|MadSci Network: Cell Biology|
Well, that's a pretty complicated question, and the subject of more than 100 years of research, but I'll give it a shot. The nucleus is usually the most prominent structure within a eukaryotic cell. (Bacteria, or prokaryotes, don't contain a nucleus at all.) The nucleus is separated from the rest of the cell by a double lipid bilayer known as the nuclear membrane. The nucleus has two main functions in the cell. First, the nucleus is the carrier of herditary information. The chromosomes, which are composed of DNA, are found within the nucleus. Each time the cell divides, the DNA information is copied and passed on the the new cells. Second, the nucleus is the supply center for the cell. The nucleus ensures that the complex molecules the cell needs to function are supplied in the amounts and variety needed. There are many experiments that confirm the role of the nucleus. In one simple experiment, a nucleus was removed from an amoeba. The amoeba stopped dividing and died within a few days. But, if a new nucleus was implanted, the single celled animal lived and divided normally. This early experiment let researchers know that the nucleus was essential for life of the cell, and for cell division. In fact, such experimental findings underlie the fields of cell biology, genetics and molecular biology. So, how does the nucleus act as the supply center for the cell? The chromosomal DNA in the nucleus contains genes. Genes are the functional units of DNA. The DNA is a complete set of instructions for making all the proteins a cell will ever need. Different genes are activated in different cells, creating the specific proteins that give a particular cell type its characteristics. The nucleus regulates the timing and amount of the proteins that are made. It does this by turning the genes off and on. Turning a gene on is called transcription. Transcription is the process by which a DNA gene is copied into an RNA message. These RNA messages exit the nucleus and are translated on the ribosomes into proteins. How does the nucleus know which genes to turn on and off? The cell senses its environment. Specialized cellular proteins called receptors detect hormones, small molecules, and even ions in the fluids surrounding the cell. By the process called signal transduction, proteins in the cell pass the information about the cell environment along to the nucleus. Signal transduction is like a relay race. For example, a cell surface receptor protein might detect a growth factor. The receptor will pass along the information that it has "seen" a growth factor outside of the cell. The passage of information is like one runner passing a baton to another runner. In this case, the "baton" is a high energy phosphate group. The baton (phosphate group) gets passed along from protein to protein, until it gets into the nucleus and is passed to a transcription factor. The transcription factors then work together to activate appropriate genes to make the cell grow, or deal with any other situation the cell experiences. For more information, I suggest you check out a textbook such as Molecular Biology of the Cell (Alberts). Hope that was helpful. Erin
Try the links in the MadSci Library for more information on Cell Biology.