Transcript Molecular Genetics of Viruses

Molecular Genetics of Viruses
• Viruses are parasites of cells.
• Typical virus
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Penetrates a cell
Takes over the metabolic machinery
Assembles hundreds of new viruses
Host cell is destroyed as the virus leaves the
cell to infect other cells
Molecular Genetics of Viruses
• Specie specific and cell specific.
• Bacteriophage (phage)
– Viruses that attack only bacteria.
• Structure
– Capsid (Protein coat)
– Envelope (phospholipids and proteins)
• Assist in penetrating host cell.
– Genetic material (DNA or RNA)
Molecular Genetics of Viruses
Reproductive Cycles (2)
• Lytic Cycle
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Penetrates the membrane
Uses host enzymes to replicate viral DNA
Transcribes viral DNA into RNA
Translates RNA to protein
Assembly of DNA and protein into virus
particle
– Erupts from cell
Molecular Genetics of Viruses
Reproductive Cycles (2)
• Lysogenic cycle
– Viral DNA is temporarily incorporated into the
DNA of the host
– Dormant state: remains inactive until some
trigger, usually an external environmental
stimulus (radiation or certain chemicals)
• Provirus
• Prophage (bacteriophage)
– Activated and starts the lytic cycle
Molecular Genetics of Viruses
• RNA virus
– Viral RNA is used directly as RNA
– Retroviruses
• Use Reverse transcriptase to make a DNA
complement
– RNA  DNA  Transcription  mRNA  Protein
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– Lysogenic cycle
Molecular Genetics of Bacteria
• Reproduction- Binary fission
– Chromosome replicates and the cell divides into
two cells, each cell bearing one chromosome.
– No spindle, microtubules, or centrioles.
• No nucleus to divide.
– Plasmids
• Short, circular DNA molecules outside the
chromosome.
• Replicate independently of the chromosome
Molecular Genetics of Bacteria
Causes of Genetic Variation in the
Genome of Bacteria
• Conjugation- DNA exchange between bacteria
– Donor bacterium produces a tube or pilus that
connects to a recipient bacterium.
– Exchange of chromosomal or plasmid DNA.
– F plasmids- genes for production of pili.
– R plasmid- provides bacteria with resistance
against antibiotics
Molecular Genetics of Bacteria
Causes of Genetic Variation in the
Genome of Bacteria
• Transduction- introduction of new DNA into a
bacteria by a virus
– When a virus is assembled during a lytic cycle, it is
sometimes assembled with some bacterial DNA in
place fo some the viral DNA.
– When this aberrant virus infects another cell, the
bacterial DNA that it delivers can recombine with the
resident DNA.
Molecular Genetics of Bacteria
Causes of Genetic Variation in the
Genome of Bacteria
• Transformation- absorption of DNA from
their surroundings.
– Specialized proteins on the cell membranes of some
bacteria facilitate this kind of DNA uptake.
Regulation of Gene Expression
• Gene regulation in prokaryotes, E. coli, is
controlled by sequences of DNA called
operons.
• Four major components of an operon.
1. Regulatory gene- produces a repressor
protein that prevents gene expression by
blocking the action of RNA polymerase.
Regulation of Gene Expression
• Four major components of an operon cont’
2. Promoter region- RNA polymerase attaches
to this region to begin transcription.
3. Operator region- can block the action of the
RNA polymerase if the region is occupied by a
repressor protein.
4. Structural genes- DNA sequences that code
for several related enzymes that direct the
production of some particular end product.
Regulation of Gene Expression
• Example: lac operon- controls the breakdown of lactose
– Regulatory gene in the lac operon produces an
active repressor that binds to the operator
region.
– When the operator region is occupied by the
repressor, RNA polymerase is unable to
transcribe several structural genes that code
for enzymes that control the uptake and
subsequent breakdown of lactose.
Regulation of Gene Expression
• Example: lac operon- cont’
– When lactose is available, some of the lactose
combines with the repressor to make it
inactive.
– When the repressor is inactivated, RNA
polymerase is able to transcribe the genes that
code for the enzymes that breakdown lactose.
Regulation of Gene Expression
• Example: trp operon- produces enzymes for the
synthesis of tryptophan
– Regulatory gene produces an inactive repressor that
does not bind to the operator.
– RNA polymerase proceeds to transcribe the structural
genes necessary to produce enzymes that synthesize
tryptophan.
– When tryptophan is available from the surroundings,
the bacteria does not need to manufacture it.
– Available tryptophan reacts with the inactive repressor
and makes it active (tryptophan = corepressor)
– Active repressor now binds to the operator region,
prevents RNA polymerase from working