DNA is a molecule that carries the genetic instructions used in the growth, development, functioning, and reproduction of all known living organisms. These sets of instructions are contained mostly in a cell’s nucleus, where they are arranged in combination with proteins called histones to form a macromolecular structure known as a chromosome. Sections of DNA that carry the instructions for particular functions are called genes.
Because DNA instructions influence all activities of a living cell, the nucleus is thought of as the control center of the cell. DNA undergoes a process called genetic transcription, during which DNA is copied (or encoded) into a similar molecule known as RNA. The DNA is transcribed one gene at a time. If the gene encoded happens to be a protein, then the transcription process produces messenger RNA (mRNA). Otherwise, it produces non-coding RNA (microRNA), ribosomal RNA (rRNA), or transfer RNA (tRNA). The transcription and the resulting gene product together are called gene expression.
In the case of mRNA production, mRNA is transported out of the cell nucleus and into the cytoplasm – the gel-like substance that holds the components of a cell together. Here it is decoded by a ribosome to construct a specific amino acid chain. This chain becomes an active protein that performs the function in the cell originally encoded for by the DNA. This second half of the process is called translation.
The main function of the nucleus is to regulate gene expression and facilitate the correct replication of DNA during the cell’s reproductive cycle. The cell’s nucleus is simply the majority of its DNA enclosed by a membrane called the nuclear membrane. All eukaryotic cells have a nucleus, and so have a nuclear membrane. Communication inside a cell is accomplished by the chemical reaction of certain cell components to specific types of proteins and chemical compounds or molecules; essentially, these special substances trigger a response in the components. If organelles were not enclosed by membranes, especially the nucleus, there would be a much greater chance for chaotic gene expression and cell activity. The nuclear envelope allows a cell to regulate gene expression and all cell activity more efficiently and accurately, as the DNA instructions intended.
The nuclear membrane is a fatty, porous two-layer envelope that contains most of the DNA of the cell, the nucleoplasm (similar in composition and function to the cytoplasm outside the nucleus) and the nucleolus. The outer layer of the nuclear membrane is directly connected to the endoplasmic reticulum, the structure just outside of the nucleus, while the inner layer is bordered by the nuclear lamina – a network of fibers and structural proteins that give the membrane mechanical support and even help regulate DNA replication and cell division. The nuclear membrane is porous throughout, allowing for exchange of proteins and other materials with the rest of the cell and facilitating the movement of ribosomes.
DNA serves as the instructions that a living cell follows, which dictates all its activities. The timely communication of these instructions to the rest of the cell is important. The existence of a nucleus provides order and efficiency to this process, reducing the chance that an error might occur. The nucleus remains intact as an organelle itself due in large part to its membrane. The nuclear envelope is a lipid (fatty protein) bilayer membrane that provides structural support, keeps the nucleus intact, and helps the nucleus in controlling gene expression and regulating all cell activity.