The nucleus (plural: nuclei) is a double-membraned organelle found only in eukaryotic cells (such as plant and animal cells) that acts as the cell's control centre. The nucleus is responsible for containing the genetic material (DNA), which holds the key to regulating all kinds of cellular activities.
In simple words, it’s the part of the cell that stores DNA and controls cell activities.
Structure of Nucleus
The nucleus is divided into different subparts, which are
1. Nuclear Membrane
- The nuclear envelope is another name for the nuclear membrane.
- It is a double membrane structure that differentiates the cytoplasm and nucleoplasm of the eukaryotic cell.
- The nuclear envelope consists of two lipid bilayers: an inner and an outer.
- The space present between the 2 layers is known as the perinuclear space.
- Porous structures are present over the nuclear membrane, which helps in the transportation of transcriptional factors, proteins, and RNAs.
2. Chromosome
- A chromosome is a structure that is composed of DNA and proteins present in the nucleus.
- These are the thread-like structures that are seen under the microscope at the time of cell division.
- The DNA is highly coiled over the histone protein that forms the chromatin.
- Chromatin can further divide into heterochromatin (highly condensed DNA) and euchromatin (loosely packed DNA).
- In humans, chromosomes are arranged in karyotypes based on size and shape the autosomes are numbered 1 to 22 with sex chromosomes listed separately.
3. Nucleic Acid
- Nucleic acids are large biomolecules, or macromolecules, present in all living cells.
- They are responsible for carrying and transmitting genetic information from one generation to the next and are essential for the function and survival of cells and organisms.
- Two types of nucleic acids are found: DNA and RNA.
- The DNA is made up of four building blocks called nucleotides - adenine (A), cytosine (C), guanine (G), and thymine (T)—which are linked together in a specific sequence to form a long, double-stranded helix.
- In the case of RNA, uracil (U) is present in place of thymine (T), and the rest are the same.
4. Nuclear Sap
- It is a clear, transparent, homogenous colloidal liquid enclosed in the nuclear membrane.
- The nucleoplasm is a complex solution containing ions, enzymes, nucleotides, and various other biomolecules necessary for the cell's genetic activity.
- The nucleoplasm contains genetic material in the form of chromatin, which is a complex of DNA, histone proteins, and other associated proteins.
- Within the nucleoplasm, there is also the nucleolus, which is a dense region that produces ribosomes, the cellular machines that synthesise proteins.
5. Nucleolus
- The unique structure inside the nucleus of eukaryotic cells is called the nucleolus.
- It mainly takes part in ribosome assembly, synthesises and stores RNA, and detects oxidative stress.
- RNA and proteins, which develop around particular chromosomal areas, make up the nucleolus.
- It is a significant part of the nucleus. The nucleolus is mainly composed of DNA, RNA, and proteins, as well as other elements.
Characteristics of Nucleus
The following are some of the general characteristics of a cell's nucleus:
- The nucleus is the unique characteristic of eukaryotic cells and is only present in eukaryotes. Prokaryotic cells don't have a true nucleus; instead, they have nucleoids.
- The nucleus is a double membrane-bound organelle that contains the cell's genetic material in the form of DNA, which is organised into structures called chromosomes.
- The nucleus is usually the largest organelle in the cell and is covered by a double membrane called the nuclear envelope, on which nuclear pores are present that allow the movement of certain molecules in and out of the nucleus.
- Inside the nucleus, the DNA is organised into long strands of chromatin, which can condense into discrete chromosomes during cell division.
- The nucleus also contains a nucleolus, which is a structure that produces ribosomes, the cellular machines that synthesise proteins.
- The presence of nuclear pores on the envelope of the nucleus facilitates the movement of substances between the nucleus and the cytoplasm.
- The chromosomes, which contain DNA, supply the genetic information that is necessary for the synthesis of various cell components as well as for the generation of life.
Types of Cells Based on the Nucleus
There are four main categories of cells based on presence-absence and number of nuclei; these are
- Enucleate Cell: Some cells like human RBCs lack a nucleus, while sieve tube elements lose their nucleus at maturity.
- Uninucleate Cell: Most of the plant and animal cells have only one nucleus.
- Binucleate Cell: Some cells, like liver cells, Paramecium, etc., during fusion or division pass through a phase called the dikaryotic phase in which they possess two nuclei.
- Multinucleate Cell: Some cells, like slime moulds, striated muscle cells, etc., contain more than two nuclei at a time and are called polynucleated cells.
Function of the Nucleus
The nucleus is responsible for containing the genetic material that regulates the hereditary characteristics of the cell. Other than that, there are some important functions that the nucleus performs.
- The nucleus regulates the cell's growth and reproduction, as it contains the cell's genetic information.
- Specific proteins and RNA move between the nucleus and cytoplasm through nuclear pores using transport signals.
- The genetic makeup of a cell is contained in the nucleus.
- In addition to serving as a DNA storage space, it also serves as the location of various significant cellular functions.
- In the nucleus, one's DNA can be duplicated first and foremost by the process of DNA replication.
- Cell division begins with creating two exact replicas of the parent DNA, with each new cell receiving a unique set of DNA and a nucleus.
- Secondly, transcription takes place in the nucleus. Different RNA types are produced by transcription from DNA.
- When cell division occurs, the chromatin fibres are arranged into chromosomes inside the nucleus that can be observed.
- The synthesis of ribosomes also occurs inside the nucleus.