Cell Biology: The Nucleus (1989)

[Music] thank you one of the great discoveries about life was the observation that living things are made of cells today we know that a cell consists of an outer plasma membrane that encases the fluid portion of the cell called the cytoplasm and a control center for the cell the nucleus the nucleus usually occupies a central position in the cell however there are other configurations in many kinds of plant cells a large vacuole fills the center portion of the cell and so the nucleus is pressed to one side in large protests the nucleus may be multiplied forming a string of nuclei all managing the Affairs of one very large cell [Music] or it may be elongated seen here wrapping around the inside of each of these cells like a long blue sausage [Music] to early cell investigators the nucleus was the single dominant structure visible with the light microscope and so a logical first question became what does this Central structure actually do in the life of a cell imagine yourself 150 years ago trying to investigate such a question you might Begin by subjecting cells to various dyes this approach showed that the nucleus contains a tangle of thread-like structures the chromosomes composed of Chromatin a substance which readily picks up dyes that do not color all the other parts an observation that surprised early investigators was that the nucleus could be removed from a cell this one has been squeezed out and the cell would go on living for some time however a cell without its nucleus could not divide and eventually died but what exactly was the connection between the nucleus and a cell's ability to divide improved staining techniques showed that the content of the nucleus undergoes a dynamic change when cells divide during division chromosomes are parceled out so that each new cell receives one complete identical set another fact each species of animal or plant contains a characteristic number of chromosomes a fruit fly four pairs of chromosomes an onion 16 pairs a human 23 pairs evidence gained by breeding different types of fruit flies supported the idea that traits were carried on the chromosomes in smaller units which came to be called genes [Music] further evidence of this was found in the salivary glands of fruit fly larvae which contain giant chromosomes looking closely at these large chromosomes shows that they are covered by bands biologists now are able to associate some of these bands with specific genes what is true of fruit flies could be applied to humans and other organisms for example this might be the position of the gene that carries the instructions for a particular enzyme this the gene for hemoglobin the genes for eye color or the gene for a muscle protein today we know that genes are segments of DNA the long molecules that make up the chromatin first seen as a Darkly stained wispy Network within the nucleus the DNA molecule was eventually found to have a double structure which permitted duplication prior to cell division understanding DNA's molecular structure also explained how it could direct the synthesis of proteins the giant molecules that make up most of the cell these include structural proteins that form the cytoskeleton the contractile proteins involved in cell movement and enzymatic proteins the organic catalysts that control a cell's chemical processes the instructions for a particular protein are written out in a sequence of DNA code words or codons each codon stands for an amino acid one of the building blocks in the protein to be assembled to begin the process of making a protein the DNA molecule opens allowing its information to be copied using another kind of molecular code letters RNA this messenger RNA carrying the codon sequence for the protein must get out of the nucleus and into the cytoplasm where proteins are assembled the electron microscope shows that the nuclear membrane contains numerous pores messenger RNA is extruded through these pores into the cytoplasm where it is used as an instruction tape for linking together the amino acids that form the protein so the nucleus is a receptacle for genetic information its DNA contains the assembly instructions for the cell's structural and enzymatic proteins [Music] but the nucleus does more than send out instruction tapes of messenger RNA it also sends out another kind of RNA called ribosomal RNA this RNA produces the tiny machines that actually assemble the proteins organelles called ribosomes ribosomal RNA is manufactured deep in the nucleus on a structure called the nucleolus a large amount of this special RNA is required to replace the thousands of ribosomes that wear out each minute in the cytoplasm of a living cell the ribosome replacement rate is particularly high in cells that have just divided and are bringing their complement of ribosomes up to a normal level [Music] when a cell divides the complete DNA instruction book must be duplicated and accurate copies passed out to each of the resulting cells as life progresses from an egg through an embryo to a newborn there is a great flurry of cell division eventually producing the many kinds of cells that make up the adult brain cells muscle cells skin cells blood cells and so on The reason an egg develops into an animal and not just a blob of trillions of similar cells is that different regions of DNA are activated in different groups of cells during development this selective reading of the DNA instruction book continues into adult life [Music] what this means is that a cell lining your cheek contains the same genes as cells in other parts of your body but in a cheek cell only the cheek cell genes are active however in adult animals there are many cells that divide only infrequently and some not at all once the animal is full grown nerve cells for example are essentially non-dividing in an adult whereas cells lining the intestinal tract May divide as often as once a day while cell division in plants and animals leads to growth and differentiation cell division in microorganisms produces population growth start with one cell and a food source and in a few days there will be a vast population of identical individuals [Music] but some single-celled organisms remain attached after division forming colonies of cells [Music] at the dawn of multicellular life such colonies may have formed an evolutionary bridge between single cells and simple multicellular organisms but no matter where they live or how rapidly they reproduce cells all go through the same kind of life cycle this cell is showing no signs of reproduction it's carrying out its life-supporting activities replacing its ribosomes and making its required proteins then something quite mysterious happens deep in its nucleus the molecules of DNA begin to open allowing new DNA copies to be made from each strand if we stain for DNA a few hours after the signal to duplicate was given we find that the color of the nucleus is about twice as intense as it was before the signal was given the reason for the change in intensity is that there is now twice as much DNA [Laughter] following DNA replication a cell May return to its normal activities for a time but sooner or later it will begin the most dynamic event in the cell's life cycle mitosis the dividing up of its chromosomes into two identical sets this is followed by division of the cell itself this process is best seen in time-lapse Motion Pictures of dividing cells here threads of Chromatin containing the strands of DNA are coalescing into chromosomes at this stage a chromosome consists of two identical halves called chromatids each of the chromatids contains one of the duplicate copies of DNA made during the copy process here in the first stage of mitosis the DNA has been squeezed into compact form ready to be parceled out into what will become two new cells during this process the nucleolus disappears and the nuclear membrane breaks down radiating out from bodies called centrioles a basket of microtubules forms around the chromosomes and by a mechanism still not fully understood all of the paired chromatids line up and are pulled apart this behavior of the chromosomes is what assures each new cell to be a complete and identical set of DNA instructions [Music] in the last stage of mitosis a new nuclear membrane forms along with a new nucleolus the nuclear events in cell division are now complete in animal cells as in this protozoan furrows pinch the cells off each half with a supply of cytoplasm creating two complete new cells [Music] the nucleus is an amazing structure within it protected from the churning chemical Factory of the cytoplasm are the DNA instructions for the cell's activities including the instructions for making cell proteins and the instructions for the eventual reproduction of the cell [Music] foreign [Music]