By: Stefanie Vorleiter

The Genetics of Viruses and Bacteria

A T4 bacteriophage is infecting the bacterium Escherichia Coli (E. coli), in this SEM.

• Molecular biology was started by the study of viruses and bacteria.

• E. Coli and other microbes are model systems.

• Provides great support that genes are made of DNA.

Model Systems mechanisms:• Useful for understanding how bacteria and viruses cause

disease. • Another use is to manipulate genes and transfer them to

other organisms. • Remember bacteria are prokaryotes, their cells are much

smaller and simply organized. • Viruses are even smaller and more simple, they are no more

than genes packages in protein coats.

A virus has a genome but can reproduce only within a host cell.

Viruses were detected before they were able to be seen.

19th century

Tobacco mosaic disease stops or shortens the growth of tobacco and gives their leaves a discoloration.

Adolf Mayer, discovered that one could transmit the disease from plant to plant by rubbing sap onto the healthy plants.

Mayer concluded that the disease was caused by small bacteria that you cannot see with a microscope.

Discovery of Viruses: Scientific Inquiry

Extracted sapfrom tobaccoplant withtobaccomosaic disease

Passed sapthrough aporcelain filter knownto trapbacteria

Rubbed filteredsap on healthytobacco plants

Healthy plantsbecame infected

This hypothesis was proved a decade later when Dimitri Ivanowsky, passed sap from the infected tobacco plant through a filter, designed to remove bacteria.

After filtering the sap still created the mosaic disease.

Ivanowsky hypothesized that bacteria cause tobacco mosaic disease. It might be that the bacteria was too small and so they filtered through.

This was proven wrong when Martinus Beijerinck, a Dutch botanist, discovered that the infectious agent of the sap could reproduce.

He rubbed plants with the filtered sap, and from these he infected plants, continuing this process through a series of infections.

Discovery of Viruses

The bacteria must have been reproducing because the disease was undiluted after several transfers.

The pathogen had only reproduced within the host it infected.

Unlike bacteria, this disease could not be cultivated in test tubes or Petri dishes.

Beijerinck believed the mosaic disease was much smaller and simpler than bacteria.

This was confirmed by Wendell Stanley, who crystallized the infectious particles, known as tobacco mosaic virus (TMV).

Discovery of Viruses

The tiniest are only 20nm in diameter, where millions could fit on a pinhead.

The largest are barely visible in light microscopes.

The crystallization of a virus by Stanley was exciting news.

The simplest cells can’t even aggregate into crystals.

-If viruses aren’t cells, what are they?

>Viruses are infectious particles consisting of nucleic acid enclosed in a protein coat.

Structure of Viruses

Viruses defy DNA’s double helix structure, they contain double stranded DNA, single stranded DNA, double stranded RNA, and singe stranded RNA depending on their nucleic acids.

The smallest virus has only 4 genes, while the largest has several hundreds.

Depending on the size and shape of the genomes capsid, will be how many genes it contains.

Structure of Viruses: Viral Genome

Protein shells enclosing the viral genome is the capsid.

Depending on the type they are: capsids can be rod-shaped, polyhedral, or more complex (T4).

Capsids are made from protein subunits, capsomeres.

The most complex Capsids are those viruses that infect bacteria, called bacteriophages.—E. Coli

Viral Envelopes are derived from the membrane of the host cell and made up of proteins and glycoproteins.

Structures of Viruses: Capsids and Envelopes

Viruses reproduce only within a host cell.

An isolated virus cannot do anything but infect its appropriate host cell.

Viruses lack metabolic enzymes, ribosome's, etc..

Each virus can only infect a limited amount of host cells called its host range.

The host range is determined by the proteins on its surface and that of the host.

General Features of Viral Reproductive Cycles

“lock and key” system is detected between proteins on the outside of the virus and receptor molecules on the cells.

-The West Nile virus can infest mosquitoes, birds, and humans.

-The equine encephalitis virus can infect mosquitoes, birds, horses, and humans.

-The measles and poliovirus only infect humans.

-Human cold virus infects only cells lining the upper respiratory tract.

-Aids virus bind to specific receptors on certain white blood cells.

(these two are limited to particular tissues.)

Features of Reproductive Cycle

Simplified viral reproductive cycle.

Mechanisms of reproductive cycle depends on the type of virus and cell.

For example: The T-even (T2,T4,T6), phages use their tail apparatus to inject DNA into a bacterium.

Inside, the viral genome can command the host, reprogramming it to copy the viral nucleic acid and create viral proteins.

Assembly into new viruses is a spontaneous process of self-assembly.

The simplest type of reproductive cycle ends with the exit of hundreds of thousands of viruses from the host cell, which usually damages of kills the host cell.

This along with the body’s response to destruction, is what causes the symptoms associated with viral infections.

The viral prodigies that exit the cell, have the ability to infect other cells, spreading the virus.

Two reproductive cycles..

A phage reproduction which causes the death of the host cell is the lytic cycle.

Lytic refers to the last stage of infection, in which the bacterium lyses (breaks open) and releases a large number of phages.

Each phage can then infect a cell and a few lytic cycles can destroy an entire bacterial population in just a few hours.

A phage that can only reproduce through the lytic cycle is called a virulent phage.

Reproductive Cycle of Phages

Attachment

Attachment

Entry of phageDNA anddegradation ofhost DNA

Entry of phageDNA anddegradation ofhost DNA

Synthesis of viralgenomes andproteins

Entry of phageDNA anddegradation ofhost DNA

Synthesis of viralgenomes andproteins

Assembly

Phage assembly

Entry of phageDNA anddegradation ofhost DNA

Synthesis of viralgenomes andproteins

Assembly

Release

Phage assembly

-phages have been used medically

Bacteria isn’t defenseless.

Natural selection favors bacterial mutants with receptor sites that aren’t recognized by a phage anymore.

When phage DNA successfully enters a bacterium, the DNA is often recognized as foreign and cut up by restriction enzymes. The bacteria's cells DNA is altered so that is prevents attacks by the restriction enzymes. Why haven ‘t phages gotten rid of all bacteria?

A virus that replicates the phage without destroying the host undergoes the lysogenic cycle.

Phages capable of using both modes of reproducing within a bacterium are called temperate phages.

Infection of an E. coli cell by λ begins when the phage binds to the surface of the cell and injects its DNA.

Within the host, the λ DNA molecule forms a circle.

Next, depending whether its lytic or lysogenic, in lytic, the viral genes immediately turn the host cell into λ producing factory, and the cell lyses and releases its viral products.

During a lysogenic cycle, the λ DNA molecule is incorporated by genetic recombination( crossing over) into a specific site on the host cells chromosome.

If this integrates, the viral DNA is a prophage, of which one gene codes for a protein that prevents transcription of other prophage genes.

Lysogenic Cycle

The phage injects its DNA.

Daughter cellwith prophage

Cell divisionsproducepopulation ofbacteria infectedwith the prophage.

Occasionally, a prophageexits the bacterialchromosome,initiating a lytic cycle.

The bacterium reproduces,copying the prophage andtransmitting it to daughter cells.

Phage DNA integrates intothe bacterial chromosome,becoming a prophage.

Lysogenic cycle

New phage DNA and proteinsare synthesized andassembled into phages.

Lytic cycleis inducedor Lysogenic cycle

is entered Prophage

Phage DNAcircularizes.

Bacterialchromosome

The cell lyses, releasing phages.

Lytic cycle

Phage

PhageDNA

Every time the E. coli cells divide, it replicates the phage DNA along with its own and passes the copies onto the daughter cells.

A single cell can quickly produce a large population of bacteria carrying the virus which enables viruses to propagate without killing the host cell.

Lysogenic implies that prophage are capable of giving rise to active phages that lyse their host cells.

This happens when the λ genome exits the bacterial chromosome.

Expression of the genes may alter the hosts phenotype, which can cause medical significance.

Lysogenic Cycle

Lytic cycle• Virulent or temperate phage• Destruction of host DNA• Production of new phages• Lysis of host cell causes release of progeny phages

Lysogenic cycle• Temperate phage only• Genome integrates into bacterial chromosome as prophage, which (1) is replicated and passed on to daughter cells and (2) can be induced to leave the chromosome and initiate a lytic cycle

The phage attaches to ahost cell and injects its DNA

ProphageBacterialchromosome

PhageDNA

Virus Lysogenic & Lytic Cycle

• Many variations on the basic scheme of viral infection and reproduction are represented among animal viruses.

• One key variable is the nature of the viral genome: It is composed of DNA or RNA? Is it double stranded or single stranded? The nature of the genome is the basis for the common classification of viruses.

• Another characteristic is the presence of absence of a membranous envelope derived from the host cell membrane.

-Nearly all animal viruses with RNA genomes have an envelope, as do some with DNA genomes.

Reproductive Cycle of Animal Viruses

An animal is equipped with an outer membrane, or a viral envelope, which it uses to enter the host cell.

Protruding from the outer surface of this envelope are viral glycoproteins that bind to specific receptor molecules on the surface of a host cell.

This reproductive cycle does not necessary kill the host, unlike the lytic cycle.

Viral Envelopes

HOST CELL

Copy ofgenome (RNA)

Capsidproteins

Viral genome (RNA)

Template

mRNA

Glyco-proteins

ER

Capsid and viral genomeenter the cellCapsid

RNA

Envelope (withglycoproteins)

New virus

Some viruses have envelopes that are not derived from the plasma membrane.

The envelopes of herpes viruses are derived from the nuclear membrane of the host. These viruses have a double stranded DNA genome and reproduce within the host cell nucleus.

The infection of other cells by these new viruses causes the blisters characteristic of herpes, such as cold sores or genital sores.

Viral Envelopes

The broadest variety of viruses are those that infect animals.

-

Like all viruses that require RNA-DNA synthesis to make mRNA, this next virus uses a viral enzyme that is packaged with the genome inside the capsid.

RNA as Viral Genetic Material

Glycoprotein

Reversetranscriptase

Viral envelope

HOST CELL

RNA (twoidenticalstrands)

Reversetranscriptase

RNA genomefor thenext viralgeneration

Capsid

Viral RNA

RNA-DNAhybrid

DNA

ChromosomalDNA

NUCLEUS

Provirus

New virus

mRNA

The RNA animal viruses with the most complicated reproductive cycles are the retroviruses.

These viruses contain the enzyme, reverse transcriptase, which transcribes a single stranded RNA template into DNA.

HIV contains two identical molecules of single stranded RNA and two molecules of reverse transcriptase.

After HIV enters the host cell, its reverse transcriptase molecules are released into the cytoplasm and catalyze synthesize viral DNA.

The new DNA enters the cells nucleus and integrates which is called a provirus, which then never leaves the genome.

The RNA polymerase transcribes the DNA into RNA molecules which can function as mRNA and as genomes.

Viruses don’t fit our description of living organisms

A virus is biologically inert, it cannot replicate its genes, or regenerate its own supply of ATP.

Even though viruses are obligate intracellular parasites that cant reproduce, their use of the genetic code makes it hard to deny them evolutionary connection to the living world.

How did viruses originate?• Because they depend on cells for their own propagation, they evolved

after cells appeared. • Many favor the hypothesis that viruses originate from cellular nucleic

acids that could move from one cell to another.• A viral genome usually has more in common with the genome of its

host, while other viral genomes have more in common with their seemingly distant related viruses.

Evolution is still a great topic of debate.

Evolution of Viruses

Viral Diseases in Animals

Viruses can damage or kill cells by the release of hydrolytic enzymes from lysosomes.

Some cause infected cells to produce toxins that lead to disease symptoms.

People recover from cold because the epithelium of the respiratory tract, which the virus infects, can repair itself.

The immune system is the basis for the major medical tool for preventing viral infections--vaccines

Viruses, viroids, and prions are formidable pathogens in animal and plants

Vaccines are harmless variants or derivatives of pathogenic microbes that stimulate the immune system to mound defenses against the actual pathogen.

Vaccination has eradicated smallpox, which was once a devastating scourge in many parts of the world.

Similar worldwide vaccination campaign are currently going on to eradicate other viruses.

Even vaccines can’t prevent certain illnesses, once they occur, medical technology can do little.

The antibiotics are powerless against viruses.Antibiotics kill bacteria by inhibiting enzymes, yet

viruses contain little or no enzymes.

Still, a few drugs combat viruses. Antiviral drugs resemble nucleosides and therefore interfere with viral nucleic acid synthesis.• Acyclovir, impedes herpes virus reproduction by inhibiting

the viral polymerase that synthesizes DNA.• AZT curbs HIV by interfering with the synthesis of DNA

reverse transcriptase.• “cocktails” are now effective, a combination of two

nucleoside mimics and a protease inhibitor.

Emerging Viruses are those that appear suddenly or come to the attention of medical scientists.

Examples are: HIV, the AIDS virus; appeared in San Francisco in the early 1980’s out of nowhere.

The Ebola virus, recognized first in 1976 in Central Africa, which caused hemorrhagic fever, often fatal.

A number of dangerous new viruses cause encephalitis (inflammation of the brain)• West Nile virus, appeared 1999 and spread to all 48 states• A more recent is the sever acute respiratory syndrome (SARS)

which started in China in 2002. Between Nov. 2002, and July 2003, 8,000 people were known to be infected of which 700 died. The cause was the corona virus, a virus unknown to humans.

Emerging Viruses

Three processes contribute to the emergence of viral diseases:

1st, the mutation of existing viruses is a major source of these new diseases.• RNA virus has a high rate of mutation because errors in replication their RNA are not corrected.• Mutation enable existing viruses to evolve, where the body is

not immune to them anymore.

2nd, the spread of existing viruses from one host species to another.• Example—The hantavirus

Emerging Viruses

3rd, Dissemination of a viral disease from a small, isolated population can lead to widespread epidemics.• AIDS went unnamed and unnoticed for decades before it began to spread

around the world.

• Technology, travel, sexual promiscuity, etc., all allowed this disease to rapidly become a global scourge.

Emerging viruses are not new, but existing viruses that mutate, spread to new host species, or disseminate widely.

Emerging Viruses

There are more than 2,000 types of viral disease known for plants and they account for about $1.5 billion loss each year in agricultural and horticultural crop destruction.

Common symptoms of viral destruction are: bleached or brown spots on leaves and fruits, stunted growth, and damaged flowers or roots.

Plant viruses are basically the same as animal viruses. Most plant viruses discovered, including the tobacco mosaic virus, have an RNA genome.

Viral Disease in Plants

Plant viral disease spread by two major routes;

Horizontal transmission-where a plant is infected from an external source of the virus and spreads to another plant.

The plant is more susceptible to damages by wind, injury, and insects.

Vertical transmission-in which a plant inherits a viral infection from a parent.

Asexual propagation or sexual reproduction.

Once a virus enters a plant cell and begins reproducing, viral components can spread throughout the plant by passing through cytoplasmic connection.

Virus Diseases in Plants

Circular RNA molecules that infect plants, are known as viroids.• One viroid disease has killed over 10 million coconut palms

in the Philippines.

Viroids do not encode proteins but can replicate in host plant cells, using cellular enzymes. These small RNA molecules cause errors in regulatory systems that control plant growth.

Prions, which cause degenerative brain disease, include scrapie in sheep, mad cow disease, and Creutzfeldt-Jakob disease in humans.

Prions are most likely transmitted in food

Viroids and Prions: The Simplest Infectious Agents.

Two characteristics of prions are very alarming.

1st, prions are slow-acting agents; the incubation period until symptoms appear is around ten years.

2nd, prions are virtually indestructible; they are not destroyed or deactivated by heating to normal cooking temperatures. • Still now, there is no known cure for prion disease.

Prions

How can a protein which cannot replicate itself, be a transmissible pathogen?• A prion is a misfolded form of a protein normally present in brain cells. • The prion converts normal protein to prion version, triggering

chain reactions, eventually leading to an increase in numbers.

Note that all my information and data, I used from the book, Biology-seventh edition, as well as from the pictures and figures from the power points.