Microbial Models

Chapter 18

The Genetics of Viruses

• Bacteria and viruses often used - reproduce quickly, have unique features.

• Bacteria - prokaryotic organisms. Cells much smaller + simply organized.

http://aquat1.ifas.ufl.edu/guide/bacecoli.jpg

• Viruses - smaller and simpler.• Most little more than clumps of

nucleic acids and protein genes in protein coat.

• Viral genomes - double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA, depending on type of virus.

http://www.tulane.edu/~dmsander/WWW/335/Papova1.gif

• Capsid - protein shell enclosing viral genome.

• Built of large # of capsomeres (subunits).

• Some viruses have viral envelopes - membranes that enclose capsids.

• Make membrane from membrane of host cell.

• Have viral proteins and glycoproteins.

• Most complex capsids found in viruses that infect bacteria (bacteriophages or phages).

• Viruses can reproduce only within host cell.

• Isolated virus unable to reproduce except in host.

http://www.mansfield.ohio-state.edu/~sabedon/images/the_bacteriophages_t4_virion.gif

• Viruses identify host cells by “lock-and-key” fit between proteins on outside of virus and specific receptor molecules on host’s surface.

• Viral infection begins when genome of virus enters host cell.

• Once inside, viral genome takes over host, reprogramming cell to copy viral nucleic acid and manufacture proteins from viral genome.

• 2 different cycles phage can go through.

• 1Lytic cycle - phage reproductive cycle ends in death of host.

• Virulent phages reproduce by lytic cycle.

• Phage breaks open cell to infect other cells.

• Viral genes turn host cell into virus-producing factory - cell soon lyses and releases viral products.

• 2Lysogenic cycle - phage genome replicates without destroying host cell.

• Gets into host’s DNA and copies pass on viral DNA.

• Sometimes viral genome exits bacterial chromosome and initiates lytic cycle.

• Switch from lysogenic to lytic may be initiated by environmental trigger.

http://genetics.hannam.ac.kr/lecture/Mgen04/images/phage-lysogen.gif

• Viruses with outer envelope use envelope to enter host cell.

• Fuses with host’s membrane, transporting capsid and viral genome inside.

• Enveloped viruses do not necessarily kill host cell.

• Some viruses have proviruses.• Provirus remains dormant within

nucleus until triggered by physical or emotional stress to leave genome and initiate active viral production.

http://basic.shsmu.edu.cn/passw/micro2/jxnr/movies/hiv4_provirus.gif

• Retroviruses have complicated life cycles.

• Carry an enzyme, reverse transcriptase - transcribes DNA from RNA template.

http://hsc.unm.edu/som/micro/images/retrovirus.jpg

• Human immunodeficiency virus (HIV - causes AIDS - acquired immunodeficiency syndrome) is a retrovirus.

• HIV enters host cell, reverse transcriptase synthesizes double stranded DNA from viral RNA.

• Transcription produces more copies of viral RNA - translated into viral proteins - self-assemble into virus particle and leave host.

• Sometimes damage from virus is irreversible (polio)

• Symptoms associated with viral infection result from body’s own efforts at defending itself.

• Modern medicine developed vaccines - harmless strains of virus that stimulate immune system.

http://www.vaccineinformation.org/photos/poliiac002.jpg

• Vaccines can fight viruses before infection, but not during.

• Antibiotics can fight bacteria, but not viruses.

• Some viral diseases (like AIDS) now have drugs to combat them.

http://news.bbc.co.uk/media/images/38246000/jpg/_38246409_hiv_spl_300.jpg

• In recent years, several very dangerous “emergent viruses” have risen to prominence.

• Ebola is one of them.

• Viruses can mutate - new strains are always evolving.

• Stronger viruses can develop.

http://www.igm.hokudai.ac.jp/vec/image/engpackaing.jpg

Plant viruses

• Plant viruses can stunt plant growth, diminish crop yields.

• Can be inherited from parent plant or caught from other plants.

• Plant cells connected by plasmodesmata so virus can spread quickly.

• Viroids, smaller and simpler than even viruses - tiny molecules of circular RNA that infect plants.

• Prions - infectious proteins that spread disease (affect brain mostly).

• Transposons (jumping genes) - DNA segments that can move from 1 location to another within cell’s genome.

http://www.anselm.edu/homepage/jpitocch/genbio/transposons.JPG

The Genetics of Bacteria

• Bacterial genome - double-stranded, circular DNA molecule.

• Tight coiling of DNA results in dense region of DNA (nucleoid) not bound by membrane.

• Many bacteria have plasmids - smaller circles of DNA.

http://www.cellsalive.com/cells/cellpix/bactcell.jpg

• Bacterial cells divide by binary fission.

• Most of bacteria in colony genetically identical to parent cell.

• Mutations - only way that bacterial DNA changes.

• Genetics recombination also causes diversity within bacterial populations.

• Recombination - combining of DNA from 2 individuals into 1 genome.

• Transformation - alteration of bacterial cell’s genotype by uptake of foreign DNA from environment.

http://faculty.ircc.edu/faculty/tfischer/images/transformation.jpg

• Conjugation transfers genetic material between 2 bacterial cells that are temporarily joined.

• One cell (“male”) donates DNA; “mate” (“female”) receives genes.

• Male determined by presence of F factor.

• F factor or its F plasmid consists of about 25 genes, most required for production of sex pili.

• F+ and F- cell meet, F+ cell passes copy of F plasmid to F- cell, converting it.

• Transposons can bring multiple copies for antibiotic resistance into plasmid by moving genes from different plasmids.

http://www.paratekpharm.com/graphics/infect/graph_spread.gif

Gene expression in bacteria

• Operon - 3 elements: genes that it controls, promotor region where RNA polymerase first binds, and operator region between promotor and 1st gene that acts as “on-off switch”.

• Operon usually on so RNA polymerase can bind to promotor and transcribe genes.

• Prevent transcription - repressor protein binds to operator (process is reversible)