Structure and Function of the Cell Nucleus
Nucellus in Interface
Presents nuclear envelope and developed chromatin fibers. The core has a very active K and occurs for later transcription translation. Also, before division occurs, DNA duplication takes place. We also find the nucleoplasm and the nucleolus or nucleoli. When the nucleus is in division, it presents some major changes. The condensed chromatin fibers give rise to structures called chromosomes. Then, the nuclear envelope will also change. In this way, the chromosomes remain immersed in the cytoplasm. Nucleoli are also disrupted.
Features
- The most standard is that there is one nucleolus per cell.
- It can occur that sink cells join, the nuclei come together, and then form a cell with multiple nuclei. This cell is a syncytium.
- Successive divisions can take place, a nucleus without dividing the cytoplasm, forming a plasmodium.
- In plant cells, it usually has a disk and is often relegated to the periphery of the vacuole.
- In animals, it is usually spherical and in a central position.
- The size is variable, usually between 5 and 25 microns.
- For each cell type, there is a nucleoplasmic relationship, which is the relation between the volume of the nucleus and the cytoplasm. When this ratio becomes too small, cell division is induced. This value depends on the type of cell.
Nuclear Envelope
It is a double membrane separating the nucleoplasm from the cytoplasm. It presents pores that allow communication between the two media. The membrane for unwanted has 70 to 90 Å and has the same structure as the plasma membrane. There is a space between the membranes, which is the perinuclear space. Below is the internal membrane, which is similar to the other. These two membranes form from the ER, and the external membrane has attached ribosomes. The membrane serves as a fibrous union between the fibrous lamina or nuclear lamina, which is a set of fibrous proteins underneath the internal membrane. The membrane pores are 800 Å in diameter. Each pore is surrounded by a set of proteins forming the nuclear pore complex. This is made both at the top and the bottom of the membrane. There are eight ribonucleoproteins that form a kind of ring. The external diameter is 700 Å and the internal is 100 Å. The space inside is occupied by eight protein particles, leaving a conical space of 100 Å that can be blocked by another central protein. This protein regulates the passage.
Function
- Separate the nucleoplasm from the cytoplasm.
- Regulate the exchange of substances symbolically through the pores.
- As a membrane, it serves as a union between chromatin fibers, involved in the constitution of the chromosomes and also in the formation of the new nuclear envelope and the distribution of chromatin masses during nuclear division.
Nucleoplasm
The internal environment of the nucleus is a colloidal dispersion composed of water and certain molecules (enzymes, proteins, nucleotides, ions). There is also a network of fibrillar proteins resembling the cytoskeleton.
Function
Involved in DNA synthesis and RNA.
Nucleolus
A spherical corpuscle with no membrane. It is 3 microns and appears only in the nuclei. Normally there is one, but there may be two. The oocytes of amphibians have hundreds. It consists of RNA and proteins. We distinguish two zones: the fibrillar zone (made up of 45S rRNA and proteins) and the granular zone (formed by tRNA and proteins).
Function
The fibrillar zone contains areas with DNA for information leading to the synthesis of nucleolar DNA. These areas are also nucleolar organizers. Genes in these areas are repeated. They transcribe genes for different RNA polymerases, thus, different rRNAs are formed. The rRNA is seen with different lengths, giving rise to a feathery structure. The granular zone is formed by breaking the rRNA, leading to different rRNAs coupled to ribosomal proteins, forming ribosomal subunits, which then pass through the pores into the cytosol. The nucleolus is a ribosome factory.
Chromatin
This consists of DNA filaments and proteins (mainly histones). There are as many strands of chromatin as chromosomes in division. These filaments form a ball and are united in certain points with the nuclear lamina or are in contact with the core.
Two Types
- Heterochromatin: Not decondensed during interphase. It has two types:
- Constitutive heterochromatin: Condensed in all cells of the body.
- Facultative heterochromatin: Condensed in some cells but not in others.
- Euchromatin: Decondensed during interphase.
The chromatin is formed by a 100 Å fiber. The DNA is wound around histone octamers to form nucleosomes. Nucleosomes can be found loosely or in a more condensed form when they bind to histone 1.
Functions
Responsible for preserving and transmitting genetic information in the DNA. Carries the information for RNA to be transcribed and translated into proteins. Only the most widespread chromatin, euchromatin, is transcribed.