Diagram of nucleus. (Source: Wikimedia Commons)
Episode 2 - The Nucleus and Transcription
The nucleus is a membrane bound structure found only in eukaryotic cells that contain most of the DNA of that cell (Mitochondria and chloroplasts also contain DNA, remember). It is the distinguishing feature of eukaryotes from prokaryotes.
Diagram of nucleus. (Source: Wikimedia Commons)
A cell’s structure and function begin with proteins, and DNA contains instructions (base pair triplets called codons) for building these proteins; Enzymes in the nucleohyaloplasm (like RNA polymerases) transcribe the DNA into messenger RNA that contains protein-building instructions.
The nucleus serves two important functions: It stores all the DNA molecules away from the metabolic machinery of the cytoplasm, which makes the DNA easier to duplicate when time comes for cell division; and it controls the exchange of substances between the cytoplasm and the DNA.
The genetic materials contained inside the nucleus are suspended in the nucleoplasm, or the nucleohyaloplasm, the structure-less fluid inside the nucleus. DNA is separated from the cytoplasm by the double phospholipid bilayer of the nuclear envelope. Nuclear pores in the nuclear envelope allow communication between the cytoplasm and the nucleus. The nucleus also contains the nucleolus, which is where the ribosomal subunits are assembled from rRNA.
The genetic materials, more commonly known as the chromatin, are attached onto the inner surface of the nuclear envelope.
In addition, the rough ER is attached directly and the smooth ER is attached indirectly to the nucleus, therefore the nucleus continuously uses proteins, produced by the ERs.
The inside of the nuclear membrane is reinforced by a layer of intermediate filaments making up the nuclear lamina.
During cell division, the chromosomes making up the chromatin duplicate themselves into sister chromatids attached at their centromere, and then condense to become visible. Next, microtubules attached to centrosomes penetrate the disintegrating nuclear envelope, and attach onto a microtubule organizing center (MTOC) called a kinetochore in the centromeres of the duplicated chromosomes. The sister chromatids are thus pulled apart, and forms the nuclei of two new daughter cells.
HeLa cells stained for DNA with the Blue Hoechst dye. The central and rightmost cell are in interphase, thus their entire nuclei are labeled. On the left a cell is going through mitosis and its DNA has condensed ready for division. (Source: Wikimedia Commons)
Transcription
In terms of cellular processes occurring the nucleus, we're hard pressed to find something more important to discuss than transcription. Using polymerase, helicase, and other specialized proteins, the DNA double helix is unzipped and read in order to produce messenger RNA - an RNA copy of the gene the cell wishes to express.
The mRNA copy can then be modified to remove introns (non-coding regions), and it is usually capped and given a poly adenine tail to protect it from degradation in the cytosol and to allow recognition by ribosomes for translation.
Nucleolus
The nucleolus may be found in the nuclei of mature, non-dividing eukaryotic cells; sometimes more than one of them may be observed. The nucleolus is seen as a dense, circular structure that has a darker color than the rest of the nucleus, and is not enclosed in membrane. It is the site in which rRNA is transcribed from DNA.
The nucleolus also imports proteins from the cytoplasm and combines them with the rRNA to make small and large ribosomal subunits.