DNA: MOLECULE OF HEREDITY

[Music] you know the rules of inheritance were first worked out by gregor mendel he was an augustinian monk this is w of the california institute of technology born and raised on a nebraska farm he first studied the then new sciences of heredity and evolution at the university of nebraska's college of agriculture in 1924 the work dr beedle has done since then has greatly added to man's knowledge of the process by which tiny particles in living cells control the forms that life will take the tiny particles are called genes the science of their study is called genetics dr beedle's work in this science won him a share in a nobel prize in 1958 you already know that each of us starts development as a tiny almost microscopic single cell the fertilized egg here it is seen through a microscope it's about the size of the point of a dull pin here's another egg about the same size as the other but this isn't a human egg it's the egg from which a sea urchin develops how does each of these egg cells know what it's supposed to develop into for instance how does the human egg cell know that it's supposed to develop into a human being it knows because the cell contains an elaborate set of directions or specifications for making people not sea urchins these specifications determine what color our hair will be whether we will be male or female to a large extent they determine what we will look like and they determine many thousands of important things about the way we will grow and develop most of these specifications are contained in the nucleus the microscopic central sphere of the egg cell located in tiny bodies called chromosomes the specifications we call genes i'm now going to ask five questions about these specifications and then tell you what we think the answers might be first how are the specifications transmitted from one generation to another i'm going to illustrate with some mice this mouse has genes in each of his cells that contain specifications that say he will produce brown pigment this one has specifications that say she will not produce pigment and the lack of pigment leaves her with white fur if these two mice mate together and produce offspring each will contribute specifications through egg and sperm cells to the offspring the mother for no pigment they're for brown pigment the sons and daughters will therefore have two sets of specifications concerning the color of their fur now how are the specifications written this mouse has specifications that say in effect i cannot produce pigment this woman is an albino she also has genes carrying specifications that say i cannot produce pigment now we know we don't have words and sentences in our cells written in the 26 letters of our alphabet but nevertheless the information is there we can write i cannot produce pigment in a simple code using only two symbols dots and dashes to someone who knows the morse code this says i cannot produce pigment the specifications for the development of the fertilized egg cell are written in a code that uses four symbols the code is written in long spiral staircase like molecules of a substance called deoxyribonucleic acid it's called dna for short now this is a model of a molecule of dna dna molecules are made of a large number of atoms and are quite complicated but they are made of smaller less complex units and it's the way these smaller units are arranged that determines what the code is saying in each particular piece of dna let's look at a simplified diagram of a dna molecule if we were to untwist the spiral the basic shape of a dna molecule would be somewhat like a ladder the long supports are made up of connected subunits of sugar and phosphates we'll indicate these with the letters s and p the rungs or connecting pieces are two units each and are of four different kinds we can simply label them a t c and g each connecting rung of the dna molecule is made up of two of them and a is always combined in a rung with a t and a c is always combined with the g however they can be combined in either direction and the rungs can be arranged in any order now the morse code has two symbols dots and dashes and the order in which they appear indicates what letter they stand for the dna code has four symbols and the order in which they are strung together is the code that spells out the specifications that tell the egg cell how to develop now of course this quote doesn't form letters and words but it does record information that controls and directs the development of an egg into a fully grown organism located on one of the chromosomes of this albino woman is a piece of dna that says in some way in its four symbol code i cannot produce pigment it's been estimated that it takes from a thousand to ten thousand of these code units arranged in a particular order to encode such a specification just imagine how many different specifications it takes to determine how you will develop there are some for the color of your hair others for the color of your eyes others for your blood type each of us has perhaps one hundred thousand separate specifications contained in the fertilized egg cell from which we develop collectively these one hundred thousand specifications add up to a kind of a recipe for us they say how to make us out of the egg cell given time food proper temperature and so forth if we expressed all this information in english it would fill about a thousand volumes yet all this information is contained in that tiny microscopic sphere that is the nucleus of the fertilized egg cell this fertilized egg cell is the beginning of a human life now let's examine how does it develop into a human being do you know what mitosis is mitosis is the way a cell is divided into two cells it's the way new cells are made that's right it's through the process of mitosis that we develop from one fertilized egg cell to an organism with millions of cells here we see a cell divided by the process of mitosis the time is sped up by the camera actually it takes much longer you'll study mitosis in some detail later but what i want you to consider now is what happens to the specifications in the nucleus of the cell when the cell divides what do you think happens do they divide some way too that's absolutely right they do when a cell divides the two new cells contain the same specifications the parent cell contained that means that the specifications that thousand volumes worth of information you remember have been reprinted into every nucleus of every single cell in your body and remember you have millions of them the reprinting of the specifications is directly tied in with the structure of the dna molecules on which the specifications are coded several kinds of experiments indicate that the reprinting occurs by separation of the molecule into two halves followed by the formation of new partners by each half now there is a complete dna molecule for the nuclei of each of the two new cells we'll look at that more closely and see exactly what happens when the halves of the dna molecule separate they separate between the two units of the connecting bonds along the molecule in the cells in which this is happening and it's happening in your cells right now there are many single subunits made from the food we eat when these come into contact with their proper partners they are held in place and a only joins with a t a c with a g now a complete new molecule is formed identical to the parent molecule the coded specification it carries has been reprinted just as it was because the sequence of the code symbols in the half that came from the old molecule controls exactly the sequence in the new half that is formed this is a molecular model of the g subunit of the dna molecule it's magnified 100 million times imagine that it's part of one of the halves left after the dna molecule has divided this is a model of a t subunit imagine that it's free in the cell if it tries to join with the g sub unit it won't hold doesn't fit but when a c sub unit comes along it fits perfectly from one dna molecule carrying its coated specification two exactly like the original are formed these go into the two daughter cells that result from the division of the parent cell in terms of molecules this is the basis of all biological reproduction this reprinting of the specifications from cell to cell is done with high precision there's perhaps only one typographical error that is only one misplaced symbol for each reprinting of the thousands of dna molecules in a single cell remember the information encoded in these molecules is equivalent to one thousand printed volumes reprinting that information amounts to a typist copying one thousand volumes and making only one significant typographical [Music] error [Music] what about the occasional mistakes in dna specifications mistakes in reprinting like typographical errors or mistakes that occur in other ways we call these mistakes mutations usually mutations give rise to wrong recipes and are harmful for example if there's a mistake in the dna specifications for a hemoglobin protein the hemoglobin made according to this specification will be defective this is a cell with normal hemoglobin the red pigment molecules of your blood that are necessary for the circulatory system this is a cell with defective hemoglobin it's called sickle cell hemoglobin because the cells containing it are distorted often in the shape of a sickle this defective hemoglobin occurs in man when there is a mistake in the dna instructions for producing hemoglobin but cells come to an untimely end and the bearer of them suffers from sickle cell anemia and often does not live to maturity now i don't want you to think that all mutations are unfavorable as a matter of fact most of them are but you'll learn that the entire process of evolution has depended upon favorable mutations we have evolved from one cell creatures of billions of years ago through errors in dna specifications that have improved the recipe for the organism in which they occurred this is the principle behind the development of hybrid seed and livestock and it's the principle nature has used in the development of such complex creatures as ourselves able to cope with our environments in a remarkably satisfactory way i hope i've arised your curiosity about these amazing dna molecules that carry your biological specifications if you want to learn more about them then there is much more no one yet knows i strongly urge you to first learn as much as possible about the chemistry of simpler molecules such study will be well worthwhile because in a real sense dna holds the secret of life itself you