A plant cell nucleus is a specialized structure, which stores the genetic information and monitors the cellular activities. Plants are the monokaryotic cells, which possess an individual nucleus. A well-developed nucleus is only found in the advanced or multicellular organisms, those having division of labour.
The number of the nucleus varies among different groups of organisms, but a plant cell only possesses a single nucleus. A plant cell nucleus has a complex structure that includes a spherical nucleolus, chromatin threads, nucleus sap and nuclear pores within the nuclear envelope.
Content: Plant Cell Nucleus
Nucleus can define as the core of eukaryotic cell that directs the overall cell’s activity important for the cell growth and differentiation. It appears as a kernel of the stone fruit. A nucleus has a two concentric membranes whose primary function is to safeguard the DNA from the reactions occuring in the cytoplasm. But, it can communicate with the cytoplasm via nuclear pore complex that allows selective import and export of the materials within the plant cell.
A plant cell nucleus appears spherical and located centrally. It constitutes 10% of the plant cell volume. The nucleus of the plant cell is surrounded by a double-layered membrane or nuclear envelope, within which nucleolus, chromatin material and karyoplasm are present. A nuclear envelope protects the contents within the nucleus and also differentiates the nucleoplasm contents from the cytoplasmic contents.
Nuclear pores on the nuclear envelope also facilitate nuclear and cytoplasmic exchange within the plant cell. A chromatin material contains the genetic information and the nucleolus participates in the synthesis of ribosomes.
Parts of the Plant Cell Nucleus
The plant cell nucleus is the specialized cell-organelle that contains chromatin, nucleolus and nucleoplasm enclosed by a layer of the perforated nuclear envelope.
Chromatin carries the DNA in a much-condensed form, because of the large content of the DNA that carries gene coding both structural and non-structural proteins. Therefore, in eukaryotic cells like plants, the DNA is closely packed like the beads (histone protein) around a strand (DNA).
Histones are the DNA-binding proteins that keep the DNA intact, by which the chromatin appears as the condensed fibres. Packing a huge content of genetic material into a small nucleus is an incredible way of packaging.
At the time of DNA packaging, each DNA strand is coiled around the small bead-like structures, or protein molecules called histones. The configuration formed by the twisting of DNA (two turns) around the histone octamer is termed as nucleosome.
Chromatins are the semi condensed structures. On microscopic observation, it appears as a “beads on a string”. The nucleosomes are condensed by a factor of six and later coiled even more by a factor of 40 , by which the localization of DNA takes less space.
A DNA carries a negative charge, which is neutralized by the positively charged histone proteins. A chromatin material generally exists as two forms, depending upon the gene transcription.
It is a highly constricted form that is transcriptionally inactive. This form of chromatin exists as constitutive or facultative heterochromatin. The constitutive heterochromatin remains compressed during the cell cycle, while the facultative heterochromatin can transform into euchromatin. It is late replicative, i.e. the heterochromatin DNA replicates later in the cell cycle.
It is a less compressed form that is transcriptionally active, and it only exists as a constitutive form. It is early replicative that replicates earlier in the cell cycle.
We sometimes get confused between the two terms (chromatin and chromosome), but there is a difference between these two. Chromatin transforms into a particular type of condensed form (Chromosome) at the time of metaphase (the cell prepares to split).
Firstly, the chromatin strands make copies of itself via DNA replication. Later, the chromatin material becomes highly compressed to the degree of a 10,000-fold and transform into chromosomes. After cell division, the chromosomes separate, giving each cell a complete blueprint of genetic information.
It appears as a spherical mass, which lacks a membrane around it. Nucleolus plays a significant role in building up ribosome subunits, which in turn helps in protein synthesis. On microscopic observation, it appears as a small patch inside the nucleus. Nucleolus is only noticeable after a cell divides. Therefore, it functions as a site that provides necessary ribosomal proteins and rRNAs for the ribosome biogenesis. In majority of the plant cells, this region contains a high concentration of iron.
It appears as the homogeneous and colloidal liquid of variable consistency. It contains an emulsion of various components like nucleoproteins, nucleic acids, proteins, enzymes etc.
It is a bi-layer membrane surrounding the nucleoplasm along with contents like nucleolus and chromatin threads. It keeps the nuclear content intact and differentiates it from the cell cytoplasm. There is the perinuclear space between the nuclear envelope separating the two concentric membranes. The perinuclear space of the nuclear envelope has a width of 10-20nm.
The outer lining of the nuclear envelope is united with the rough endoplasmic reticulum (RER). The inner layer is lined by the protein fibres (called nuclear lamina), which anchors the nuclear components. Therefore, nuclear envelop guards the genetic material and disintegrates at the time of mitosis.
As we have discussed earlier, a nuclear envelope is punctured with several cavities called nuclear pores. Nuclear pores are surrounded by the large complex of proteins that regulates the nuclear and cytoplasmic exchange of the specific materials within the plant cell.
The nuclear pores permit free diffusion of polar molecules, metabolites and ions within the cell, by blocking the majority of the large molecules. Thus, the nuclear pore complex provides a channel for the back and forth exchange of material within nucleoplasm and cytoplasm instead of a barrier, i.e. nuclear envelop.
Large biomolecules can also enter into and exit from of the nucleus, which carries specific amino-acid sequences that represent they belong in the nucleus. These particular sequences are called nuclear localization signals or nuclear import sequences, which label specific proteins that can be taken up by the nucleus.
Therefore proteins (like histones) can enter the nucleus for the packaging of DNA, from the cytoplasm. Similarly, RNA transcript and the precursors of protein possess nuclear export sequences that can be taken up by the cell cytoplasm.
The nucleus is a room allotted for the storage of genetic material, which performs a significant role inside a plant cell:
- It anchors the genetic material (DNA) inside the chromatin fibres and keep it safe from the cytosolic reactions via a bilayer nuclear membrane.
- Secondly, nucleus directs all the cellular activities occurring inside the cell, such as cell replication, metabolism, growth, and differentiation etc.
- Nucleus also decides the genetic attributes of the cell, as it stores the hereditary material within itself.
- It also acts as a site of transcription, where the DNA is transcribed into mRNA that translates the blueprint of the information into the proteins by the association of tRNA and rRNA.
- Nucleolus within the nucleus has a significant role in synthesizing the protein-producing units called ribosomes.
- It also facilitates to and fro transportation of the selective molecules and regulatory factors through the small passage termed as nuclear pores.
Therefore, we can conclude that the nucleus of the plant cell possesses a complex structure that contains the nuclear contents very precisely inside a small nucleus. Apart from the small size of the nucleus, it acts as a command centre for all cellular activities that decides the cell physiology, morphology, growth and division.