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6.11.2. The interphase nucleus

Core structure

The nucleus of a typical cell has the following components:

Nuclear envelope (or karyotheque)

The nucleus is delimited by a double membrane, which leaves between them an intermembrane or perinuclear space. In some areas, both membranes merge leaving a space or pore (pore complexes).

It presents the following structure:

  • The outer nuclear membrane is 70 to 80 Å thick. On its cytoplasmic side, it has ribosomes attached by the largest subunit. The perinuclear space and the outer nuclear membrane are continuous with the membrane of the endoplasmic reticulum, and in cells with little endoplasmic reticulum, they fulfill their function.
  • The perinuclear or intermembrane space is between the two nuclear membranes, and is 100 to 200 Å thick, although in some places it can present dilations of up to 700 Å. It communicates directly with the lumen of the endoplasmic reticulum.
  • The inner nuclear membrane is associated with it, on the face of the nucleoplasm, a network of protein fibers called the nuclear lamina or nuclear cortex. This network of intertwined protein filaments has the function of serving as an anchoring point for chromatin and regulating the growth of the nuclear envelope.

The nuclear pores are protein complexes that cross the nuclear envelope. Pores form or disappear depending on the functional state of the cell. They regulate the exchange of molecules between the nucleus and the cytosolWater-soluble molecules pass freely through the pores, and macromolecules such as RNA or proteins, which are not water-soluble, intervene in active transport mechanisms.

The nuclear envelope appears in the telophase from cisternae of the endoplasmic reticulum organized by the nuclear lamina.

Functions of the nuclear membrane

  • Protects DNA from the action of cytoplasmic enzymes and cytoskeletal movements.
  • It separates the RNA transcription and maturation process that occurs in the nucleus, from the translation process that occurs in the cytoplasm, which allows better control and regulation of both processes.
  • Regulates the bidirectional traffic of substances through the pores.

Nucleoplasm (karyoplasm or nuclear matrix)

The internal environment of the nucleus is called the nucleoplasm or nuclear matrix, and it is similar to the cytosol. It is formed by a colloidal dispersion composed of a great variety of immediate principles such as: protides (amino acids, peptides, histones, protamines and enzymes involved in the transcription and replication of DNA ...), nucleic acids (nucleotides, nucleosidestRNAMRNA, rRNA ...), lipids (proteolipids), carbohydrates ( glycogen), mineral salts, and ions.

Within this matrix are the nucleolus  and chromatin, separated by a three-dimensional protein network, similar to the cytoskeleton, which extends throughout the nucleus.

The function of the nuclear matrix is  not only structural, since many enzymes are associated with it.

Nucleolus

It is a spherical structure, located inside the nucleus, which lacks a membrane. There is usually one per cell, although some cells may have more ( amphibian oocytes, for example, contain more than a thousand nucleoli).

The nucleolus is where all types of rRNA are synthesized. The main function of the nucleolus is the synthesis of ribosomes , transcribing ribosomal RNA to form its components. It is related to the synthesis of proteins. In cells with very active protein synthesis there are many nucleoli.

The nucleoli are more prominent at the interface, and disappear when the chromosomes are condensing. It disappears in prophase and reappears in telophase.

Video: Difference between nucleus and nucleolus.

Cromatin

The genetic material of the interphase nucleus is called chromatinEach chromatin fiber is made up of DNA associated with histones and other non-histone proteins. Chromatin has already been described in the topic of nucleic acids. It is called chromatin because it is stained with dyes such as acetic carmine or acetic orcein. When the cell divides, chromatin condenses and forms chromosomes, so it can be said that chromatin and chromosomes are two different states of the same substance. They are different for these reasons:

  • At the interface, the synthesis of proteins and RNA, and the duplication of DNA, takes place, so it is necessary that the genetic information is accessible, and the genetic material will be found extended (chromatin).
  • In cell division, genetic material is distributed evenly to daughter cells. The material will be found individualized chromosomes.

Fundamental ideas about the core

Some of the most important parts of the nucleus are the following:

1- Nuclear pore. 2-Nuclear membrane; 3-Euchromatin; 4- Heterochromatin; 5- Nucleolus.

Core structure in interface

    • Nuclear envelope (or karyote) .
      • Double membrane with an intermembrane space crossed by nuclear pores.
      • It appears in telophase .
    • Nucleoplasm (karyoplasm or nuclear matrix). Similar to cytosol, it contains:
      • Nucleolus .
        • Here the rRNAs are synthesized.
        • Function: synthesis of ribosomes.
        • It disappears in prophase and appears in telophase.
      • Chromatin .
        • When the cell divides, it condenses to form chromosomes.
        • Two types of chromatin:
          • The heterochromatin or dense chromatin (with fragments of DNA that are not transcribed).      
          • The  euchromatin  or  chromatin diffuse (with DNA fragments are transcribed).