Chapter  13 and 21.
Why study bacterial genetics?
•	Its an easy place to start
•	history
•	we know more about it
•	systems better understood
•	simpler genome
•	good model for control of genes
•	build concepts from there to eukaryotes
•	bacterial genetic systems are exploited in biotechnology
Bacteria
•	Bacteria review 
•	one-celled organisms
•	prokaryotes
•	reproduce by mitosis
•	binary fission
•	rapid growth
•	generation every ~20 minutes
•	108 (100 million) colony overnight!
•	dominant form of life on Earth
•	incredibly diverse
Bacterial diversity
Bacterial genome 
•	Single circular chromosome
•	haploid
•	naked DNA
•	no histone proteins
•	~4 million base pairs
•	~4300 genes
•	1/1000 DNA in eukaryote
No nucleus!
•	No nuclear membrane
•	chromosome in cytoplasm
•	transcription & translation are coupled together
•	no processing of mRNA
•	no introns
•	but Central Dogma 
still applies
•	use same 
genetic code
Binary fission
•	Replication of bacterial chromosome
•	Asexual reproduction
•	offspring genetically identical to parent
•	where does variation come from?
Variation in bacteria
•	Sources of variation
•	spontaneous mutation
•	transformation
•	plasmids
•	DNA fragments
•	transduction
•	conjugation
•	transposons
Spontaneous mutation
•	Spontaneous mutation is a 
significant source of variation 
in rapidly reproducing species
•	Example: E. coli
•	human colon (large intestines)
•	2 x 1010 (billion) new E. coli each day!
•	spontaneous mutations
•	for 1 gene, only ~1 mutation in 10 million replications
•	each day, ~2,000 bacteria develop mutation in that gene
•	but consider all 4300 genes, then:
4300 x 2000 = 9 million mutations per day per human host!
Transformation 
•	Bacteria are opportunists
•	pick up naked foreign DNA wherever it may be hanging out
•	have surface transport proteins that are specialized for the uptake of naked DNA 
•	import bits of chromosomes from other bacteria
•	incorporate the DNA bits into their own chromosome
•	express new gene
•	form of recombination
Swapping DNA
•	Genetic recombination by trading DNA
Plasmids 
•	Plasmids
•	small supplemental circles of DNA
•	5000 - 20,000 base pairs
•	self-replicating
•	carry extra genes
•	2-30 genes
•	can be exchanged between bacteria
•	bacterial sex!!
•	rapid evolution 
•	antibiotic resistance
•	can be imported 
from environment
Plasmids & antibiotic resistance
•	Resistance is futile?
•	1st recognized in 
1950s in Japan
•	bacterial dysentery 
not responding to antibiotics
•	worldwide problem now
•	resistant genes are 
on plasmids that are swapped between bacteria
Biotechnology  
•	Used to insert new genes into bacteria
•	example: pUC18
•	engineered plasmid used in biotech
Transduction 
Conjugation
•	Direct transfer of DNA between 2 bacterial cells that are temporarily joined
•	results from presence of F plasmid with F factor
•	F for “fertility” DNA
•	E. coli “male” extends sex pilli, attaches to female bacterium
•	cytoplasmic bridge allows transfer of DNA
Chapter  13 and 21.
Viral diseases 
Emerging viruses
•	Viruses that “jump” host
•	switch species
•	Ebola, SARS, bird flu, hantavirus
What is a virus? Is it alive?
•	DNA or RNA enclosed in a protein coat
•	Viruses are not cells
•	lack enzymes for metabolism
•	lack ribosomes for protein synthesis
•	need host “machinery”
Viral protein coat
•	Capsid 
•	crystal-like protein shell
•	1-2 types of proteins
•	many copies of same protein
Viral genomes
•	Viral nucleic acids 
•	DNA 
•	double-stranded
•	single-stranded
•	RNA
•	double-stranded
•	single-stranded
•	Linear or circular
•	smallest viruses have only 4 genes, while largest have several hundred
Viral envelope
•	Lipid bilayer membranes cloaking viral capsid
•	envelopes are derived from host cell membrane
•	glycoproteins on surface
Generalized viral lifecycle
•	Entry
•	virus DNA/RNA enters host cell
•	Assimilation
•	viral DNA/RNA takes over host
•	reprograms host cell to copy viral nucleic acid & build viral proteins
•	Self assembly
•	nucleic acid molecules & capsomeres then self-assemble into viral particles
•	exit cell
Viral hosts
•	Host range 
•	most types of virus can infect & parasitize 
only a limited range of host cells
•	identify host cells via “lock & key” fit 
•	between proteins on viral coat & 
receptors on host cell surface
•	broad host range
•	rabies = can infect all mammals
•	narrow host range
•	human cold virus = only cells lining upper respiratory tract of humans
•	HIV = binds only to specific white blood cells
Bacteriophages 
•	Viruses that infect bacteria
•	ex. phages that infect E. coli 
•	lambda phage
•	20-sided capsid head encloses DNA 
•	protein tail attaches phage to host & injects phage DNA inside
Lytic lifecycle of phages
Lysogenic lifecycle of phages
Defense against viruses
•	Bacteria have defenses against phages
•	bacterial mutants with receptors that are no longer recognized by a phage
•	natural selection favors these mutants
•	bacteria produce restriction enzymes 
•	recognize & cut up foreign DNA
•	It’s an escalating war!
•	natural selection favors phage mutants resistant to bacterial defenses
RNA viruses 
•	Retroviruses 
•	have to copy viral RNA into host DNA
•	enzyme = reverse transcriptase 
•	RNA  DNA  mRNA
•	host’s RNA polymerase now transcribes viral DNA into viral mRNA
•	mRNA codes for viral components
•	host’s ribosomes produce new viral proteins
Retroviruses
•	HIV
•	Human ImmunoDeficiency Virus 
•	causes AIDS
•	Acquired ImmunoDeficiency 
Syndrome
•	opportunistic diseases
•	envelope with glycoproteins 
for binding to specific WBC
•	capsid containing 2 RNA 
strands & 2 copies of 
reverse transcriptase
HIV infection
•	HIV enters host cell
•	macrophage & CD4 WBCs
•	cell-surface receptor
•	reverse transcriptase synthesizes double stranded DNA from viral RNA
•	high mutation rate
•	Transcription produces more copies of viral RNA 
•	translated into viral proteins
•	proteins & vRNA self-assemble into virus particles 
•	released from cell by “budding” or by lysis
Cancer viruses
•	Viruses appear to cause certain human cancers
•	hepatitis B virus 
•	linked to liver cancer
•	Epstein-Barr virus = infectious mono
•	linked to lymphoma
•	papilloma viruses 
•	linked with cervical cancers
•	HTLV-1 retrovirus 
•	linked to adult leukemia
Cancer viruses
•	Transform cells into cancer cells after integration of viral DNA into host DNA
•	carry oncogenes that trigger cancerous characteristics in cells
•	version of human gene that normally controls cell cycle or cell growth
•	Most tumor viruses probably cause cancer only in combination with other mutagenic events
Prions
•	Misfolded proteins
•	infectious
•	make plaques (clumps) & 
holes in brain 
as neurons die
Protein as information molecule?! 
•	Prions challenge Central Dogma
•	transmit information to other proteins
Any Questions??