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

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!

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

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

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!

Lytic vs. Lysogenic Infections

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) http://www.youtube.com/watch?v=B7ITZgag6w0&feature=related New virus

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 http://www.npr.org/templates/story/story.php?storyId=114075029

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 1749-1823

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)

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.

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).

LE 18-13 http://learn.genetics.utah.edu/content/begin/dna/prions/

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!!

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

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.

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

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!

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

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

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

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!

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.