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What Does a Nucleus Do?

The most obvious internal structure seen by early microbiologists was the nucleus. Although the function of the nucleus was yet to be determined, cells could easily be distinguished by the presence or absence of a nucleus. Cells without a nucleus were termed prokayotic, and cells with a nucleus were termed eukaryotic. Later, scientists began to understand the function of the nucleus as a repository for the cell's DNA. Within the nucleus, the DNA is replicated to make copies for daughter cells and transcribed into RNA for eventual translation into proteins.

The Nucleus Basics
- The nucleus is a universal feature of organisms with eukaryotic cells, albeit some organisms such as Giardia have two nuclei, and some specialized cells such as mammalian red blood cells are absent a nucleus. This specialized organelle consists of a membrane surrounding the genetic material of the cell. By contrast, prokaryotes, which include Eubacteria and Archaeabacteria, lack a membrane-bound nucleus, though even these organisms concentrate their genetic material into a region of their cell called the nucelolus. It should be noted that organelles such as chloroplasts and mitochondria possess their own genetic material independent of the DNA in the nucleus.
DNA
- DNA is the genetic blueprint for the cell, a code book for the variety of proteins used throughout the cell. The nucleus helps preserve the DNA and functions much as the reference section of a library. Pieces of the code may be copied and carried out of the nucleus, but the DNA itself remains in the nucleus just as pages of a reference book may be copied and the copies carried out of the library. The process of transferring these messages is collectively called transcription and the subsequent interpretation of these messages is called translation. Additionally, a process known as replication allows copies of the DNA to be synthesized with each copy of the DNA subsequently passed on to daughter cells during cell division.
Replication
- Since DNA is the instruction manual for the cell, it is important for a complete copy of the DNA to be made prior to cell division. Even a slight error can have dramatic and even fatal consequences to the daughter cell. While the enzyme DNA polymerase may garner much of the attention since it makes the actual copy of the DNA, other enzymes also play a role in unzipping and re-zipping the DNA. Additional enzymes serve a role as proofreaders helping to reduce the risk of errors in the copy. In animal cells, the nuclear membrane degrades during cell division to facilitate the transfer of each copy of DNA to the prospective daughter cells.
Transcription
- Interpreting the code within the nucleus to create proteins involves two processes called transcription and translation. During transcription, a section of DNA containing a particular gene is translated into messenger RNA (mRNA) which passes out of the nucleus through pores in the nuclear membrane. While this may seem analogous to photocopying a page in a book, in eukaryotes the mRNA contains introns--sections of DNA that must be removed prior to translation. Removing the introns involves additional enzymes to cut the mRNA and remove the intron and then reconnect the mRNA fragments
Translation
- Outside the nucleus, translation occurs using the mRNA to sequence the amino acids in the protein. Each protein is coded for a separate gene within the nucleus. The complexity of this transcription and translation is illustrated by the fact that the cell contains a complete copy of the genetic material but may only need a specific portion of it as in the case of specialized cells or need certain sections only at particular times. The nucleus not only stores all this information, but it also allows the appropriate section to be accessed when needed, though the exact mechanisms for this are not yet fully understood.

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