1. The Genetics of Viruses and Bacteria – Today viruses,
Chapter 18
The Genetics of Viruses and Bacteria – Today viruses,
Cold, flu, warts, cancer?

2. Structure of Viruses
Viruses are not cells (no membrane, ribosomes – they are parasites)
Viruses are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope obtained from its host.
* May contain DNA or RNA (single or dbl stranded) but not together!

3. Capsids and Envelopes – see side poster
A capsid is the protein shell that encloses the viral genome – this structure fits into target cell receptors, so viruses are cell specific. Influenza? Polio?
A capsid can have various structures

4. 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 new viruses
A phage that reproduces only by the lytic cycle is called a virulent phage
Bacteria have natural defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA

5. The Lysogenic Cycle
The lysogenic cycle replicates the phage genome without destroying the host
The viral DNA molecule is incorporated by genetic recombination 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, doesn’t necessarily destroy host cell and can last a long time!

6. Lytic vs. Lysogenic Infections

7. Capsid and viral genome enter cell RNA
LE 18-8
Capsid
Capsid and viral genome
enter cell
RNA
HOST CELL
Envelope (with
glycoproteins)
Viral genome (RNA)
Template
mRNA ER
Capsid
proteins
Glyco-
proteins
Copy of
genome (RNA)
New virus

8. Viral Diseases in Animals
Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes
Some viruses cause infected cells to produce toxins that lead to disease symptoms

9. Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen
Vaccines can prevent certain viral illnesses (named for vaca – cowpox). Edward Jenner

10. Emerging Viruses
Emerging viruses are those that appear suddenly or suddenly come to the attention of scientists
Severe acute respiratory syndrome (SARS) recently appeared in China, Swine flu 2009
Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host territory (mutate to infect human cells)

11. Viroids and Prions: The Simplest Infectious Agents
Viroids are circular RNA molecules (no capsid) that infect plants and disrupt their growth – even smaller than virus.

12. Prions – scary!! Read “The Deadly Feast”
Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals (Mad Cow, Scrapie, CJD, Kuru, CWD)
Prions propagate by converting normal proteins into the prion version. Can occur by random mutation or by eating infected flesh (brain).

13. LE 18-13

14. Origins?
Viruses are found wherever there is life and have, it is therefore presumed, existed since living cells first evolved. The origin of viruses is unclear because they do not form fossils, so molecular techniques have been used to compare the DNA or RNA of viruses and are a useful means of investigating how they arose. There are several theories!!

15. On to Bacterial Gene Expression:
Remember bacteria:
Prokaryotes
Formerly Kingdom Monera (1) – now Bacteria & Archaea (2)
3 shapes (cocci, bacilli, spirilla)

16. The Bacterial Genome and Its Replication
The bacterial chromosome is usually a circular DNA molecule with few (no?) associated proteins
Many bacteria also have plasmids, smaller circular DNA molecules that can replicate independently of the chromosome – carry few BUT IMPORTANT genes.

17. Mechanisms of Gene Transfer and Genetic Recombination in Bacteria
Bacterial cells divide by binary fission (asexual), which is preceded by replication of the chromosome
Three processes bring bacterial DNA from different individuals together (sexual):
Transformation
Transduction
Conjugation

18. Transformation
Transformation is the alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environment
For example, harmless Streptococcus pneumoniae bacteria can be transformed to pneumonia-causing cells (remember R vs. S strains). Showed in Griffiths famous experiment!

19. Transduction In the process known as transduction, phages carry
bacterial genes from one
host cell to another as they move
throughout a system.

20. Conjugation and Plasmids
Conjugation is the direct transfer of genetic material between bacterial cells that are temporarily joined via sex pili
The transfer is one-way: One cell (“male”) donates DNA, and its “mate” (“female”) receives the genes

21. F plasmid Bacterial chromosome F+ cell F+ cell Mating bridge F– cell
LE 18-18_1
F plasmid
Bacterial chromosome
F+ cell
F+ cell
Mating
bridge
F– cell
F+ cell
Bacterial
chromosome
Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient

22. R plasmids and Antibiotic Resistance
R plasmids confer resistance to various antibiotics
When a bacterial population is exposed to an antibiotic, individuals with the R plasmid will survive and increase in the overall population causing evolution – humans drive this by taking antibiotics but shortcutting etc.. It’s coevolution – like insects and pesticides!

23. Transposition of Genetic Elements
Transposons are sequences of DNA that can move around to different positions within the genome of a single cell, a process called transposition. In the process, they can cause mutations and change the amount of DNA in the genome. Transposons were also once called jumping genes. They were discovered by Barbara McClintock early in her career, for which she was awarded a Nobel prize in 1983.