1. Viruses

2. Viruses Nonliving particles
Very small (1/2 to 1/100 of a bacterial cell)
Do not perform respiration, grow, or develop
Are able to replicate (only with the help of living cells)
Host cell—a cell where a virus replicates

3. T4 bacteriophage infecting an E. coli cell
0.5 m

4. Comparing the size of a virus, a bacterium, and an animal cell
Animal cell nucleus

5. Viral Structure 2 main parts: inner core of nucleic acid (DNA or RNA)
instructions for making copies of the virus
outer coat of protein (capsule)
determines shape of virus

6. (a) Tobacco mosaic virus
Viral structure
18  250 mm
70–90 nm (diameter)
80–200 nm (diameter)
80  225 nm
20 nm
50 nm
(a) Tobacco mosaic virus
(b) Adenoviruses
(c) Influenza viruses
(d) Bacteriophage T4
RNA
Capsomere of capsid
DNA
Capsomere
Glycoprotein
Membranous envelope
Capsid
Head
Tail fiber
Tail sheath

7. Infection by tobacco mosaic virus (TMV)

8. Replication: Attachment to Host Cell
Order of events:
1.virus recognizes host cell
2.virus attaches to receptor site on membrane of host cell
a. Receptor site on host matches with viral proteins (like a puzzle)
3.virus enters host cell
4.virus replicates inside host cell

9. Simplified viral reproductive cycle
VIRUS
Capsid proteins
mRNA
Viral DNA
HOST CELL
Entry into cell and
uncoating of DNA
Replication
Transcription
DNA
Capsid
Self-assembly of new virus particles and their exit from cell

10. Attachment is Specific
viruses have specifically shaped attachment proteins
each virus infects only certain types of cells
most are species specific
Smallpox, polio, measles—affects only humans
although some are not
West Nile virus—mosquitoes, birds, humans, horses
some are cell-type specific
polio—affects intestine & nerve cells

11. Lytic cycle of phage T4, a virulent phage
Attachment. The T4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of an E. coli cell.
Entry of phage DNA and degradation of host DNA. The sheath of the tail contracts, injecting the phage DNA into the cell and leaving an empty capsid outside. The cell’s DNA is hydrolyzed.
Synthesis of viral genomes and proteins. The phage DNA directs production of phage proteins and copies of the phage genome by host enzymes, using components within the cell.
Assembly. Three separate sets of proteins self-assemble to form phage heads, tails, and tail fibers. The phage genome is packaged inside the capsid as the head forms.
Release. The phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter. The cell swells and finally bursts, releasing 100 to 200 phage particles. 1 2 4 3 5
Phage assembly
Head
Tails
Tail fibers

12. Lysogenic vs. Lytic Lytic cycle (virulent phage)
Release of virus bursts and kills host cell (lysis)
Lysogenic cycle (temperate phage)
Viral DNA integrates into host genome (provirus)
Can be transmitted to daughter cells
Can initiate lytic cycle in response to environmental signal (stress)

13. The lytic and lysogenic cycles of phage , a temperate phage
Many cell divisions produce a large population of bacteria infected with the prophage.
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
Phage DNA integrates into the bacterial chromosome, becoming a prophage (provirus).
New phage DNA and proteins are synthesized and assembled into phages.
Occasionally, a prophage exits the bacterial chromosome,
initiating a lytic cycle.
Certain factors
determine whether
The phage attaches to a
host cell and injects its DNA.
Phage DNA
circularizes
The cell lyses, releasing phages.
Lytic cycle
is induced
Lysogenic cycle
is entered or
Prophage/Provirus
Bacterial
chromosome
Phage
DNA

14. Karyotypes

15. All the chromosomes in a cell can be stained.

16. Matching Bands

17. This set of chromosomes is called a Karyotype.

18. Karyotypes are used to determine: 1. Gender 2. Chromosome Disorders