viral and bacterial genomes. review of viruses are viruses alive? contain genetic material (dna or...
TRANSCRIPT
Are Viruses Alive?
• Contain genetic material (DNA or RNA)
• Cannot live outside of a cellular host
• Do not contain ribosomes• Do not carry out any type of
respiration• Are not made of cells
Virus-Host Specificity• Viruses have different shapes that
are specific to the attachment sites of their host cells
• Most viruses are species & cell specific
• Examples, adenovirus—respiratory cells; polio—nerve cells; HIV—T-cells
• Some viruses (such as rabies) can cross species
Viral Reproduction—Lytic vs. Lysogenic Cycles
• Lytic cycle– Attachment– Entry– Destruction of host cell DNA– Production of viral proteins & viral
DNA– Assembly of viruses– Lysis of Cell and Viral release
Lysogenic Cycle
• Attachment• Entry• Integration of Viral DNA in host
chromosome• Reproduction of host cell and viral DNA• “Activation” of viral DNA• Destruction of host cell DNA• Production of viral proteins & viral DNA• Assembly of viruses• Lysis of Cell and Viral release
Two Types of Viruses
• DNA Viruses• RNA Viruses
– RNA serves as mRNA– RNA serves as template for mRNA– RNA serves as template for DNA– These viruses are called
RETROVIRSES– HIV is an example of a RETROVIRUS
“Emerging Viruses”
• Viruses that cause NEW diseases• HIV• Ebola• Hantavirus• Nipah virus• Influenza Virus
Viroids
• Pieces of circular RNA that infect plants
• They reproduce and disrupt the metabolism of plant cells and stunt the growth of the entire plant
Prions
• An infectious protein molecule• Cause a number of degenerative
brain diseases including “mad cow disease”
DNA of Bacteria
• One circular chromosome• The dense region of the DNA is
called the nucleoid—not membrane bound
• Also, may have one or more plasmids
Bacterial Reproduction
• Bacteria reproduce by binary fission
• This asexual reproduction results in a colony of “clones”
• However, because of their fast proliferation (every 20 minutes for E. coli), rare mutations still result in significant bacterial diversity
Transformation
• Remember Fred Griffith!• Alteration of the bacterial cell’s
genotype by the uptake of naked, foreign DNA from the environment
Conjugation
• Direct transfer of genetic material between two bacterial cells that are temporarily joined.
• The bacterial version of sex
Plasmids
• Small, circular, self-replicating DNA molecules separate from the bacterial chromosome.
• Has only a small number of genes• These genes not required for the
survival & reproduction of the bacterium
• However, they can give bacteria an advantage for survival
Most Famous Plasmids
• F plasmid—25 genes, most required for the sex pili
• R plasmids—contain genes conferring antibiotic resistance
Transposons
• A piece of DNA that can move from one location to another in a cell’s genome.
• Sometimes called “jumping genes”
• The process “scatters” genes throughout the genome
Metabolic Control of Gene Expression in Bacteria
1. Cells can vary the numbers of specific enzyme molecules made, i.e. they can regulate the expression of genes
2. Cells can adjust the activity of enzymes already present
Operons• E. coli synthesizes tryptophan in five steps,
each step catalyzed by a specific enzyme• The genes for the five enzymes are
clustered together on the bacterial chromosome
• A single promoter serves all five genes• Transcription gives rise to one mRNA
molecule that represents all five genes of the pathway with start and stop codons throughout
Operons
• A single “on-off” switch controls the whole cluster of related genes
• The switch is called the OPERATOR
• It is positioned within the promoter and controls access of RNA polymerase to the genes
• The operator plus promoter plus genes is called an OPERON
Control of Operons
• The Operator is normally “open” meaning the series of genes can be transcribed
• It can be turned off with a REPRESSOR—a protein that binds to the operator and blocks attachment of RNA polymerase
• This regulatory protein’s gene is located away from the operon
Control of Operons• The regulatory protein is transcribed
continuously and there are always some of these in the cell
• However, they are allosteric proteins• In their “normal” shape, they will not bind
to the operator• However, when the end product of the
operon pathway (i.e. tryptophan) attaches to the allosteric site, the shape changes and it will bind to the operator and switch it off