Genetics of Viruses and Bacteria

Viral structure  Virus: “ poison ” (Latin); infectious particles consisting of a nucleic acid in a protein coat (there are MANY, MANY types of viruses)  Composition of virus  Capsid: protein shell that encloses the viral genome (the protein subunits are called capsomeres)  DNA or RNA that is inserted into infected cells

Examples of viruses

Virus structure (cont.)  Other accessories for viruses/virus types:  Membranous envelope that allows a virus to “fool” a cell membrane and allow the virus to enter the cell ( viral envelope )  Bacteriophage (phage): viruses that are able to infect bacteria

General features of viral reproduction  Viruses are intracellular parasites  They need a host cell to reproduce  They lack enzymes, ribosomes and all other machinery needed to make proteins  Viruses can only infect a limited range of cells (host range)  This is why diseases are usually species or tissue specific

Lytic Cycle  The lytic cycle is a viral reproductive strategy that results in the death of the host cell  Attachment: virus binds to a specific receptor site on the outer membrane  Injection: the viral DNA/RNA is inserted into the cell membrane  Synthesis: the viral DNA directs the production of viral proteins and the synthesis of viral nucleotides  Assembly: the synthesized viral material is assembled  Release: the viral particles are released from the organism, thereby destroying the host cell  Virulent virus (phage reproduction only by the lytic cycle)

Lytic cycle

Lysogenic Cycle  Genome replicated w/o destroying the host cell  Very similar to the lytic cycle  Key differences:  Genetic material of virus becomes incorporated into the host cell DNA by recombination (uses crossing-over)at a specific chromosomal loci  The incorporated viral DNA is known as a prophage  Once the prophage synthesizes its material, it circulates in the cell  Temperate virus (phages capable of using the lytic and lysogenic cycles)  May give rise to lytic cycle

Lysogenic cycle

Animal Viruses  Viruses that infect animals are extremely varied  They can be double stranded or single stranded  They can be made of DNA or RNA  They can have an outer membrane (viral envelope) or not  PURPOSE: The reason for the extreme variability in viral composition is to enter cells and utilize their reproductive machinery

Retroviruses (class of RNA Viruses  Retroviruses : a class of RNA virus that can use an RNA template to transcribe its nucleotides into the DNA template  Uses an enzyme called reverse transcriptase  One deadly example of a retrovirus is HIV  This is the virus that leads to the disease known as AIDS

Retrovirus (HIV)

HIV (cont.)  Unlike a prophage in bacteria, the integrated viral DNA ( provirus ) is a permanent part of the cells genotype  The cell will continue to synthesize the virus for the life of the cell

How do we fight viruses?  Viruses are extremely damaging  They utilize our own cellular machinery to produce, infect and destroy our own cells  With the creation of vaccines (harmless variants of pathogenic microbes), we can condition our body to destroy the infection before it can result in illness

Why do we still have viruses?  With the advent of vaccination, a lot of diseases have become extinct (polio or small pox)  Yet, viruses have a high level of mutation  They are constantly changing to “fool” your bodies immune system  Even the influenza virus (flu) mutates every year so that you must get a new flu vaccine each season  Also we do not understand enough about some viruses to create a vaccine

Viroids and prions  Viroids: tiny, naked circular RNA that infect plants; do not code for proteins, but use cellular enzymes to reproduce; stunt plant growth  Prions: “infectious proteins”; “mad cow disease”; trigger chain reaction conversions; a transmissible protein

Bacterial genetics  Nucleoid: region in bacterium densely packed with DNA (no membrane)  Plasmids: small circles of DNA (separate from bacterial genome)  Reproduction: binary fission (asexual )

Bacterial DNA-transfer processes  Transformation: genotype alteration by the uptake of naked, foreign DNA from the environment  Transduction: phages that carry bacterial genes from 1 host cell to another  Generalized: random transfer of host cell chromosome  Specialized: incorporation of prophage DNA into host chromosome  Conjugation: direct transfer of genetic material; cytoplasmic bridges; pili; sexual

Bacterial Plasmids  Small, circular, self-replicating DNA separate from the bacterial chromosome  F (fertility) Plasmid: codes for the production of sex pili (F+ or F-)  R (resistance) Plasmid: codes for antibiotic drug resistance

Transposable elements  Transposable elements : nucleotide sequences that can move from one site in a chromosome or plasmid to another site  Insertion sequence : (only in bacteria) can move one gene from one site to another  Transposons: transposable genetic element; piece of DNA that can move from location to another in a cell’s genome (chromosome to plasmid, plasmid to plasmid, etc.); “jumping genes”  This allows genetic information to be incorporated or passed on to other bacteria

Incorporation of a plasmid

Operons (the basic idea)  For many proteins, there is a segment of DNA where all of the necessary genes are grouped together  Therefore, you only need a single promoter site where RNA polymerase can begin to transcribe the DNA code  Near the promoter site is a stretch of DNA that controls whether RNA polymerase can bind. This is called the operator  The promoter site, the operator and the stretch of DNA that codes for the protein(s) is called the operon

Operons (the trp operon)  An example of an operon is the tryptophan (trp) operon in E. coli that produces the amino acid, trp  The way it works  Trp operon is usually ‘on’... RNA polymerase has access to the promoter  To stop the production of trp, the operon has to be turned ‘off’  A protein called the trp repressor binds to the operator and blocks the attachment of RNA polymerase  This repressor protein is specific to the trp operator site and stops transcription  The trp repressor is the product of another regulatory gene with its own operon  When trp is absent, the repressor is inactive and the production of trp proceeds normally  When trp is present in higher concentrations, it acts as a corepressor  It binds with the repressor protein and “activates” it so that it can bind to the operator and turn off transcription

Repressible operon  The trp operon is called a repressible operon  This means that the trp operon is usually in the “on” condition... it can transcribe the DNA normally  Transcription can only be inhibited when trp binds with the repressor protein  This allows the repressor protein to bind to the operator and prevent transcription

Inducible operon  In an inducible operon, the operon is usually “off”  It is not possible to transcribe the DNA  There must be some sort of signal (molecule) that can turn the operon on  An example of an inducible operon is the lactose (lac) operon

Operons (the lac operon)  In E. coli, the enzyme beta-galactosidase is needed to break lactose into glucose and galactose  Normally, E. coli does not have a large amount of this enzyme present  The operon to create beta-galactosidase is normally in the “off” position  A regulatory gene, lacI, creates a repressor protein that is normally bound to the operator of the lac operon  When lactose is present, it will bind to the repressor protein and inactivate it  Since this molecule is needed to start DNA transcription, it is called an inducer