Viruses:Genes in a Box

Living or Non-living?Living Characteristics

Highly organized Possess genes

(hereditary)Non-living

Characteristics Not made of cells Cannot reproduce on

their ownWhich is it?

Non-living

General Characteristics

Consist of a bit of nucleic acid wrapped in a protein coat – called a capsid

Survive by infecting a cell and using the cell’s reproductive

processes to make more viruses.Are VERY small, approximately 50µmAffect all living organisms—plants, animals, and bacteria.

Bacteriophage Viruses

“Bacteria eater”Bacteria cell

Bacteriophages

Phage Reproduction Reproduce one of two ways Phage bores a hole in the cell membrane and injects its DNA

1. Lytic Cycle Produces many little

copies of the phage within the bacteria cell

Then the bacteria cell lyses (breaks-open) and phages are released

Kills bacteria cells in the process

2. Lysogenic Cycle Viral DNA is inserted into

the bacterial DNA; becomes known as a prophage

Viral DNA is replicated, transcribed, and translated with the bacterial DNA

Does NOT kill the bacterial cell, no phages are created

Will be passed on to daughter cells with host DNA – creates a large # of bacteria cells carrying the phage/viral DNA

Environmental factors may trigger a switch from the lysogenic to

the lytic cycle

Lytic & Lysogenic Cycles

Plant VirusesCan stunt plant growth

and diminish crop yieldsMost contain RNA rather

then DNA as their genetic material

Genetic engineering has been used to create plants resistant to some of the viruses

Ex: Tobacco Mosaic Virus, Potato X Virus

Animal VirusesCommon causes of diseaseContained within an

envelopeSpikes helps the virus enter

and leave the host cellContain DNA or RNA as their

genetic materialVaccines help prevent viral

infections but most have no cures

Ex: Influenza, measles, mumps, herpes, HIV, small pox, chicken pox, etc…

Flu

Lipid By-layer

Receptor Proteins

DNA

Capsid

Enveloped Virus Reproduction

Protein spikes attach to cell and envelope fuses with the cell’s membrane

Virus uses the cell to reproduce and leaves without killing the cell

Not all animal viruses reproduce in the cytoplasm, many do so in the host cell’s nucleus

Mumps Virus

HIV Reproduction Virus resembles flu or

mumps viruses physically, but has a special system of reproduction

Retrovirus, an RNA virus that reproduces with DNA

Contains an enzyme called reverse transcriptase, which converts RNA into DNA

HIV (Human Immunodeficiency Virus) causes AIDS (Acquired Immune Deficiency Syndrome)

Experiments with Viruses

Relatively easy to study Viral symptoms in organisms Reproduce very quickly

Provided the first glimpses of how DNA works and the role it plays in heredity

In the early 1900s, the effects of viruses could be seen in the form of outbreaks of small pox or the disturbing behaviors of animals affected by rabiesOnly had simple microscopes, were unable to see the viruses

Dmitri Iwanovski 1892, Russian Dmitri Iwanovski

began research studying a disease effecting tobacco plants

Crushed plants to extract juices

Healthy plants were exposed to the extract and they then became infected

The liquid was then filtered in order to remove the infection, but it still caused the plants to become diseased

Concluded there must be something in the infected plants that would ‘poison’ healthy ones

Martinus Beijerinck1898, Professor of Microbiology

in the NetherlandsAlso experimented with Tobacco

Mosaic VirusObserved:

‘Microbe’ was smaller then bacteria Could only be cultured on living

plantsSuggested that some ‘microbes’

are not cellularNamed the pathogen virus,

Latin for toxin or poison

Frederick Griffith 1928, British Microbiologist Studied the affects of two versions of the bacteria that

causes pneumonia, one was pathogenic (smooth) and the other harmless (rough)

Experimental Findings

The invention of the electron microscope allowed better views of tiny objectsWendell Stanley (Princeton, NJ), in 1935, isolated the nucleoprotein linked to causing the tobacco mosaic virus by crystallizationRealized a specific molecule was the root of inheritanceFocused attention on the chemical make-up of

chromosomesEukaryotic chromosomes are made of DNA and

proteinBut which was responsible for heredity?

Inheritance: DNA or Protein?DNA Protein

4 Nucleotides 20 Amino Acids

Simple Elaborate

Lacking variety Varied Structures

Avery, MacLeod, & McCarty1944, NYCRevisited Griffith’s experimentsTook extract from heated S strain and

treated it with DNAase (digests DNA), then mixed with rough bacteria and injected into rats; the rats lived

Treated extract with protease (digests proteins), mixed with rough bacteria and injected into rats; rat died

This showed that DNA, not protein, has ability to transform cells—thus identifying DNA as the “transforming” factor

Alfred Hershey & Martha Chase1952, University of Southern CaliforniaExperimented with E. coli to see which part of the

T2 phage was transferred during infectionLabeled vDNA and viral protein with radioactive

isotopes vDNA—phosphorus-32

not found in proteins v-protein—sulfer-35

not found in DNA

Hershey & Chase Experiment