The pores cross both nuclear membranes, providing a channel through which larger molecules must be actively transported by carrier proteins while allowing free movement of small molecules and ions. The nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression-the nucleus is, therefore, the control center of the cell.īecause the nuclear envelope is impermeable to large molecules, nuclear pores are required to regulate nuclear transport of molecules across the envelope. The genes within these chromosomes are structured in such a way to promote cell function. Nuclear DNA is organized as multiple long linear molecules in a complex with a large variety of proteins, such as histones, to form chromosomes. The cell nucleus contains all of the cell's genome, except for the small amount of mitochondrial DNA and, in plant cells, plastid DNA. The main structures making up the nucleus are the nuclear envelope, a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm and the nuclear matrix (which includes the nuclear lamina), a network within the nucleus that adds mechanical support, much like the cytoskeleton supports the cell as a whole. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells, have no nuclei, and a few others including osteoclasts have many. nuclei from Latin nucleus or nuculeus, meaning kernel or seed) is a membrane-bound organelle found in eukaryotic cells.
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