The cytoplasm of the eukaryotic cell encompasses the matrix inside the cell membrane and outside the nucleus. The cytoplasm is the substance in which various cellular components are found. A major difference between eukaryotic and prokaryotic cytoplasm is that eukaryotic cytoplasm has a complex internal structure, consisting of exceedingly small rods called microfilaments and intermediate filaments and cylinders called microtubules. Together, they form the cytoskeleton.
The cytoskeleton and other cytoplasmic components are held together by a three-dimensional membrane of fine filaments called the microtrabecular lattice (Thomas, 2004). It provides support and shape, organizes chemical reactions that occur in the cytoplasm, and assists in transporting substances through the cell and even moving the entire cell, as in phagocytosis. The movement of eukaryotic cytoplasm from one part of the cell to another, which helps to distribute nutrients and move the cell over a surface, is called cytoplasmic streaming (Ferris & Beveridge, 2005). E. Nucleus
The most characteristic eukaryotic organelle is the nucleus. The nucleus is usually spherical or oval, is frequently the largest structure in the cell, and contains almost all of the cell’s hereditary information (DNA). Some DNA is also found in mitochondria and chloroplasts of photosynthetic organisms. The nucleus is separated from the cytoplasm by a double membrane called the nuclear envelope. Each of the two membranes resembles the plasma membrane in structure. Minute pores in the nuclear membrane allow the nucleus to communicate with the membranous network in the cytoplasm, called the endoplasmic reticulum.
Substances entering and exiting the nucleus are believed to pass through the tiny pores (Henrichsen, 2003). Within the nuclear envelope is a gelatinous fluid called nucleoplasm. One or more spherical bodies called nucleoli are also present (singular is nucleolus). These structures are a center for the synthesis of ribosomal RNA, an essential constituent of ribosomes. Finally, there is the DNA, which is combined with a number of proteins, including several basic proteins called histones and nonhistones.
The combination of about 165 base pairs of DNA and 9 molecules of histones is referred to as a nucleosome. When the cell is not reproducing, the DNA and its associated proteins appear as a threadlike mass called chromatin. Prior to nuclear division, the chromatin coils into shorter and thicker rodlike bodies called chromosomes. Nucleosomes are the basic structural units of chromosomes (Ferris & Beveridge, 2005). Eukaryotic cells divide by two elaborate mechanisms called mitosis and meiosis.