Nucleus

Introduction

Nucleus is usually the most conspicuous organelle of eukaryotic cell. However, well defined nucleus is absent in prokaryotic cells. Nucleus is the repository of genome and the source of informational macromolecules that govern the synthetic activities of the cytoplasm.

Discovery

A Dutch Microscopist, Antonie van Leeuwenhoek observed nucleus in 1710, as a centrally placed clear area in the blood cells of amphibians and birds.

Fontana (1781) recorded an ovoid structure in each of the isolated epidermal cells of eel’s skin.

However, Robert Brown (1831) was the first to use the term nucleus for a prominent body present in the orchid cell. He stated that nucleus was the regular feature of the cells.

Number

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 in shape, but can be ellipsoidal, fusiform, convulated, lobbed and branched.

Area: about 10-15 µm²

A nucleus generally occupies about 10-15% of a cell.

Exception

Heart muscle cell of Ox, nucleus generally occupies about only 6% of cell.

Nucleus occupies almost the whole portion of a sperm cell.

Physical Structure

A nucleus may have these following structural components:

1. Nuclear membrane

Kite-infiltrated a microneedle into a nucleus collapsed the nucleus

Feldharr and Feldharr– They injected fluorescent-labelled  γ-globulin into egg cell. They observed that only cytoplasm fluoresced under certain wavelength and nucleus remain unchanged.

We can tell that there is a membrane covering nucleus which is semi-permeable in nature and it has selectivity.

2. Nucleoplasm

Amorphous, jelly like substances present in the nucleus called protoplasm of nucleus. A number of chemical components are present in nucleoplasm.

3.Chromatin net

Previously it was thought that, a densely net like structures are present in interphase nucleus. But there is actually nothing like chromatin net in the nucleus.

4. Chromocenter

Few darkly-stained bodies are often found in the nucleus specially in interphase nucleus. These are known as Chromocenter.

5.Nucleolus

The nucleolus is the site of ribosomal RNA (rRNA) transcription and processing. It is also the site of processing of several other noncoding RNAs.

6. Chromosome

A DNA molecule with its attendant proteins that moves as an independent unit during mitosis and meiosis. Before DNA replication, each chromosome consists of a single DNA molecule plus proteins and is called a chromatid. After replication, each chromosome consists of two identical DNA molecules plus proteins. These are sister chromatids. Chromosomal DNA molecules are usually linear but can be circular in bacteria, and viruses.

Chemical Composition

Meischer– He isolated a chemical “Nuclein” form the nucleus of Salmon fish. This “Nuclein” is actually nucleoprotein (Nucleic acid + Protein).

Types of nucleoprotein

1. DNA-histone
2. DNA-protamine
3. DNA- basic protein

Soluble materials

1. Most of the soluble proteins are enzymes
2. RNP (Ribo-nucleo protein)
3. Free RNA

Among all the proteins, histone is 20% and protamine is 27% of the total protein in nucleus.

DNA: Histone – 1:1

Co-enzyme

NAD, ATP, Acetyl co-A, AMP, GMP

The nucleus is composed of about 9-12% DNA, 5% RNA, 3% lipids, 15% simple basic 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++.

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Nuclear membrane

Discovery

Callan and Tomlin- They took photograph of nuclear membrane by electron microscope in 1950.

They observed that:

• Nuclear membrane is rough and porous
• It is continuous
• It is two layers

-Outer layer (have pore)

-Inner layer (do not have pore)

Physical Structure

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Nuclear pore

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

Chemical structure

Callan and Tomlin in 1950- They told that “protein is the only only component of nuclear membrane”.

Ponder in 1961- “Nuclear membrane composed of Lipoprotein”.

Lipid: Protein = 1.7:1

Merrian (1961), Feldher (1964), DuPrew (1965)- They treated nuclear membrane with three enzymes.

RNase + Nuclear Membrane = No effect

Dnase + Nuclear Membrane = No effect

Protease + Nuclear Membrane = Nuclear membrane destroyed

So, protein is present in nuclear membrane.

Then, they added trypsin with nuclear protein.

Nuclear protein + trypsin = Basic protein which breaks peptide bonds between arginine and lysine.

So, basic protein is present in nuclear membrane.

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Nucleo Bleb

Helen Gay (1955, 1956)

It transport some elements and synthesized materials from nucleoplasm to cytoplasm.

It maintains the connectivity between nucleoplasm and cytoplasm.

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Nucleolus

Discovery

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 regions as Nucleolus (pl. Nucleoli).

Occurrence

ØGenerally found in interphase nucleus.

ØIn rare case, 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 chromosome (named by Heitz in 1931).

ØNucleoli bearing chromosomes are also called “SAT-chromosomes”.

ØThe position of chromosomes where the nucleoli are attached, called  “Sine-Acido-Thymonucleinico (SAT)”.

Number

Each cell must have at least one nucleolus.

Range 1-10 per cell.

Two or more nuclei may join to form a bigger one.

Origin

Nucleoli originate from SAT.

Generally SATs are present at small arm of chromosome.

In 1934, Barbara McClintock worked on Zea mays.

She proved that SAT has direct role in nucleolus formation.

Chemical Structure

Wilson (1925): Nucleoli are the reservoir of nucleic acids.

Till 1940, people believed that RNA is present in the nucleoli.

Caspersson and Schultz (1940): They proved that DNA is present in the nucleolus in a very little 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 (Ribonucleo-protein).

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 m-RNA.

Fine structure

Anderson and Beams (1956): They found some granules of 150-200Å in diameter in nucleolus of a kinds of bug. But they didn’t find any membrane like structure and those were more or less similar to the granules present in cytoplasm and nucleoplasm.

Marangoni and Bernhard (1963): They 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): They found four regions in vertebrate nucleolus.

1. Pars chromosoma:

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 r-DNA (ribosomal DNA).

However, in rare case, the chromosomal-DNA may be present.

2.Pars fibrosa

Pars fibrosa is not always distinct, because sometimes it may merges with pars chromosoma region.

Two types of fibrils are present.

1.Round fibril of 40-80Å in diameter

2.Elongated fibrils of 200-400Å in diameter

Actually these fibrils are RNP (Ribo-nucleo protein).

3.Pars granulosa

These are composed of many granules and surrounds the pars fibrosa region. Some RNP of 150-200Å in diameter may present which are 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 nucleolus. Protein, carbohydrate, lipid, NAD, NAD-synthetase, 45S RNA, etc. may present.

Functions

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 by ribosomal proteins.

4.The ribosomal subunits pass out through the nuclear pores into the cytoplasm. Here the subunits join to form ribosomes when needed. Thus, it is the nucleolus which provides machinery (ribosomes) for protein synthesis.

5.The nucleolus also plays a role in cell division.

6. Produce some t-RNA and m-RNA.

7.It can able to synthesize protein independently.

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SAT-region

DNA at SAT is repeated 50-17,000 times called repetitive sequence.

Some scientists believed that nucleolus may help in the formation of spindle apparatus.