Cell Nucleus: Structure and Functions
The Cell Nucleus
The cell nucleus is the largest organelle of the eukaryotic cell. It is a small spherical or oval body and is usually placed in the center of the cell but can also be located in the periphery. Prokaryotic cells have no nuclear membrane, unlike eukaryotes, where it exists and also has the structures explained below:
Structure
In its structure, we can appreciate:
- Nuclear envelope
- Chromatin
- Nucleolus
Nuclear Envelope
This can include:
- Outer membrane: It is continuous with the endoplasmic reticulum (ER) and can be covered with polysomes (clusters of five or six ribosomes).
- Inner membrane (smooth appearance)
- Nuclear Lamina (fibrous filaments that support): They are vital to the reorganization after mitosis.
- Nuclear pores: These are formed where the inner and outer membranes unite. They are flower-shaped structures consisting of eight sections in the manner of petals. At its apex, there is a small hole. The pores allow the exchange of substances between the nucleus and cytoplasm.
Chromatin
During interphase, the genetic material is in a state of chromatin. The chromatin filaments are long, thin, and fine genetic material.
Within the nucleus, chromatin can be appreciated in two ways:
- Heterochromatin: Corresponds to a higher level of chromatin condensation while remaining chromatin. It is usually placed near the nuclear envelope and is transcriptionally inactive.
- Euchromatin: It is more abundant, has a lower level of compression, and is believed to be transcriptionally active during interphase.
We must remember that there is only chromatin during interphase, and at the initiation of mitosis, it is condensed on chromosomes to enable mobility with ease.
Nucleolus
The most obvious substructure within the nucleus is the nucleolus. This is due to the large amount of DNA and RNA that it contains. Here, rRNA (ribosomal) transcription occurs, which produces ribosomes. When salts are partly constituted, the nucleus moves to the cytoplasm where their training ends. Then there is the ARNn (nucleolar), which originates from different segments of DNA, one of which is called the nucleolar organizer region (RON). This ARNn is associated with cytoplasmic proteins, many of which make up the ribosome.
In the nucleolus, we can see different areas:
- Granular zone: Ribosomal subunits near completion.
- Fibrillary Area: Places where RON is surrounded by dense filaments.
Protein Synthesis
Protein synthesis requires 3 types of molecules:
- mRNA (transmits information from DNA to the cytoplasm)
- rRNA (constitutes a major part of the ribosome where synthesis occurs)
- tRNA (puts in place the amino acid in the polypeptide chain that is growing)
First, the mRNA is created using DNA from the nucleus. Before leaving the cytoplasm, the mRNA undergoes a maturation called splicing, where the sequences of bases that are certain genes are eliminated (introns) and others are conserved and rearranged (exons). The mRNA is directed to the ribosomes where it binds to tRNA, which possesses amino acids for protein synthesis. Once in the ribosomal protein, synthesis occurs.
Core Functions
- At the core are kept genes on chromosomes (in mitosis) and chromatin (during interphase).
- Organizes genes on chromosomes, which allows cell division.
- It transports regulatory factors and genes through nuclear pores.
- Produce messages (mRNA) that encode proteins.
- Produce ribosomes in the nucleolus.