Discovery of Viruses A.Meyer (1883): disease is contagious; infectious agent is a very small...

Discovery of VirusesA.Meyer (1883): disease is contagious; infectious agent is a very small bacterium that cannot be seen with a microscopeD.Ivanowsky (1890): tobacco mosaic disease is caused by a bacterium too small to be trapped by a filter

M.Baijesinck (1897): disease caused by reproducing particle (much smaller and simpler) than bacterium

W. Stanley (1935): crystallized infection particle known an the tobacco mosaic virus (TMV)

Viruses:• single or double stranded DNA• single or double stranded RNA

Capsid= protein coat that encloses viral genesEnvelope= membrane that surrounds viral capsule

• Many viruses use their tailpieces to inject DNA into the host cell• The new viral DNA takes over the cell and reprograms it to copy the viral genome and to manufacture capsid protein.•Most viruses contain the ability for self-assembly.

Host Range= limited # of host cells that a parasite can infect

* Viruses recognize host cells by a complementary fit between external viral proteins and specific cell receptor sites

Zoonoses= some viruses can infect several species

Bacterial Viruses: Bacteriophages• helped demonstrate that DNA

was the genetic material• established phage-bacterium system as an experimental tool

RNA Viruses: viruses related to RNA

RETROVIRUSES: RNA virus that uses reverse transcriptase to transcribe DNA from the viral RNA genome

AIDS is caused by HIV which is a retrovirus.

Viruses cause disease by:• damaging and killing cells• producing toxins• are indirectly responsible for disease symptoms

Can be fought by:Vaccines: harmless variants of pathogenic microbes that mobilize a host’s immune system against the pathogen

*polio, rubella, mumps, measlesAntiviral drugs: interfere with viral reproduction or viral nucleic acid synthesis

* influenza

Oncogenes: genes found in viruses or normal eukaryotic cells, that trigger transformation of a cell to a cancerous state

* Tumor viruses are usually turned on by CARCINOGENS.

Viruses do NOT fit cell theory:• cannot reproduce independently, but• have a genetic code• can mutate and evolve

Bacteria: smaller than eukaryotes, but larger than viruses

• DNA found in a single circular bacterial chromosome = GENOPHORE• double stranded chromosome• nucleoid region• extrachromosomal DNA found in plasmids

Bacteria reproduce by binary fission:* Replication of bacterial

chromosome begins at a single origin of replication* Two replication forks move in opposite directions until they meet

Genetic Recombination:Transformation: process of gene

transfer during which a bacterial cell incorporates genetic material from its surroundings* Some bacteria take up naked DNA from the surroundings (Ca++ helps this process)* Foreign DNA can be integrated by crossing over

Transduction: gene transfer from one bacterium to another by a bacteriophage

General Transduction: random pieces of host cell DNA are packaged w/in a phage capsid during the lytic cycle

* When the phage particle infects a new host cell, donor DNA recombines w/ recipient cell DNA

Conjugation: direct transfer of genes between two cells that are temporarily joined

Sex pili

Plasmid: double stranded ring of DNA that carries extrachromosomal DNAEpisomes: genetic components that can replicate independently as free molecules in the cytoplasm or incorporated into the bacterial chromosome

Transposons = DNA sequences that can move from one chromosomal site to anotherJUMPING GENES

Conservative Transposition: movement of pre-existing genes from one genomic location to anotherGenes are not copied, so the

number of genes stays the same

Replicative transposition: movement of gene copies from original site of replication to another location in the genome

*transposons genes are inserted at some new site w/o being lost from the original site

* Transposons scatter genes throughout the genome w/o an apparent, single target

Control of Gene Expression:Operons: a cluster of regulatory genes that control the function of structural genes

Genes that code for a polypeptide

* Prokaryotic cells have a single promotor region – so RNA polymerase transcribes all structural genes on an all or none basis

Produces a single polycistronic mRNA with coding sequences for all enzymes

* In eukaryotes, responsible for “unpacking” the euchromatin

Less condensed chromatin (DNA and histones) which are uncoiled and transcribed

* Does not transcribe heterochromatin Very condensed

chromatin that remains condensed and is not transcribed

Operator: DNA segment between operon’s promoter region and structural genes – controls access of RNA polymerase to structural genes

Repressors: specific protein that binds to an operator and blocks transcription of an operonBinds reversiblyreversibly to DNA

Regulatory genes: genes that code for repressor or regulators of other genes

* Metabolism can cue a repressor.

Tryptophan: if absent• repressor protein is in inactive conformation• Trp operon is turned on

Tryptophan: if present• repressor is in active conformation• binds to operator• Trp operon is switched off

Repressible enzymes: enzymes which have their synthesis inhibited by a metaboliteNegative Feedback:• genes are switched on until a specific metabolite activates the repressor• function in anabolic pathways• end product switches off its own production by repressing enzyme synthesis

Inducible enzymes: enzymes which have their synthesis stimulated (induced) by metabolitesNegative Feedback:• genes are switched off until a specific metabolite inactivates the repressor• function in catabolic pathways• enzyme synthesis is switched on by the nutrient pathway it uses

Lac Operon: inducible enzyme affectedLACTOSE

GLUCOSE + GALACTOSE

ß-galactosidase

Allolactose

Inactivated repressor loses affinity for lac operon

Operon is transcribed

Enzymes for lactose metabolism are produced.

Recombinant DNA:* Uses restriction enzymes that recognize short, specific nucleotide sequences and cut DNA into small segments

* Sticky ends are used to join DNA pieces of different sources.

* Unions are made permanent by joining with DNA ligase.

Gel Electrophoresis: used to separate restriction fragments on the basis of size

Using Plasmids:1. Treat bacterial DNA w/ a restriction enzyme; treat plasmid w/ the same restriction enzyme

DNA and plasmid gets cut into thousands of tiny fragments with sticky ends.

2. Mix DNA w/ plasmids.Sticky ends and plasmids join.

3. Add DNA ligaseCatalyzes formation of covalent bonds.

4. Allow bacteria to reproduce.

Sequencing: uses restriction enzymes to cut DNA into small sequencesEach fragment is treated w/ a

radioactive primer – helps identify gene sequence

PCR (polymerase chain reaction): using special primers and DNA polymerase molecules, billions of copies of DNA can be produced very quickly

RFLP (restriction fragment length polymorphisms) Analysis:Looking at differences in

restriction fragment length occurring in homo-logous DNA sequences

* Show differences in different individuals.

Uses of DNA Technology:

A. ResearchB. Human Genome ProjectC. MedicineD. ForensicsE. Agricultural

ETHICS:• WHO?• WHY?• HOW?• WHAT FOR?