#cellstructure

•𝐂𝐄𝐍𝐓𝐑𝐈𝐎𝐋𝐄

Centrioles are found only in animal cells, generally located near the nucleus. Centrioles occur in pairs. They are composed of nine sets of triplets, all arranged lengthwise, creating a cylindrical structure.

Not all cells contain centrioles, but most animal cells have them, and they are always composed of the same typical pattern: a cylinder formed by nine sets of triplets that, when cut in a transverse section (cross-section), looks like a pinwheel.

•𝐍𝐔𝐂𝐋𝐄𝐔𝐒

The nucleus contains most of the genes in the eukaryotic cell. (Some genes are located in mitochondria and chloroplasts.) It is generally the most conspicuous organelle, averaging about 5 μm in diameter. The nuclear envelope encloses the nucleus, separating its contents from the cytoplasm. The nuclear envelope is a double membrane. The two membranes, each a lipid bilayer with associated proteins, are separated by a space of 20–40 nm.

The envelope is perforated by pore structures that are about 100 nm in diameter. At the lip of each pore, the inner and outer membranes of the nuclear envelope are continuous. An intricate protein structure called a pore complex lines each pore and plays an important role in the cell by regulating the entry and exit of proteins and RNAs, as well as large complexes of macromolecules.

Except at the pores, the nuclear side of the envelope is lined by the nuclear lamina, a netlike array of protein filaments that maintains the shape of the nucleus by mechanically supporting the nuclear envelope. There is also much evidence for a nuclear matrix, a framework of protein fibers extending throughout the nuclear interior. The nuclear lamina and matrix may help organize the genetic material so it functions efficiently.

Within the nucleus, the DNA is organized into discrete units called chromosomes, structures that carry the genetic information. Each chromosome contains one long DNA molecule associated with many proteins. Some of the proteins help coil the DNA molecule of each chromosome, reducing its length and allowing it to fit into the nucleus.

The complex of DNA and proteins making up chromosomes is called chromatin. Chromatin inside a eukaryotic cell nucleus can be classified into two major types based on its state of condensation: heterochromatin and euchromatin.

A prominent structure within the nondividing nucleus is the nucleolus (plural, nucleoli), which appears through the electron microscope as a mass of densely stained granules and fibers adjoining part of the chromatin.

The nucleus is involved in both mitosis and meiosis. Several events required for cell division occur in the nucleus, including DNA replication and transcription of DNA into precursor RNA molecules.

All of the enzymes required for replication and repair of newly synthesized DNA, as well as for transcription and processing of precursor RNA molecules, are found in the nucleus.

•𝐄𝐍𝐃𝐎𝐏𝐋𝐀𝐒𝐌𝐈𝐂 𝐑𝐄𝐓𝐈𝐂𝐔𝐋𝐔𝐌

The endoplasmic reticulum (ER) is such an extensive network of membranes that it accounts for more than half the total membrane in many eukaryotic cells. (The word endoplasmic means “within the cytoplasm,” and reticulum is Latin for “little net.”) The ER consists of a network of membranous tubules and sacs called cisternae (from the Latin cisterna, a reservoir for a liquid). The ER membrane separates the internal compartment of the ER, called the ER lumen (cavity) or cisternal space, from the cytosol. And because the ER membrane is continuous with the nuclear envelope, the space between the two membranes of the envelope is continuous with the lumen of the ER.

There are two varieties.

• 𝐑𝐨𝐮𝐠𝐡 𝐄𝐑 is the site of protein synthesis resulting from the attached ribosomes.

• 𝐒𝐦𝐨𝐨𝐭𝐡 𝐄𝐑 has three functions:

1. Assists in the synthesis of steroid hormones and other lipids

2. Connects rough ER to the Golgi apparatus

Learn about what is the cell