Viruses
Very, very small -- 20 nm in diameter
Consist of nucleic acid enclosed by a protein coat
Viral Genomes: Double stranded DNA, Single stranded DNA, Double stranded RNA, or Single
stranded RNA
Organized as single nucleic acid molecule, linear or circular
4 to 100s of genes
Capsid = Protein coat that encloses the viral genome
Capsomeres = protein subunits made from only one or a few types of protein
Envelope = Membrane that cloaks some viral capsids
Help viruses infect their host
Derived from host cell membrane, usually modified
Viruses are obligate intracellular parasites, meaning that they can only reproduce within a host
cell
Host Range = Limited number or range of host cells that a parasite can infect
Viral replication: 1. Infect host with viral genome. 2.Use host cell resources to replicate viral
genome, manufacture capsid protein, and assemble new viruses
Three patterns of viral genome replication
DNA -> DNA
(eg. Pox and PapovaViruses)
Use DNA polymerase produced by host
RNA ->RNA
(eg. Rabies and influenza)
Use RNA replicase (a viral gene product)
RNA->DNA->RNA (eg. HIV, HTLV-1)
Use reverse transcriptase (a viral gene product)
Viral nucleic acid and capsid proteins assemble spontaneously into new virus particle= selfassembly
Bacteriophages or Phages = Viruses that infect bacteria
Two reproductive cycles: 1. Lytic cycle = A viral replication cycle that results in death or lysis of
the host cell
Virulent phages = phages that lyse their host cells
Bacterial defenses
Change receptor sites used for virus recognition
Restriction enzymes cut up foreign DNA
2. Lysogenic Cycle = A viral replication cycle that involves the incorporation of viral genome
into host cell genome.
Temperate viruses = Viruses that can integrate their genome into host chromosome and
remain latent until they initiate a lytic cycle (eg. Lambda Phage)
Prophage = A phage genome that is incorporated into a specific site on the bacterial
"chromosome”
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Most prophage genes are inactive. However, one active gene codes for a repressor
protein which switches off most other prophage genes
Lysogenic cell = Host cell carrying a prophage in its "chromosome"
Some prophage genes may change host’s phenotype, a process called lysogenic
conversion
diphtheria, botulism, scarlet fever -- results from toxins coded for by prophage genes
Animal viruses:
Some have envelopes
1. Attachment: glycoproteins. 2. Entry: receptor mediated endocytosis. 3. Uncoating: Viral
RNA and Protein Synthesis. 4. Assembly and Release
Provirus =Viral DNA that inserts into a host chromosome
RNA Viruses
+ and - strands
+ = the strand that corresponds to mRNA
- = strand that is a template for synthesis of a plus strand
RNA viruses are classified by type of nucleic acid (1-6)
Retrovirus = RNA virus that uses reverse transcriptase to transcribe DNA from the viral RNA.
(HIV, HTLV-1)
Reverse transcriptase = type of DNA polymerase that transcribes DNA from RNA
template
Viral Disease
Vaccines = harmless variants or derivatives of viruses that activate hosts immune
mechanisms
antiviral drugs = interfere with viral nucleic acid synthesis
Cancer
Some viruses can transform infected cells to a cancerous state by permanently inserting
viral nucleic acids
into host DNA. (Epstein-Barr Virus, HTLV-1, RSV (chickens)
Oncogenes = genes found in viruses or as part of normal eukaryotic genome, that trigger
transformation
of a cell to cancer
Often code for cellular growth factors
Carcinogens probably also act by turning on oncogenes
Plant viruses
Horizontal transmission = receives virus from external source (pruning shears,
insects…)
Vertical transmission = inherited from parents (Gametic cells, or through plant
clippings)
Viroids = plant pathogen that is smaller and simpler than viruses
(naked RNA molecules)
Prions = pathogenic proteins
may be defective versions of a normal protein
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catalyze conversion of normal protein to prion. (Cause CJD, Kuru, Mad Cow
Disease, Scrapie)
Bacterial DNA
Bacterial chromosome (genophore) is usually circular
Some bacterial contain plasmids = extrachromosomal genes consisting of a doublestranded ring of DNA
Binary fission = asexual bacterial reproduction that produces clones- genetically
identical offspring
Genetic recombination
Transformation =process of gene transfer during which a bacterial cell assimilates
foreign DNA from the surroundings
e.g. Insulin
Transduction= gene transfer from one bacterium to another by a bacteriophage
Generalized vs Specialized
Conjugation and Plasmids
Conjugation = The direct transfer of genes between two cell that are temporarily joined
Plasmid = small double stranded ring of extrachromosomal DNA -- only a few genes, replicate
independently
Episomes = plasmids that can reversibly incorporate into cell’s chromosome
Resistance plasmids = carry genes that confer resistance to certain
antibiotics
Transposons = DNA sequences that can move from one chromosomal site to another
Conservative transposition = movement of preexisting genes from one genomic location
to another - genes are not replicated before the move (number of gene copies is conserved)
Replicative transposition = movement of gene copies from original site to another
location in the genome - transposon’s genes are inserted in new site w/o being lost from original
site
Insertion sequence = simplest transposons which contain only the genes necessary for the
process of transposition
sequence coding for transposase, the enzyme that catalyzes insertion of
transposons
inverted repeats = short noncoding sequences of DNA that are repeated in
reverse order on opposite ends of a transposon
e.g
ATCCGGT.............ACCGGAT strand 1
TAGGCCA............TGGCCTA strand 2
DNA polymerase helps form direct repeats
Complex transposons = transposons which include additional genetic material besides that
required form transposition
Control of gene expression
Regulation of enzyme activity
Feedback inhibition
Regulation of gene expression
gene repression
Structural gene = gene that codes for a polypeptide
Operon = regulated cluster of adjacent structural genes with related functions
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Has a single promoter so RNA polymerase transcribes the whole cluster
polycistronic mRNA = large mRNA molecule that is a transcript of several genes
Operon can be controlled by a single Operator = DNA segment between an operon’s
promoter and structural genes, which controls access of RNA polymerase to structural genes
Repressor = specific protein that binds to an operator and blocks transcription of the operon -blocks attachment of RNA polymerase to the promoter
Regulatory genes = genes that code for repressor or regulators of other genes
Corepressor = a molecule, usually a metabolite, that binds to a repressor protein
Repressible enzymes = enzymes which have their synthesis inhibited by a metabolite
Inducible enzymes = enzyme synthesis stimulated by metabolites
Positive control
catabolite activator protein (CAP)
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