PRESENTED BYVina Aulia (100210103051)

Misrai Farauk (100210103057)

VIRAL GENOME

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.

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)

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.

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

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

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.

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

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

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.

The lambda phage which infects E. coli demonstrates the cycles of a temperate phage.

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

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.

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

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.

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.

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

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.

RNA Viruses

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

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

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

Company Logo

ENVELOPED NAKED

Double – stranded Double – stranded Single – stranded

Icosahedral Complex Icosahedral Icosahedral

HERPES POX PAPOVA PARVO HEPADNA ADENO

DNA VIRUSESDNA VIRUSES

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

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)

Transcriptional/translational Transcriptional/translational challengeschallengesAccess to RNA

polymeraseMonogenic

expression in eukaryotes

Temporal control of gene expression

Competition with host for ribosomes

Replication of DNA Viruses