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.

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.

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.

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. 1 Lytic 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. 2 Lysogenic 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.

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.

Retroviruses have complicated life cycles. Carry an enzyme, reverse transcriptase - transcribes DNA from RNA template.

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.

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.

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.

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.

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.

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.

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.

Gene expression in bacteria Operon - 3 elements: genes that it controls, promotor region where RNA polymerase first binds, and operator region between promotor and 1 st 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)