Eukaryotic cells protect their DNA in the nucleus. It contains most the genes of eukaryotic cells except some are located in mitochondria and chloroplast. It is about 5µm in diameter.
Nucleus has double-membrane system. It composes of two phospholipid bilayers where numerous protein molecules are embedded inside the bilayers. It surrounds the fluid portion of nucleus, called nucleoplasm. The nuclear envelope has pores to control the movement of large substances cross the nuclear envelope. It allows the water solubles substances to move through.
Nucleolus (plural, nucleoli) is the dense mass material when look under microscope. One or more nucleoli form inside the nucleus. Protein and RNA molecules are synthesized here. They form the subunit of ribosomes. The subunits pass through nuclear pores and reach cytoplasm where the translation (synthesis of protein) happen.
DNA is organized into discrete units called chromosomes. It carries genetic information. Chromatin, protein and DNA make up of chromosome. Thin chromatin fibres coil up and become dense and thick when the cell starts to divide. Each eukaryotic species has a characteristic number of chromosomes. For example, 46 chromosomes from human cell and 8 chromosomes in fruit fly cell.