THE NUCLEUS
The nucleus is the 'control center' of the cell. It primarily is composed of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), and sulfur (S) in some proteins (particularly enzymes). The nucleus contains nucleic acids, DNA, and RNA, which in turn contain C, H, O, N, and P. The nucleus controls cell functions via DNA, which contains the instructions. The DNA can be compared to computer software, controlling the cell organelles, which are comparable to computer hardware. DNA provides the code for protein synthesis. This code is communicated by messenger RNA (mRNA) to the protein-manufacturing organelles: ribosomes.

When DNA is used to produce mRNA, it is stretched out in the form of chromatin so its code can be 'read.' When it is moved during cell division (mitosis followed by cytokinesis), it is coiled and packed in the form of chromosomes.

The nucleus consists of a nucleoplasm, similar to the cell's cytosol. The nucleoplasm contains primarily water and dissolved organic compounds used in the synthesis of DNA and RNA. The nucleoplasm is bounded by the nuclear envelope, a double-membrane structure with pores that enable the transport of materials into and out of the nucleus. The phospholipids of the nuclear envelope contain C, H, O, and P.

The nucleolus is the site of ribosomal RNA synthesis and ribosome subunit assembly. Ribosomes are sent to the cytoplasm to carry out protein synthesis. Ribosomes can attach to the endoplasmic reticulum (ER) to form rough ER, or can be in the cytoplasm, depending on the proteins needed.