The nucleus is usually the most conspicuous organelle of a eukaryotic cell. However, a well-defined nucleus is absent in prokaryotic cells. The nucleus is the repository of the genome and the source of informational macromolecules that govern the cytoplasm’s synthetic activities.
A Dutch Microscopist, Antonie van Leeuwenhoek observed the nucleus as a centrally located clear area in amphibian and bird blood cells in 1710.
Fontana (1781) recorded an elliptical shape in each eel epidermal cell.
The term nucleus was first used by Robert Brown (1831) to describe the main body in orchid cells. He said the nucleus is a regular feature of cells.
Most cells contain a single nucleus, known as mono or uninucleate cells. Cells with two nuclei are known as binucleate cells, e.g., Paramecium. Sometimes more than two nuclei are present in a single cell. Such cells are called polynucleate or multinucleated cells. e.g., Vaucheria
Shape & Size
Generally spherical or round, but can be ellipsoidal, fusiform, convoluted, lobbed, and branched.
Area: about 10-15 µm²
A nucleus generally occupies about 10-15% of a cell.
The nucleus of the heart muscle cell of an Ox generally occupies only 6% of the cell.
The nucleus occupies almost the whole portion of a sperm cell.
A nucleus may have the following structural components:
- Nuclear membrane
Kite-infiltrated a microneedle into a nucleus collapsed the nucleus
Feldharr and Feldharr- They injected fluorescent-labeled γ-globulin into an egg cell. They observed that only the cytoplasm fluoresced under a specific wavelength, and the nucleus remained unchanged.
We can tell that a membrane covers the nucleus, which is semi-permeable and has selectivity.
Amorphous, jelly-like substances present in the nucleus are called protoplasm of the nucleus. Several chemical components are present in the nucleoplasm.
3. Chromatin net
Previously it was thought that densely net-like structures were present in the interphase nucleus. But there is nothing like a chromatin net in the nucleus.
Few darkly-stained bodies are often found in the nucleus, especially in the interphase nucleus. These are known as Chromocenters.
The Nucleolus is the site of ribosomal RNA (rRNA) transcription and processing. It is also the site of processing several other noncoding RNAs.
Each chromosome comprises a single DNA molecule plus proteins, and each is called a chromatid before it replicates. Chromosomes move independently during mitosis and meiosis. A sister chromatid is a pair of identical DNA molecules and proteins forming a chromosome after duplication. In bacteria and viruses, chromosomal DNA molecules are usually circular but can also be linear.
Mischer- He isolated a chemical “Nuclein” from the nucleus of Salmon fish. This “Nuclein” is nucleoprotein (Nucleic acid + Protein).
Types of nucleoprotein
- DNA- basic Protein
- Most soluble proteins are enzymes
- RNP (Ribo-nucleoprotein)
- Free RNA
Among all the proteins, histone is 20%, and protamine is 27% of the total Protein in the nucleus.
DNA: Histone – 1:1
NAD, ATP, Acetyl Co-A, AMP, GMP
The nucleus is composed of about 9-12% DNA, 5% RNA, 3% lipids, 15% simple essential proteins such as histone or protamines, about 65% complex acid or neutral proteins, including enzymes such as polymerases for the synthesis of DNA and RNA, organic phosphates and inorganic salts or ions such as Mg++, Ca++, and Fe++.
Callan and Tomlin took photographs of nuclear membranes in an electron microscope in 1950.
They observed that:
- The nuclear membrane is rough and porous
- It is continuous
- It is two layers
-Outer layer (have more)
-Inner layer (do not have more)
ü1o,ooo pores may be present in a whole nuclear membrane (maximum case).
ü40-80 pores may be present per cubic micron.
üSize and shape of pores are variable.
Can be elastic
Callan and Tomlin, in 1950- Said that “protein is the only component of the nuclear membrane.”
Ponder in 1961- “Nuclear membrane composed of Lipoprotein.”
Lipid: Protein = 1.7:1
Merrian (1961), Felder (1964), DuPrew (1965)- They treated nuclear membranes with three enzymes.
RNase + Nuclear Membrane = No effect
Dnase + Nuclear Membrane = No effect
Protease + Nuclear Membrane = Nuclear membrane destroyed
So, Protein is present in the nuclear membrane.
Then, they added trypsin with nuclear Protein.
Nuclear Protein + trypsin = Basic Protein, which breaks peptide bonds between arginine and lysine.
So, essential Protein is present in the nuclear membrane.
Helen Gay (1955, 1956)
It transports some elements and synthesized materials from nucleoplasm to cytoplasm.
It maintains the connectivity between nucleoplasm and cytoplasm.
Fontana in 1781: He found some darkly stained regions within the nucleus by Eosine dye (a basic dye).
Bowman in 1840: He termed this darkly stained region Nucleolus (pl. Nucleoli).
ØGenerally found in the interphase nucleus.
ØIn rare cases, it may present at prophase.
ØDisappear during cell division and reappear after telophase.
ØIt is well established that nucleoli are attached to some chromosomes. These chromosomes are called nucleoli-bearing chromosomes (named by Heitz in 1931).
ØNucleoli bearing chromosomes are also called “SAT-chromosomes.”
ØThe position of chromosomes where the nucleoli are attached is called “Sine-Acido-Thymonucleinico (SAT).”
Each cell must have at least one Nucleolus.
Range 1-10 per cell.
Two or more nuclei may join to form a bigger one.
Nucleoli originate from SAT.
Generally, SATs are present at the small arm of the chromosome.
In 1934, Barbara McClintock worked on Zea mays.
She proved that SAT has a direct role in nucleolus formation.
Wilson (1925): Nucleoli are the reservoir of nucleic acids.
Till 1940, people believed that RNA was present in the nucleoli.
Casperson and Schultz (1940): They proved that DNA is present in the Nucleolus in a minimal amount by UV spectrophotometer.
Porter and Bernhard (1954): They proved that nucleoli are not only the reservoir of RNA but also RNAs are synthesized within the nucleoli from RNP (Ribonucleoprotein).
The amount of Protein is more than RNA.
Vincent (1965): From isolated Nucleolus, he found two types of RNA.
i.r-RNA (about 25-40%)
ii. t-RNA (about 25%)
iii. Protein (about 25-30%)
The function of Protein that has been synthesizing within the Nucleolus is to protect mRNA.
Anderson and Beams (1956): They found some granules of 150-200Å in diameter in the Nucleolus of kinds of bug. But they didn’t find any membrane-like structure, which was more or less similar to the granules in cytoplasm and nucleoplasm.
Marangoni and Bernhard (1963) reported two regions in the Nucleolus.
i.Fibrous: inner side, 80-100Å diameter fibrils
ii. Granular: peripheral side, consists of 150-200Å diameter ribosome-like densely arranged particles.
D. Hay and J. B. Gurdon (1968) found four regions in the vertebrate Nucleolus.
- Pars chromosome:
Mainly composed of nucleolar chromatins. Two types of nucleolar chromatins are present.
1. Peri-nucleolar chromatin
2. Inter-nucleolar chromatin
In addition, there are many highly-coiled fibrils of 200-250Å diameter; these are rDNA (ribosomal DNA).
However, in rare cases, chromosomal DNA may be present.
2. Pars fibrosa
Pars fibrosa is only sometimes distinct because sometimes it may merge with the pars chromosome region.
Two types of fibrils are present.
1. Round fibril of 40-80Å in diameter
2. Elongated fibrils of 200-400Å in diameter
These fibrils are RNP (Ribo-nucleoprotein).
3. Pars granulosa
These are composed of many granules and surround the pars fibrosa region. Some RNP of 150-200Å in diameter may be variable in size. 28S and 18S RNA may present.
4. Pars amorpha
The portion of the Nucleolus without fibrils and granules or the matrix of the Nucleolus. Protein, carbohydrate, lipid, NAD, NAD-synthetase, 45S RNA, etc., may present.
1. The Nucleolus synthesizes and stores rRNA.
2. It also stores ribosomal proteins received from the cytoplasm.
3. It forms ribosomal subunits by wrapping the rRNA with ribosomal proteins.
4. The ribosomal subunits pass through the nuclear pores into the cytoplasm. Here the subunits join to form ribosomes when needed. Thus, the Nucleolus provides machinery (ribosomes) for protein synthesis.
5. The Nucleolus also plays a role in cell division.
6. Produce some t-RNA and mRNA.
7. It can able to synthesize Protein independently.
DNA at SAT is repeated 50-17,000 times, called a repetitive sequence.
Some scientists believed that the Nucleolus might help form the spindle apparatus.