Ultrastructure

A typical interphase nucleus is 5-25 mm in diameter and is differentiated into the following parts-nuclear envelope, nucleoplasm, nuclear matrix, chromatin, and nucleolus.

1. Nuclear Envelope or Karyotheca-It separates the nucleus from the cytoplasm. The nuclear envelope is made up of two lipoprotein and trilaminar membranes. The inner membrane is smooth. The outer membrane may be smooth, or its cytoplasmic membrane may bear ribosomes like the rough endoplasmic reticulum. Perinuclear space (100-700 Angstrom wide) is an electron transparent space that separates the two membranes of the nuclear envelope. The outer membrane is often connected to the endoplasmic reticulum.
2. Nucleoplasm-It is a transparent, semifluid, and colloidal substance filling the nucleus which contains nucleoside and a number of enzymes (e.g., DNA polymerase, RNA polymerase, nucleoside phosphorylase) required for the synthesis and functioning of DNA, RNA, nucleoproteins, etc. Some proteins present in nucleoplasm are essential for spindle formation.
3. Nuclear Matrix-It is a network of the fine fibrils of acid proteins which function as a scaffold for chromatin. It forms a dense fibrous layer called nuclear lamina below the nuclear envelope, on the periphery. In this lamina, terminal ends of chromatin fibers are embedded.
4. Chromatin-Chromatin is a hereditary DNA-protein fibrillar complex that contains DNA, RNA, and proteins. It has the ability to get stained with basic dyes. It occurs in the form of fine overlapping and coiled fibers which produce a network called chromatin reticulum. 
5. Nucleolus-A nucleolus is a naked, round, or slightly irregular structure that is attached to the chromatin at a specific region called the nucleolar organizer region (NOR). About 1-4 nucleoli are found commonly in a nucleus. A covering membrane is absent around the nucleolus.
6. Nuclear lamina-The nuclear lamina is a structure near the inner nuclear membrane and the peripheral chromatin. It is composed of the lamina, which is also present in the nuclear interior, and lamin-associated proteins. Its function is to maintain the stability of the nucleus, organize chromatin and anchor nuclear pore complexes.
7. Nuclear bodies-Nuclear bodies (also known as nuclear domains, or nuclear dots) are membrane-less structures found in the cell nuclei of eukaryotic cells. Nuclear bodies include Cajal bodies, the nucleolus, and promyelocytic leukemia protein (PML) nuclear bodies (also called PML oncogenic dots).
8. Nuclear pores-The nuclear pore is a protein-lined channel in the nuclear envelope that regulates the transportation of molecules between the nucleus and the cytoplasm. In eukaryotic cells, the nucleus is separated from the cytoplasm and surrounded by a nuclear envelope. This envelope safeguards the DNA contained in the nucleus.

The cell is the basic structural and functional unit of living organisms. In biology, the cell has the same central position as that of an atom in the physical sciences. The bodies of living organisms are made up of microscopic units called cells. Cells have evolved a variety of different lifestyles. Many organisms, such as bacteria (both archaebacteria and eubacteria), protozoa(e.g., amoeba), and yeasts consist of single cells (called unicellular organisms) that have the ability to perform a function like self-replication. More complex organisms, called multicellular organisms, those consists of collections of cells that perform particular functions.

Term cell was discovered by Robert Hook in his book Micrographia. The size of the cell is 10-16 micrometers. The shape of the cell may be polygonal, disc-like amoebic, thread-like, cuboid or irregular, oval, hexagonal, circular, branched, elongated, etc. The cells are too small to be seen with naked eyes. So, it is studied with the help of microscopes.