THE THREAD OF LIFE

[Music] the harmless chemical we call ptc i want each of you to put a strip in his mouth now some of you will be able to taste it and others won't if you can taste it it's going to taste bitter don't worry it's harmless the ability to taste ptc is a dominant inherited trait just like tallness and mendel's pea plants let's consider the case of this baby who can't drink milk most babies of course thrive on a milk diet but this one has a peculiarity in his body chemistry his body can't handle one of the sugars found in milk sugar called galactose medical records showed that the baby's ailment is inherited a matter of genes genes that control the way sugar is used in the body go ahead one question yes well that is is it genes that tell whether a baby will be a boy or a girl yes they do a little while ago you remember we found that the genes are on the chromosomes we also got the idea that all of our 46 chromosomes are in 23 matched pairs actually there's a very important exception in every male human being the 23rd pair of chromosomes is a mismatch one large partner and one short one we call them an x chromosome and a y in the cells of every female human being there are two x chromosomes and no y a fertilized egg that has two x's will grow into a girl one that has an x and a y can only grow into a boy which parent determines which types the fertilized egg will have can't be the mother because she has no choice she can only contribute an x chromosome but father in his cell has one of each an x and a y and the sperm that fertilizes the egg can carry either an x or a y the chesses are 50 50. if the father's sperm cell contains an x the baby will be a girl if a y a boy will be born father's chromosomes decide i suppose that's why fathers brag so much well fathers may brag but on the other hand the y seems to carry nothing much except the genes that determine male sex while the x chromosome carries a variety of genes this arrangement leads to interesting consequences for instance women are a lot sapped to be colorblind than men yeah i've heard that's true and i've always wondered why well the gene for color blindness is a recessive that's carried on an x chromosome if a girl inherits this gene she usually has a dominant normal gene on her other x chromosome to suppress it so this girl would have normal color vision while if a boy inherits the same gene there's no dominant gene for normal color vision on the little y to suppress it so he'll be colorblind if you think about it you'll recall that nearly all the colorblind people you've known have been men or boys now you know the reason and for this reason color blindness is called a sex-linked trait say wouldn't baldness be a sex-linked trait you might think so but it's not the baldness gene is on a different pair of chromosomes but for some reason it produces its effect only in the presence of the male sex hormone when a woman carries the gene for baldness nothing happens when a man carries the gene then you get this eminently resplendent effect dr baxter hello this is your first appearance i believe the question young lady yes sir everything you've shown us and talked about is heredity isn't environment really more important of course environment is enormously important every gardener knows that plants grow better when they're watered and tended mendel's tall peas grew even taller when he planted them in good soil and your good health and your good looks are partly due to good genes but they're also due to good food and happy surroundings you're a product of both your heredity and your environment well then let me ask you this question can you inherit the effects of a good or a bad environment that question has been studied and discussed for a long time and the answer is no we can have our blood replaced by a transfusion of somebody else's blood or develop spectacular muscles by weightlifting but none of these changes can be passed on to our children well yes but heredity has to change has to well i mean well not in one generation but over the long haul a lot of genes must have changed otherwise people today would look like they did in the days of the caveman yes genes do sometimes change heredity changes the changes are called mutations such changes down through the ages of the basis of evolution the camel that you see nowadays once looked like this every now and then on rare occasions nature strikes a wrong key well now back up please nature strikes a wrong key every now and then about once in a hundred thousand generations or more a gene doesn't copy itself exactly pulls a boner and this is called a mutation yes and if this mistake occurs in a germ cell it can be inherited these mice are victims of gene mutations one dwarf one two fat one born hairless and one with skin like a rhinoceros [Music] and this poor creature because of a mutation affecting his nervous system is condemned to spend his whole life running in circles mutations like these are harmful of course but sometimes mutations show up in pleasant or beneficial changes these stunning goldfish are descendants of mutated fish so were the mink that produced this beautiful pastel coat we don't know yet precisely how a gene changes though we have some pretty good ideas but once a gene does change it can be passed on only in its new form it becomes a regular part of heredity from that time on plant breeders have taken a lot of practical advantage of nature's mutations by discovering new strains and breeding them it's been possible to grow more and better food in many countries farmers often lose crops of wheat because of wheat rust breeders have discovered some strains that are resistant to rust and perpetuated them this crop is immune all over the world hybrid corn genetically bred has saved farmers and consumers millions of dollars greatly increased our supply of food the principles of genetics have also been put to work in breeding poultry for market and for egg production in breeding hogs and beef cattle in growing oranges and grapefruit lumber trees in sweden and rice in japan witness this most fabulous genetic creation a seedless watermelon developed by dr hitoshi kahara and his associates in japan oh looks mighty good dr baxter i can see how scientists do wonderful things with nature's mutations but i was wondering do they know how to cause mutations yes they do and man-made mutations are one of the important scientific questions for the modern world it's found that mutations can be artificially induced in several ways back in 1926 working in his lab at the university of texas dr herman j muller began some new experiments he set up an x-ray machine in his laboratory his purpose was to see if mutations could be caused by radiation he conducted his x-ray experiments on the fruit fly x-rays can produce some striking abnormalities among the descendants of irradiated fruit flies nearly wingless flies flies with extra wings [Music] flies with curly wings [Music] flies with black bodies white eyed flies found that x-rays can also have a deadly effect when he x-rayed fruit flies a certain proportion of their descendants died as embryos and the eggs never hatched again i suppose the mutation means that something happens in the cell have they found out what happens scientists have photographed irradiated cells and find that strange things happen this is a normal cell dividing the chromosomes split draw apart and two new cells are formed these are cells that have been irradiated they cannot divide normally some of the chromosomes are broken some are misshapen sometimes the chromosomes fail to split and the cell cannot reproduce but lighter radiation may cause changes that can't be seen in the chromosomes apparently by producing changes in the genes themselves thus the descendants of irradiated germ cells give us albino corn distorted ears of corn and stunted plants but dr baxter are all the effects of radiation bad most of them are therefore we must be very careful when we use radiation but scientists are discovering ways to use artificial mutations for the good of mankind to cite one case irradiation was used during world war two to produce new strains of the mold which makes penicillin these artificially created strains increased production of penicillin several times over thousands of lives were saved radiation is sometimes used to check the abnormal cell division we know as cancer cancer cells multiply wildly in an abnormal way scientists are trying to find out what has gone wrong [Music] we know that all cells are more easily affected by radiation when they are dividing that is why cancer cells can sometimes be knocked out by x-ray treatments to me the greatest service any science performs therefore its most practical aspect is what it adds to man's fund of knowledge the knowledge of ourselves and the world we live in one of the most exciting fields of science today is the continuing search to find and identify those invisible units of heredity that mendel imagined a hundred years ago you think they'll ever see what those units look like well they're getting warm and the road they're taking is leading them close to the secret of life itself the first big clue turned up in switzerland the scientist frederick measure experimenting with rhine river salmon discovered unknown chemicals in the salmon sperm one of these chemicals was later found to be present in the chromosomes of all kinds of cells one called deoxyribonucleic acid known by the letters dna [Music] [Applause] [Music] could this complicated substance with the long name be mendel's particle of heredity the gene itself it looks pretty much as if dna is the material that carries our heredity and here is an experiment that shows why we think so this demonstration is by dr harriet afroosi taylor about the smallest cells we know of are bacteria but they too have heredity we have discovered that some bacteria become resistant to drugs to antibiotics this resistance is hereditary in other words the bacteria have acquired a mutation for resistance this is how we can demonstrate what happens both of these plates contain an antibiotic that kills bacteria this dish has been spread with ordinary pneumococcus bacteria the kind that causes pneumonia none of the cells here have grown because the antibiotic has killed them the second dish has been spread with the same kind of bacteria except that this strain is resistant these cells are flourishing now we can extract the dna from the resistant bacteria and we get these pure fibers of dna here you can see the fibers more clearly we have put a little of this dna into a tube now we add to it the bacteria which are not resistant and let them grow for a little while finally we spread these treated bacteria from the tube on a dish containing the antibiotic spelling out the letters d n a and leave it in a warm place for a day by the next day something surprising has happened the bacteria have multiplied their heredity has been changed some of the bacteria we treated have picked up the dna and from it have gained the ability to resist the drug what is more they have passed the resistance along to their descendants unbelievable as it seems we can transfer heredity from one strain to another through the substance of dna well gee what kind of stuff is dna remember what we said about seeing a gene this is the most important photograph a close-up of the dna molecule magnified more than 100 000 times this molecule may be the gene itself looks like a tangle of thread yes and the way that molecule looks long and thread like is a key part of our story using physical and chemical clues scientists have been able to figure out how the dna molecule looks in detail this concept one of the most brilliant theories of modern science was formulated by two young men an american j.d watson and an englishman fhc crick this model represents only a short piece of a dna molecule magnified many times its proportions are deceiving the dna molecule is only 1 10 millionth of an inch in diameter but the dna molecules in your body if laid end to end would reach to the sun and far beyond the thread of life [Music] notice the structure the pieces are linked together in two intertwined chains forming a framework like a long spiral staircase the steps of the staircase are of four different kinds one two three four we've made models in school those balls represent atoms don't they yes there are hundreds of thousands of atoms carbon hydrogen oxygen nitrogen and phosphorus linked together in a fantastic array that spells the living gene but can you say a bunch of atoms like that are alive can they grow no but put them all together into a dna molecule and in this molecule you have an essential quality of living matter the ability to reproduce to make copies of itself and of all the molecules known to chemistry only dna and its relatives have this ability the theory of watson and crick assumes that when a gene is ready to reproduce itself to divide as it must do if the chromosomes divide the whole dna molecule comes apart in two pieces the twin chains that is made of unwinding like the strands of a rope floating around it in the cell are smaller molecules which make up a sort of spare part supply for dna they are drawn by chemical forces to find open spots to join forming the steps of the staircase until each strand of dna has made itself a new partner and now two double chains two identical molecules exist where only one existed before this is the theory of how heredity is passed on mind you this is just a theory but it makes sense many scientists are at work trying to see if it is really correct that's extremely interesting dr baxter but there's something i don't understand you said before genes make us different yes and if all genes are made of dna why aren't we all a light you've put your finger on an intriguing part of the puzzle if all dna molecules are made up of the same parts how can people differ the evidence is strong that the two interlinked spirals are the same in all creatures but the difference which makes you you and me me is to be found on the stair steps of the spiral there are only four kinds of stair steps but the order in which they are arranged can be varied enormously from one dna molecule to another so it may be the arrangement of the steps in this fantastic stairway that spells out the difference between you and me between a fish and a bird between a grasshopper and a redwood tree could the sequence of the stair steps be a kind of message to the cell a genetic code telling it what to do one combination might be part of the code for producing your curly hair another combination might decide that you'll be left-handed or that you'll inherit a white forelock [Music] the number of possible combinations is fantastic millions of such combinations working together could produce the countless complex traits that make up our inheritance research on dna is one of the great new challenges in science for the young scientists of the future there are many exciting questions ahead can we learn how to read the codes in dna how the gene passes its orders along to the rest of the cell why certain genes work only at certain ages infancy childhood [Music] adulthood an old age [Music] astronomers tell us there may be millions of planets capable of supporting life is dna a basic chemical of life on other planets as it is on earth [Music] someday we may know the answers to these questions and when we do science always pushing forward we'll find new mysteries to be solved [Music] for the physical nature of life around us and all its wonderful variety is a constant marvel to mankind [Music] life reproducing itself in its own likeness provides endless challenges to human curiosity the power of the human mind to observe to inquire to reason to imagine the existence of particles that are too small to see or things too great to measure that power of the mind is unlocking the deepest secrets of nature for the mind is the great lover of all things human thought is the process by which human ends are ultimately answered the bell system is grateful to the many distinguished scientists who helped to prepare this story of genetics our thanks to the advisors who supplied and checked the scientific content dr james f crowe [Music] dr norman h horowitz and to the individuals and organizations who provided special material our thanks to the advisory board which reviews the scientific aspects of these programs its members represent the broad range of modern science biology and genetics medicine bacteriology and botany chemistry geophysics physics anthropology electronics and acoustics mathematics engineering to all these men the bell system is indebted for their support of this venture in public education through entertainment you