The nucleus is a cellular organelle that is only present in eukaryotes. Its main function is to regulate gene expression; it also controls cellular growth and replication.
One of the main functions of the nucleus is to carry DNA in the form of chromatin. It also duplicates DNA. While eukaryotes have a nucleus packed with DNA, prokaryotes and archaea do not. The nucleus is usually the largest organelle present in a cell, taking up about ten percent of the space. The DNA contained in a nucleus is tightly packed into chromosomes, which must be unwound prior to cell division and DNA replication. If one were to unravel the DNA from each cell, it would be over six feet long. DNA transcription into mRNA takes place in the nucleus. Translation of the mRNA into peptides is done by ribosomes in the cytoplasm.
Inside the nucleus is the nuclelous, which creates ribosomes. Ribosomes create cell proteins. Cell proteins are used for many functions, including the organization of DNA within the nucleus. Another function of the nucleus is to store these RNA and proteins. Also, the nucleolus is responsible for rDNA transcription, mRNA oversight and senses stress.
The nucleus creates mRNA, which is necessary for protein synthesis. These pre-messenger ribonucleic acids, also known as mRNA, are an important part of the cell replication process. They move from the nucleus to the cytoplasm.
The nucleus is protected by a structure called the nuclear envelope. The nuclear envelope is impermeable to larger molecules, but nuclear pores in the envelope allow the passage of small molecules, like mRNA, into the nucleus. The main role of the nuclear envelope is to separate the nucleus from the cytoplasm. During cell division, the nuclear envelope disappears, just like the nuclelous. It creates itself again once it becomes a new cell.
Inside the nuclear envelope are nuclear pores. These pores act as a gateway between the nucleus and cytoplasm, allowing certain things like DNA and RNA and the necessary components to build DNA and RNA. Everything that should come into the nucleus has nuclear localization signals, known as NLS, and everything that should leave the nucleus has a nuclear export signal, known as NES.
Cells Without Nucleus
Not every eukaryotic cell has a nucleus. In many multicellular organisms, such as humans, the nucleus is removed naturally and is normal for the function of a particular cell. Two examples of this are human red blood cells and platelets. Since these cells do not contain a nucleus, they are unable to replicate. Other cells may contain multiple nuclei. This phenomenon is seen in some protozoa and fungi.
Overall Function of a Cell
The nucleus is just one component of a cell. Cells are considered the building blocks for all living things. In addition to the nucleus, there is the cytoplasm, mitochondria and ribosomes. The cytoplasm is closely related to the nucleus, since it is separated by the nuclear envelope from the nucleus. The mitochondria creates food for the cell, and it can replicate itself. On the other hand, ribosomes create proteins.