Viruses

Characteristics of Viruses Viruses are NOT alive They do not have a cytoplasm or organelles Cannot carry out cellular functions They do not divide (reproduce) outside host cell

Viral Size and Structure Viruses are super small! The shape of a virus is the result of its genome and protein coat – capsid Some viruses have a bilipid membrane called an envelope that surrounds the capsid

Classification of Viruses Viruses can be classified by: – if the have RNA or DNA – If genome is single stranded or double stranded – The nature of their capsid – Presence or absence of envelope Example: SARS is a single-stranded, linear RNA and envelope virus with lollipop-shaped proteins that make the envelope look like a crown

Viral Replication Viruses need a host cell! They can spread from host to host through the air, water, food, or bodily fluids (yuck) Viruses infect both prokaryotes AND eukaryotes

Viral Replication Viruses need to recognize a proper host cell before they can infect it. Viruses are obligate intercellular parasites – they replicate only by using host cell enzymes and organelles to make more viruses

Viral Replication Pathway “lock-and-key” fit between proteins and receptor molecules of host cells A viral infection begins when a virus enters the host cell. The viral genome takes over the metabolic machinery of the cell and makes new viruses

Replication in DNA Viruses When DNA viruses enter a host cell, it makes mRNA, which is the template for making proteins Other DNA viruses insert their DNA into the host cell’s chromosome – This inserted viral DNA is known as a provirus – The host cell’s enzymes then transcribe the provirus into mRNA and create viral proteins

Replication in RNA Viruses The genome of some RNA viruses enter host cells and acts as mRNA They are then translated into new viral proteins! Yikes! Retroviruses contain the enzyme reverse transcriptase in addition to RNA

Retroviruses Reverse transcriptase uses RNA as a template to make DNA which then inserts into the host cell’s genome. Reverse transcriptase reverses the normal process of making proteins Htw

Replication in Viruses That Infect Prokaryotes Bacteriophages (phages) are viruses that infect bacteria

The Lytic Cycle During the lytic cycle, a virus invades a host cell, produces new viruses, and ruptures (lyses) the host cell – Lysis is the process of phage particles bursting from the bacteria cell and destroying it! Viruses that reproduce only by the lytic cycle are virulent. – Virulent viruses destroy the cell that they infect

The Lytic Cycle

The Lysogenic Cycle Allows viruses to hid in their host cell for days, months, or years! These viruses are called temperate viruses Phage DNA that is integrated into a specific site of the host cell’s chromosome is called a prophage

The Lysogenic Cycle

Viruses: Tools for Biotechnology They are important research tools! We can replace large pieces of DNA of a phage with DNA of a particular interest!

The Origin of Viruses Most scientists believe viruses evolved from early cells – One hypothesis is that the first viruses were probably naked pieces of nucleic acid that could travel from cell to cell – The viruses entered cells through damaged cell membranes – Over time genes evolved that coded for protective protein coats as well as signaling for specific cells

The Origin of Viruses Evolution by Natural Selection! Think of some examples!

Vectors of Viral Disease Because viruses are lifeless particles, their spread depends on other agents. A vector is an intermediate host that transfers a pathogen or a parasite to another organism Examples of vectors for viruses: animals, mosquitoes, ticks, fleas, YOU!

Vectors of Viral Disease The West Nile virus, (causes fever, headache, coma, paralysis, and even death) infects mainly birds. If a mosquito bites a bird infected with West Nile virus, then bites a human, the virus can spread!

Examples of Viruses in Humans flu Chickenpox Measules Polio Viral hepatitis Cold Yellow fever West Nile And much much more 

Viruses and Cancer Cancer is the result of: Some viruses contain viral oncogenes, genes that cause caner by blocking the normal controls on cell reproduction

Viruses and Cancer Other viruses cause cancer because the viral DNA inserts itself into a host’s chromosome near a proto-oncogene, which usually controls cell growth. The proto-oncogene is converted to an oncogene.

Emerging Viral Diseases Emerging diseases are illnesses caused by new or reappearing infectious agents that typically exist in animal populations –often in isolated habitats- and can infect humans who interact with these animals Example: Recent Ebola outbreak blamed on a bat infecting a human

Chickenpox and Shingles pg. 489 Tamar

VIRAL HEPATITIS PG. 489 Benny

Acquired Immune Deficiency Syndrome (AIDS) Pg. 490 Hadar

Short Essay Vaccinations Vector Control and Drug Therapy Emerging Viral Diseases