Microbial Models of DNA

Microbes : microscopic organisms like bacteria and viruses, can include fungi and protista

Bacteria : kingdoms Eubacteria and Archeabacteria (formerly Monera), Prokaryotic cell, unicellular, one circular piece of DNA and multiple plasmids

Viruses : nonliving – DNA or RNA and a protein coat

Usefulness

• While we are familiar with disease causing bacteria and viruses (pathogens), most bacteria are harmless and actually helpful

• Because the have simple systems – the study of their genetics has helped us understand more complex systems, like our own.

• They have some specialized systems that have helped us study disease

• They transfer genetic material between cells – an important area of genetic research.

Discovery of Viruses

• Late 1880’s – Mayer, Ivanowsky and Beijerinck; plants can spread disease to each other, disease is smaller than a bacteria and can be within the sap

• Early 1900’s - Stanley ; crystallized TMV (tobacco mosaic virus)– Crop was still big part of economy

• Actually saw virus with electron microscope

Viruses

• 20 nm (nanometers) – smaller than ribosome• Cells can not be crystallized• Nucleic acid plus protein

– Protein called a capsid– Variety of shapes…. Including the icosahedron– dsDNA, ssDNA, ssRNA and dsRNA– Linear or circular– 4 – 100’s genes

• Bacteriophages are viruses that infect bacteria– Phages named T1 – T7

Viral Reproduction

• Viruses are host specific (range)– Phages only bacteria, some only E.coli– Rhinovirus (cold) only humans– Rabies – all mammals

• Proteins fit with receptor cells– Some viruses of eukaryotes are tissue

specific (URT)

• Lytic vs lysogenic life cycles – See handouts

Viral Life Cycles

LYTIC cycle• Virus attaches to host• Injects nucleic acid• Host reads nucleic acid

and then• manufactures viral DNA

and proteins• Virus is assembled• Escapes host cell• Infects other cells

LYSOGENIC cycle• Same as lytic UNTIL• Cell replicates – BEFORE

copying and making viral instructions

• This takes along viral nucleic acids to new generations of host, as part of the host genome

• At some point, lots of host offspring are triggered to start lytic cycle

• “latent” phase

Control of Viral Genes

• Bacteria posses restriction endonucleases, which are enzymes that degrade viral DNA as it enters

• Go lysogenic instead – if phage is added to a specific site within bacterial DNA then it’s a prophage and it codes for a protein that represses the other phage genes.

• Some bacteria are actually more harmful because they posses phages that make toxins

• Viruses can also be ‘enveloped’ by host to help them hide

Retroviruses

• These viruses double stranded RNA that is a template from which an enzyme called reverse transcriptionase can make DNA

• “backwards”

• An example is HIV

Vaccines• Colds – epithelial cells in throat can repair themselves efficiently• Polio – attaches to nerve cells that cannot be repaired• 1796 – Edward Jenner – noticed that cow pox (mild) makes one

immune to small pox (more harmful).• Used fluid from cow (vacca) pox blister to give people cow pox,

making them immune to small pox. • Small pox has since been eradicated (WHO)• Fewer cases of polio, rubella, measles and mumps since vaccines• Rabies vaccines for pets• Newer vaccines for chicken pox (varicella) and HPV (gardasil)• Flu shots are vaccines against the flu – varies from year to year and

location to location– Globalization– Evolution of viral genome– CDC and WHO

Emergent Diseases

• More cases, cases in new location, more deadly cases and spreading to a new host make a disease an emergent disease– HIV– Hantavirus– Ebola ! ! ! !– The plague (emergent diseases aren’t all viral) – not

currently emerging…. But still exists– Lyme disease– Asian Bird Flu– ???? Certain cancers

Link between cancer and virus

• Some viruses do cause cancers in animals– Example is feline leukemia

• Versions of oncogenes are found in normal cells – something triggers them

• Viruses may activate proto-oncogenes

• Viroids = circle of RNA that only infects plants, replicate in plant cell to interfere with enzymes

• Prions = infectious proteins that convert normal proteins to infectious proteins – Ex. “Mad Cow” aka Crueutzfeldt –Jacob disease

History

• Viral genomes often have much in common with the hosts DNA– Could have evolved as either plasmids or

transposons– Plasmids are small circular pieces of DNA that

replicate rapidly with in bacteria– Transposons are pieces of DNA that move

from location to location with in a genome

Bacterial Genetics

• Bacteria are cells – prokaryotic cells with a cell membrane, cytoplasm with ribosomes, and a circular piece of DNA in the nucleoid region. May also have plasmids

• Bacteria are m, • genome is about 4300 genes which is 100 x virus and

1/1000 of a euk. Cell• DNA is 500 longer than the cell• Divide by binary fission• Adaptable

– Lots of habitats and host and modes of nutrition– Natural selection happens quickly because of generation time

(hours to days)

Genetic Recombination• Transformation

– Uptake of DNA from the environment– S. pneumonia coats

• Transduction– Phages carry genes from one bacterial host to another– May have research possibilities

• Conjugation– Bridge called pili between two bacteria and plasmids are transferred….

No new cells, not reproduction– Transfer resistance to antibiotics – called R factor for resistance– Conjugation requires bacteria to have F (fertility) factor

• Transposons– Regions of DNA that move within a bacteria, either to a new place on

the chromosome or between the chromosome and the plasmids– Originally called ‘jumping genes’ by McClintock, they moved within the

genome of indian corn, turning the color genes on and off

Control of Bacteria genes

• Restriction endonucleases• Transposons• Feed back inhibition from OPERONS

– Transcription unit– Promoter : RNA polymerase attaches– Operator : acts as switch– Regulatory gene– Repressor : keeps operator off– Inducer : lets operator be on

OperonsOff to on / and on to off

• trp operon• See pg. 338• If no tryptophan –

then operon on and making tryp.

• If lots of trp – then repressor in operator, no need to make if you already have

• lac operon• See pg. 339• No lactose, no need

for lactase enzyme.• Lactose actually

inactivates the repressor to lactase can be made