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PRESENTED BYVina Aulia (100210103051)
Misrai Farauk (100210103057)
VIRAL GENOME
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The tiniest viruses are 20 nm in diameter. (smaller than a ribosome)
They consist of nucleic acids enclosed in a protein coat and sometimes a membranous envelop.
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Nucleic Acid Of Viruses Viral genomes can be either DNA or RNA, never
both.
This genome, once inside a host cell, directs synthesis of new viral proteins, and also replication of new viral genomes
Viral genomes come in all kinds:• Single- or double-stranded, RNA or DNA• Linear or circular• One piece or segmented (in several fragments)
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The genomes (sets of genes) maybe1. Double stranded DNA2. Single stranded DNA3. Double stranded RNA4. Single stranded RNA
(+) single stranded DNA(--) single stranded DNA
They are called either a DNA or RNA virus depending on the type of nucleotide in the make-up.
They may be linear or circularThe smallest have only 4 genes and
largest have several hundred.
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Capsid – a protein shell that covers the viral genome. They may beRod-shapedPolyhedralMore complex
Capsids are built from large numbers of protein subunits called CAPSOMERES
The most complex capsids are found in viruses that infect bacteria – BACTERIOPHAGES (T1-T7). They have a protein tail piece with tail fibers that attach to the bacterium
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ReproductionViruses are obligate
intracellular parasites that can reproduce only within a host cell.
They do not haveEnzymes for
metabolismDo not have ribosomesDo not have the
equipment to make proteins
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Each type of virus can infect and parasitize only a limited range of host cells called its HOST RANGE.Some are broad based while others are not.
Swine flu virus can infect swine or humansRabies can infect may mammals
Some can parasitize only E. coliEukaryote viruses are usually tissue
specificViruses use a “lock and key” fit to identify
hosts.
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Reproduction occurs using lytic or lysogenic cycles
The Lytic CycleCulminates in the
death of the host cellVirulent viruses
reproduce only by lytic cyle.
Natural selection favors bacterial mutations with receptor sites that are resistant to a particular phage or that have restriction enzymes to destroy the phages.
The Lysogenic CycleReplication of the
viral genome without destroying the host cell.
A temperate virus may reproduce by either cycle.
Lambda virus: resembles T4 but only has a single short tail fiber
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While phages have the potential to wipe out a bacterial colony in just hours, bacteria have defenses against phages.Natural selection favors bacterial
mutants with receptors sites that are no longer recognized by a particular type of phage.
Bacteria produce restriction nucleases that recognize and cut up foreign DNA, including certain phage DNA.Modifications to the bacteria’s own
DNA prevent its destruction by restriction nucleases.
But, natural selection favors resistant phage mutants
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In the lysogenic cycle, the phage genome replicates without destroying the host cell.
Temperate phages, like phage lambda, use both lytic and lysogenic cycles.
Within the host, the virus’ circular DNA engages in either the lytic or lysogenic cycle.
During a lytic cycle, the viral genes immediately turn the host cell into a virus-producing factory, and the cell soon lyses and releases its viral products.
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The lambda phage which infects E. coli demonstrates the cycles of a temperate phage.
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Lambda reproductionInfects an E. coli cell by injecting its DNAThe lambda DNA molecule forms a circle.Lytic or lysogenic cycles beginIn a lytic cycle, the cell is turned into a
lambda producing factory, the cell lyses and releases its products.
In a lysogenic cycle, the viral genome is incorporated into by genetic recombination into a specific site on the host cell’s chromosome.
It is now known as a prophage
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Every time the E. coli divides, it replicated the phage DNA and passes it along to the daughter cells.
This enables the phage to replicate without destroying the host.
The phages may at some point in time become active phages that lyse their host cell and releasing infectious particles.
There is usually an environment trigger.There may be other prophages released as
well and this may change the phenotype of the host. This is of medical importance. Examples: diphtheria, botulism and scarlet fever.
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Regardless of the type of virus, the parasite diverts the host cell’s resources for viral production.
The host cell provides:Nucleotides for nucleic acid production
EnzymesRibosomestRNAAmino acidsATP
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Modes of infection and replication of animal viruses
Focus on animals viruses with a viral envelopThe envelop is outside the
capsid and helps the virus enter the host cell.
Generally a lipid bilayer with glycoprotein spikes
The envelop fuses with the cell membrane
The ER of the host cell makes the membrane proteins which are transported to the membrane
New viruses exits the host in a process similar to exocytosis.
This reproductive cycle does not kill the host.
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Some viruses have envelopes that are not derived from the plasma membrane.
Herpesvirus has an envelop that is derived from the nuclear membrane.
These become integrated into the host genome as a provirus. Once these viruses are acquired they tend to reoccur through out a person’s life.
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RNA as Viral Genetic MaterialThe broadest variety of RNA
genomes is found among viruses are those that infect animals.
There are three types of single stranded RNA genomes
The genome of class IV can directly serve as mRNA and can be translated into viral protein immediately after infection
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RETROVIRUSESMost complicatedGenetic information
flows in the reverse direction
Have the enzyme reverse transcriptaseTranscribes DNA from an
RNA templateThe newly made DNA
than integrates as a provirus into the nucleus of the animal cell
The host’s RNA polymerase transcribes the virual DNA into RNA molecules.
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RNA Viruses
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Single Strand RNA Viruses• (+) strand
– Genome used directly as mRNA can be translated immediately
– Polio– Ex: Rhinoviruses
•( -) strand– mRNA must be
transcribed from genome template
– Must carry RNA dependent RNA polymerase enzyme
–Ex: Measles
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Replication of a (+) sense RNA virus1. (+) sense RNA acts like
mRNA, gets directly translated into viral proteins
2. To make new copies of the virus genome, a complementary RNA is made (-- sense) from + RNA
3. (-) sense RNA is template for synthesis of many copies of the (+) sense RNA genome that goes into the new viruses
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Replication of -- sense RNA viruses:
Inside the infecting virus capsid is the RNA genome and a transcriptase
that makes + sense RNA from the – sense genome.
The + sense RNA then:•acts like mRNA to direct protein synthesis•serves as a template for synthesis of many new
Copies of the original – sense RNA genome
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Company Logo
ENVELOPED NAKED
Double – stranded Double – stranded Single – stranded
Icosahedral Complex Icosahedral Icosahedral
HERPES POX PAPOVA PARVO HEPADNA ADENO
DNA VIRUSESDNA VIRUSES
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DNA (genome) replication DNA (genome) replication strategies similar in all and strategies similar in all and similar to host similar to host
ssDNA becomes dsDNA
5’ to 3’ synthesis; need for primer
Variety of enzymes of host or viral origin : DNA polymerase (proofreading), helicases, ss binding proteins, ligases
In nucleus except for poxviruses
Phage T4 replisome
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Replication Challenges for Replication Challenges for DNAVirusesDNAViruses Access to nucleusCompeting for
nucleotidesCell cycle control in
eucaryotes - S phase dependent materials for some
Primer removal and replacement (completing ends)
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Transcriptional/translational Transcriptional/translational challengeschallengesAccess to RNA
polymeraseMonogenic
expression in eukaryotes
Temporal control of gene expression
Competition with host for ribosomes
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Replication of DNA Viruses