Mitosis (1961)

the reproduction of New Life the growth of all living things both plant and animal is basically the same where is it more dramatically portrayed than in the growth of an embryo on the first day the future chicken is merely a speck in the Yol on the second the embryo has taken on a recognizable shape day by day new tissue is added and growth continues in just 21 days a chicken has been formed a complete organism that is originated from a single cell in the same way this cell about the size of a pin head is the beginning of a human embryo and develops into something far removed from the original but however complex their structure all animals and plants are composed of cells and all have been brought into being by the most basic type of reproduction the reproduction of cells by division this is the single egg cell that will develop into a rabbit shortly after fertilization it divides into two these in turn divide to make four soon a whole complex of cells is formed and they keep multiplying when the embryo is sufficiently developed the dividing cells start to grow and this process of cell division and cell growth continues to maturity but there is nothing halfhazard in this business of cells splitting in two on the contrary the whole operation is one that Demands a high degree of organization if the proper result is to be achieved basically a cell is composed of a complex mixture of substances called cytoplasm in which is suspended the nucleus inside the nucleus are the all important chromosomes the chromosomes contain molecules of a substance called DNA these molecules of DNA are thought to be the genes all these different genes are the specifications which determine an organism's different characteristics whether it will develop into a chicken or a bean plant have blue eyes or brown be a boy or perhaps a girl they are a sort of recipe for growth thousands of different Gene specifications are carried by each chromosome so to distinguish these different chromosomes we'll give them different colors now let's see what would happen if during the growth process the cell were to be arbitrarily divided in two each new cell would now be short of many of the specifications contained in the original obviously if this were to happen in practice the whole character of the new cells would be changed instead of this arbitrary division what actually happens is that a second and identical set of chromosomes is formed this is achieved by duplicating each chromosome already present then the nuclear membrane dissolves away and the double chromosomes line up on the center of the cell when division takes place one complete set of chromosomes moves into each of the new cells that is being formed this highly organized division which results in two daughter cells that are genetically identical is the process that we call mitosis in order to observe the actual event of mitosis in a living organism the noted biologist Andre berer chose to experiment with the blood lily the seed pods of the blood lily contain cells with very large chromosomes the material is therefore an ideal subject for observation with the phase microscope we can photograph the mitotic process as it actually takes place in a living cell the division which in fact takes about 5 hours in this type of cell has been compressed into just over a minute here the cell is in the so-called resting stage although they are not yet visible a second set of chromosomes is already being formed the activity of the cell increases as the time four division grows nearer the chromosomes become visible soon we can identify them as duplicate sets of chromosomes they start to organize themselves in a line down the center of the cell ready for their division the double chromosomes then separate an identical set of daughter chromosomes moving to each end of the cell the cell wall gradually forms in the middle creating two new daughter cells that have the same hereditary characteristics as the original although mitosis is in fact one continuous process we normally study it one step at a time before mitosis takes place the cell is in the resting stage actually the only thing it's resting from is mitosis for in every other way it's very much occupied it's living its normal life and in particular it's growing in this resting stage the chromosomes are not the rodlike bodies that we normally associate with the word instead they are in the form of a twisted mass of threadlike material called chromatin the First Act of this natural drama is the prophase and it starts with the doubling of the chromatin content then the chromatin threads gradually ually condense into the conspicuous double chromosomes these identical twins are joined at but a single point called the centrom miror now the nuclear membrane dissolves away while all this is going on a remarkable structure of radiating fibers begins to appear this is known as the spindle and runs from one pole to the other a spindle fiber anchors itself to each Central Mir the spindle can be thought of as a sort of scaffolding on which the mitotic division will take place before we go on to the next phase let's look at the prophase as it actually takes place in a living plant cell the chromatin can be seen condensing to form the chromosomes then the nuclear membrane dissolves and the cytoplasm and nuclear material freely intermingle these find threads are the spindle fibers unfortunately they're not easy to see in a living cell now the nucleoli disappear at the same time the twin chromosomes untwist but they still remain held together by the centrom Mir up to now the chromosomes have been scattered at random in the central part of the cell in this next stage the metaphase they line up on the equator ready for actual division the lineup is at right angles to the spindle and at the Midway point between the two poles the next stage the anaphase is the climax of the mitotic process for now each centromere splits in half and the two daughter chromosomes separate completely independent of one another they will move apart apparently pulled by the spindle fibers attached to their centrom mirors the two groups of chromosomes each set identical to the one moving in the other direction migrate to the opposite poles of the cell here is a blood lily cell in metaphase the spindle fibers are slowly aligning the chromosomes along the Equator of the cell anaphase and the daughter chromosome separate the spindle fibers pulling them to the opposite poles of the cell here it is again in another cell the chromosomes line up down the center then the daughter chromosomes separate from one another the fin Act of this natural drama is the telophase in plant cells like this one a cell plate begins to form cutting through the spindle the spindle The Guiding mechanism of the mitotic process now subsides its job completed while the cell is being cut in two the chromosomes resume their original thread-like appearance and new nuclear membranes form around them here is the actual telophase taking place the cell plate begins to form and the spindle subsides here it is again in another cell note how the chromosomes aggregate at the poles no longer visible as individuals in this example you will see the chromosomes being enveloped by the nuclear membranes the new clei reappear the Mother cell is now completely cut in half by the cell walls the two new cells will return to the resting stage where they will develop and grow up to now we've only looked at mitosis in a plant cell and although animal and plant mitosis are basically the same it's interesting to observe some of the differences these are the heart cells of a n prepared in a tissue culture the duplicate sets of chromosomes can be seen gradually emerging from the chromatin Mass the nucleoli disappear and the nuclear membrane dissolves the chromosomes now line up down the center of the cell anaphase and the daughter chromosom separate each set moving to the opposite of the cell in an animal cell there is no cell plate that cuts the cell in two instead the two daughter cells pull apart from one another as the two cells pull apart eruptions can be seen around the cell membranes these are the result of physical and chemical adjustments that have to be made in the new daughter cells finally the chromosomes Mass together in each new nucleus and the division of the original cell is completed in addition to helping an organism grow cell division also acts as a repair service the blood thickens and clots over the wound forming a protective scab underneath new cells are formed to replace those that have been destroyed and gradually the tissue is healed as well as healing wounds cell replacement has to offer a continuous 24-hour service to the bloodstream every second about 10 million of our red blood cells are destroyed and they have to be replaced then two the epithelium or Surface cells and these include the skin are constantly dying and being sloughed off as the old skin cells are discarded new ones are constantly being formed to take their place sometimes however this Replacement Service runs wild and for some unknown reason cells start to multiply more rapidly than they are discarded this is cancer and the uncontrolled growth of these abnormal cells replaces healthy [Music] tissue another significant departure from the normal takes place when living cells are subjected to [Music] radiation this blood lily cell has been treated with radioactive strum and the bombarding Rays have broken the chromosomes one broken piece has even been left stranded on the cell plate in this cell most of the chromosomes are already migrating to one pole even before division has taken place place when the cell eventually does divide the disorganized and broken chromosomes can only result in the death of both the injured cells in this irradiated cell you can see the that some pieces of chromosomes will now carry too many genetic instructions those are the long ones others will not carry enough the short ones as neither of the new cells will now have the correct amount of genetic information they too cannot function properly and will soon die dividing cells have also been subjected to various chemicals in order to determine how they would be affected it was found that cells treated with culine had the mitotic event arrested at metaphase the chromosomes just did not separate the chromosomes line up on the metaphase plate ready for division and start to divide but the action of the culine on the cell prevents the chromosomes from moving apart freezing them as it were at The crucial moment of division these are but two examples of the Intensive research that is gradually increasing our knowledge of a fascinating process fortunately however the natural Perfection of normal mitosis is rarely Disturbed starting from the original egg it reprints precise instructions for growth and development into the nuclei of every one of the millions of cells that make up the body it is one of the most important biological events for with careful accuracy it builds and maintains both plant and animal it is an event which is constantly reenacted an event which maintains the continuity of life itself for e e