral nucleic acids are more similar to host’s nucleic acids but there are some viral genes that are similar for all viruses.