Viruses: Threshold Of Life (1967)

[Music] foreign [Music] no study in biology deals more closely with the very origin and existence of life itself than does the study of viruses though virologists have learned a great deal about viruses they remain certain basic questions that they still have not answered to their entire satisfaction for example looking at an electron micrograph of a virus the most experienced of scientists may still wonder what exactly is a virus looking at a tobacco leaf attacked by rapidly reproducing viruses he may wonder how do viruses exist and reproduce looking at a culture of disease bacteria being destroyed by viruses he may ask himself how can we use our knowledge of viruses to improve our own health and well-being as a biological scientist he knows that these questions have not been fully answered yet he knows that he and his colleagues have a great deal of information about viruses and their effects he knows of much that has been done to answer his first question what exactly is a virus to find out scientists have located viruses and can describe the work this entails the optical microscope is practically valueless for observing individual viruses the only true viruses large enough to be barely visible through the highest powered light microscope are viruses of smallpox and certain other pox diseases others may be 10 or 20 times smaller but there is a way to locate viruses and collect them for study we begin with tissues infected by viruses say tobacco leaves infected by tobacco mosaic virus we freeze the infected material this ruptures cells in which viruses are located we use a grinder to rupture the cells further this releases juices which we filter the small viruses pass through the filter with a liquid to separate viruses from other particles in the liquid we use an ultracentrifuge after an hour or so of spinning at 20 000 revolutions per minute we have a sediment made up of everything as large as or larger than a virus now by adding water we prepare a suspension of the sediment to remove particles larger than viruses we spin this suspension in a centrifuge that moves relatively slowly this leaves viruses in the fluid another run through the high speed ultra centrifuge gives us a sediment that's almost entirely viruses to be sure viruses are there we can examine the sediment with an electron microscope the picture shows viruses each of these bodies is not a single virus but a crystal made up of many small viruses a number of viruses tend to form crystals in this way a researcher knows that various viruses have different shapes some are shaped like cubes or like compound cubes others like helixes spirals still others are constructed according to complex geometric ratios viruses outside a living host are apparently lifeless bodies simple or complex in structure the scientist can further explain virus structure the simplest viruses have just two parts an outer coat of proteins and a core of genes either dna or rna never both the most complex viruses contain little more than this there may be very small amounts of fats and carbohydrates and sometimes toxins and enzymes this partially answers the scientists first question what is a virus and he knows that other studies have provided a partial answer to his second question how do viruses exist and reproduce the question invites us to consider whether viruses are alive for though we've seen that viruses outside living cells are apparently lifeless yet the same viruses inside the cells of a living host reproduce and spread so viruses may be said to be living or non-living depending on the point of view a scientist can describe how he learns about virus activity we may introduce certain viruses into the cells of living plants such as the maize plant viruses that affect animals such as the ferret may be grown in living specimens however it's often impractical to keep animals in the lab so we grow animal viruses in embryos or in living tissues removed from developing chicken eggs or from animals these tissue cultures are kept alive in the lab working with them we've learned that particular viruses tend to grow best in certain kinds of host cells not in others some viruses animal viruses grow best in cells of only certain animals other viruses plant viruses grow best in only certain plant cells still other viruses known as bacteriophages successfully infect only particular species of bacteria the action of bacteriophages can be used to demonstrate reproduction in a virus a model of a cell may be used to illustrate the reproductive cycle the virus attaches itself to the host cell and injects its genetic material other viruses may enter differently but the results are similar once inside the genetic material behaves like part of the cell virus genes join with the cells genes once they have joined they redirect the cell so it no longer produces new cells like itself instead it produces viruses finally the cell bursts and the viruses are freed perhaps to infect other cells one can observe the destruction of an entire bacterial culture in about 20 minutes a microscopic representation shows what is happening one by one cells are bursting liberating viruses the technical name for this kind of cell destruction is lysis the spots on the culture are areas where viruses have lysed bacteria these viruses are cell killers other cell killing viruses cause diseases such as curly top and beets foot and mouth disease in cattle and influenza and polio in human beings these viruses kill cells however researchers know of viruses that are not cell killers some cause the cells they infect to grow and reproduce very rapidly in a living animal or plant this kind of growth may result in tumors and though no cells are destroyed the host organism may die men study other viruses that sometimes cause no harm at all to their hosts a scientist can describe such a situation we can raise a mouse that has leukemia viruses in itself yet examination of the animal's blood reveals no sign of leukemia we believe the viruses are hidden in the cells we believe they live and reproduce along with the cells without harming the animal this state of affairs known as latency may go on for years and for generations however exposing such a mouse to x-rays or radiation may change the relationship between virus and cell the exposed mouse may develop leukemia as viruses which are in the cells become active some virologists believe that latency is more common than cell destruction such men have discovered instances in which viruses actually benefit their hosts a scientist can describe such a case we prepare a culture of bacteria that have the ability to swim into this culture we inject a virus this virus destroys the bacteria now if the very same virus is placed in a culture of related non-swimming bacteria these bacteria are not destroyed instead some gain the ability to swim viruses have carried genes for swimming from bacterium to bacterium a process known as transduction that helps to distribute bacterial traits demonstrations like these show that men know quite a lot about viruses and their ways of life so much that a scientist may justifiably ask how can we use our knowledge of viruses to improve our own health and well-being to answer this question he may cite the human implications of some virus studies because viruses are so simple in structure it's been relatively easy to catalog their genetic characteristics like rate of maturation and distribution of dna the locations of these genes are charted on gene maps maps that give us insight not only into virus genetics but into the makeup of all cells including our own also studies of tumor-causing viruses have led many of us to believe that viruses may cause cancer and related diseases what's more our study of bacteriophages has over the years led some men to believe these may be used to kill disease bacteria inside the body but perhaps the most exciting studies in virology are those in which we have created viruses from virus genes and non-living chemical media the implications of these studies are far-reaching perhaps men of science will someday be able to create genes from chemicals to create living cells to create and repair human tissues and organs because of the work these men are doing perhaps our grandchildren will live in a world in which plants and animals are less harmful to mankind and where because of our study of viruses life can be more pleasant and rewarding you