Viral ReplicationViral Replication

• In order to reproduce a virus must invade or infect a living host cell.

•To infect a cell, the virus must first attach to it.

•This attachment is specific – the virus must find the type of cell that it can “fit” onto

•2 types of virus life cycles1. Lytic Viruses 2. Lysogenic Viruses

Bacteriophage: Lytic Bacteriophage: Lytic CycleCycle

• Attachment:Phage attaches by tail fibers to host cell

• Penetration:(injection) of viral DNA or RNA into host cell

• Replication/Biosynthesis: reproduction of new viral proteins and nucleic acid (DNA)

4 Maturation:Viral components are assembled into virions.

Tail

5 Release:Host cell lyses and new virions are released.

DNA

Capsid

Tail fibers

Assembly/Maturation: of new virusesRelease: Host cell lyses and new viruses released

Viral LatencyViral Latency•Some viruses have the ability to become dormant inside the cell•Called latent viruses•They may remain inactive for long periods of time (years)•Later, they activate to produce new viruses in response to some external signal•HIV and Herpes viruses are examples

Lysogenic Cycle

The virus does not reproduce and lyse its host cell- at least not right away

Lysogenic CycleLysogenic Cycle•Phage DNA injected into host cell Viral DNA joins host DNA forming a prophage

• Viral DNA may stay inactive in host cell for long periods of time

• Replicated during each time infected cell divides

• Over time, many cells form containing the prophages

•The prophage may remain part of the host cell DNA for many generations

•Eventually the prophage stage will become active and remove itself from the host DNA and direct synthesis of new virus particles.

•Factors like sudden changes in temperature or availability of nutrients can activate the prophage.

•When an activation signal occurs, the phage DNA starts replicating

•Once a prophage cell is activated, host cell enters the lytic cycle•New viruses form a & the cell lyses (bursts)

Virus said to be virulent (deadly)

HOST CELL

LYSES & DIES

Watch Glencoe video clip

• Some eukaryotic viruses remain dormant for many years in the nervous system tissues

Eg. Chickenpox (caused by the virus Varicella zoster) is a childhood infection. It can reappear later in life as shingles, a painful itching rash limited to small areas of the body

SHINGLES

•Herpes viruses also become latent in the nervous system

•A herpes infection lasts for a person’s lifetime

•Genital herpes (Herpes Simplex 2)

•Cold sores or fever blisters (Herpes Simplex1)

SKIN TO SKIN CONTACT

PASSED AT BIRTH TO BABY

Bacteriophages

•Viruses that attack bacteria are called bacteriophage or just phage•T-phages are a specific class of bacteriophages with icosahedral heads, double-stranded DNA, and tails

•some infect E. coli , an intestinal bacteria•Six small spikes at the base of a contractile tail are used to attach to the host cell •Inject viral DNA into cell

Escherichia Coli Bacterium

T - EVEN PHAGES ATTACK THIS BACTERIUM

RetrovirusesRetroviruses

RetroviruseRetrovirusess

•HIV, the AIDS virus, is a retrovirus

•Feline Leukemia Virus is also a retrovirus

Characteristics of Characteristics of RetrovirusesRetroviruses

•Contain RNA, not DNA•Family Retroviridae•Contain enzyme called

Reverse Transcriptase•When a retrovirus infects a

cell, it injects its RNA and reverse transcriptase enzyme into the cytoplasm of that cell

ENZYME

RetrovirusesRetroviruses•The enzyme

reverse transcriptase (or RTase), which causes synthesis of a complementary DNA molecule (cDNA) using virus RNA as a template RTase

Watch Glencoe video clip

Treatment for Treatment for Viral DiseaseViral Disease

• 1) Interferons • are naturally occurring proteins

made by virus-infected cells• Make it more difficult for

viruses to infect other cells • inhibit viral replication within

host cells, activate natural killer cells and macrophages, increase the resistance of host cells to viral infection.

2. 2. VaccinesVaccines

•An attenuated virus is a weakened, less vigorous virus

•“Attenuate" refers to procedures that weaken an agent of disease (heating/genetically altering)

•A vaccine against a viral disease can be made from an attenuated, less virulent strain of the virus

•Attenuated virus is capable of stimulating an immune response and creating immunity, but not causing illness

3) Antiviral drugsInterfere with: •Virus attachment to host cell•nucleic acid replication•capsid formation

Use of natural poisons that kill virus infected cells only

Viruses: The Good…..

• Used to implant new DNA for disease resistance

• Manufacture harmless viruses that will “crowd out” disease-causing ones

• Create viruses that ignore healthy cells but destroy cancer cells