Eukaryotic Cell Nucleus: Structure and Function
The Nucleus: Center of Eukaryotic Cell Control
The nucleus is the central control center of the eukaryotic cell. It contains the majority of cellular DNA and genetic information for almost every gene function.
Morphology and Composition
In animal cells, the nucleus is typically spherical, while in plant cells, it tends to be more irregular in shape. The nucleus contains the majority of cellular DNA within the eukaryotic cell. DNA molecules vary in size, each forming a chromosome. The nucleus also contains a large number of proteins and varying amounts of RNA.
Structure of the Nucleus
The nucleus can be divided into four main parts:
Nuclear Envelope
The nuclear envelope is a double membrane structure. The inner membrane defines the core, while the outer membrane is continuous with the endoplasmic reticulum (ER). Pore complexes are present, which are not simple openings but have a complex ring structure involving nucleoporins and other proteins. A central communication channel allows for exchange between the nucleus and cytoplasm.
Nucleoplasm
The nucleoplasm is the internal environment of the nucleus. Its main component is chromatin. Within the nucleoplasm, there is a spherical region called the nucleolus. The nucleoplasm contains most of the cellular DNA, varying amounts of RNA, and many proteins.
Nuclear Matrix
The nuclear matrix is a network of proteins analogous to the cytoskeleton. Through its association with the nuclear matrix, chromatin is organized into specific regions. Another component is a fibrous protein that contacts the inner membrane of the nucleus.
Nucleolus
The primary function of the nucleolus is to produce ribosomes from ribosomal proteins and RNA. It is a granular structure, roughly spherical, very bright, and without membranes. It is composed of proteins and RNA. Its structure can be divided into:
- Cortex: A granular peripheral zone.
- Fibrous Central Area: Formed by nucleolar RNA and proteins.
Chromatin and Chromosomes
During interphase, DNA adopts the structure of chromatin filaments and nucleosomes. Chromatin is formed by DNA and associated proteins. The proteins associated with DNA are of two types: histones and non-histone proteins.
Histones
Histones are proteins with low relative molecular mass and are very basic. They aid in the packing of DNA by providing:
- Packages of 8 molecules (octamer).
- The strand of DNA is wrapped around the histone octamer with a fixed number of nucleotides around each octamer.
Non-Histone Proteins
Non-histone proteins are very heterogeneous and can be grouped into two categories:
- Some have a structural function.
- Others have functions related to DNA activity, such as transcription, replication, and their regulation.
When the cell prepares to divide, chromatin condenses to form chromosomes. These are further wound to form thick filaments. Non-histone proteins, called scaffolding proteins, play an important role in stabilizing this structure. They form a skeleton around which the thick filaments are structured to reach the state of chromosome condensation.
A chromosome has a narrowing called the centromere, which divides the chromosome into two equal or unequal arms. At the centromere is a structure called the kinetochore, to which microtubules can bind, playing an important role in the movement of chromosomes during mitosis. The ends of chromosomes are called telomeres and are formed by specific DNA sequences.
Types of Chromosomes
Chromosomes are classified according to the position of the centromere:
- Metacentric: The centromere is more or less centered.
- Acrocentric: The centromere position results in unequal arms.
- Telocentric: The centromere is so close to one of the telomeres that there is only one arm.