Nucleus – Definition, Structure & Function, Cellular Vs Atomic Nuclei

The nucleus is the part of the cell where DNA is stored. This is also where proteins are manufactured.

Nucleus – Definition, Structure & Function, Cellular Vs Atomic Nuclei

In addition to these functions, the nucleus plays a role in regulating gene expression, cell division, and apoptosis (programmed cell death).

There are two types of nuclei: cellular and atomic.

Cellular nuclei contain DNA, whereas atomic nuclei only contain protons and neutrons. Both types of nuclei are important for life because they provide the building blocks for cells.

In this article, we will discuss what is a nucleus, how it’s structured, and its function. We’ll also look at the differences between cellular and atomic nuclei.

Definition Of Nucleus

The word ‘nucleus’ comes from the Latin word “nux”, meaning nut. It was used by the ancient Greeks to describe the hard center of an apple or other fruit.

The word ‘nucleus’, however, has a more specific definition.

A nucleus is defined as the central part of a cell that contains all the genetic material necessary for the development of the organism.

A nucleus is an organelle that contains DNA. It can be found inside all eukaryotic cells.

The nucleus consists of three parts: the inner membrane or nuclear envelope, the chromatin, and the cytoplasm.

The nuclear envelope surrounds the nucleus and separates it from the rest of the cell. Chromatin is made up of DNA and protein.

Inside the nucleus, there are many structures called chromosomes. These are bundles of DNA that carry genes. They’re organized into pairs, one coming from each parent.

Each chromosome has a centromere on either end. A centromere is a region of the chromosome where microtubules attach.

Microtubules are long threads of protein that help move molecules within the cell. When the centromeres come together, the cell divides into two new cells.

Structure Of The Nucleus

The nucleus is an irregularly shaped organelle with a diameter of about 5-10 micrometers. It is surrounded by a double layer of membranes called the nuclear envelope.

The outermost membrane of the nucleus is called the lamina. The innermost membrane is called the pore complex.

The pore complex is composed of pores that allow small molecules to enter and exit the nucleus.

The lamina is attached to the pore complex through filaments called lamins. Lamins are proteins that form the scaffolding around which the nucleus is built.

The lamin network provides structural support for the nucleus.

The nucleus is divided into several compartments. The most prominent compartment is the nucleolus. The nucleolus is a dense body located near the periphery of the nucleus.

It contains ribosomes, which are the sites where RNA is synthesized. Another major component of the nucleus is heterochromatin.

Heterochromatin is highly condensed chromatin. It is usually associated with the centromere.

Other components include the nuclear matrix, the nuclear pore complexes, and the interchromatin granule clusters.

The nucleus is not uniform. Its largest dimension varies from 15 micrometers to 50 micrometers. The nucleus is approximately 10 times larger than the cell itself.

The nucleus is roughly spherical. However, it does have a definite outline. The nucleus is also asymmetrical. Some regions of the nucleus are much smaller than others.

This variation in size is due to differences in gene activity. Regions of the nucleus that contain active genes tend to be large.

In contrast, inactive genes are often concentrated in small areas of the nucleus.

The structure of the nucleus depends on the type of nucleus. There are four basic structures:

1. Proton-rich nucleus

2. Neutron rich nucleus

3. Isotope

4. Fissile nucleus

Proton-Rich Nucleus

A proton-rich nucleus has more protons than neutrons. These are unstable nuclei.

They can be broken apart easily. Examples include hydrogen, helium, carbon, nitrogen, oxygen, etc.

Neutron Rich Nucleus

A neutron-rich nucleus has more neutrons than protons. These are stable nuclei. They cannot be broken apart easily.

These are found in heavy metals such as gold, silver, lead, mercury, platinum, palladium, rhodium, iridium, osmium, ruthenium, rhenium, etc.


Atomic nuclei can be divided into isotopes based on their mass numbers. The mass number is the total number of protons plus neutrons.

If you add up the number of protons and the number of neutrons in a nucleus, then you get its mass number.

For example, the mass number for uranium 238 is 236. That means there are 238 protons and neutrons in this nucleus.

You could also say that there are 235 neutrons and 23 protons in this nucleus.

Fissile Nucleus

A fissile nucleus has enough neutrons so that if one of them were to decay, it would release energy. This causes the nucleus to become unstable.

The Function Of The Nucleus

The nucleus controls the transcription of DNA. Transcription involves copying the information encoded in DNA into RNA.

The process begins when a molecule of messenger RNA (mRNA) binds to a specific sequence of DNA known as a promoter.

Promoters are sequences of DNA that control the expression of particular genes.

Once bound to the promoter, mRNA becomes a template for making complementary strands of RNA.

One strand of this newly formed RNA is then used as a template for making more RNA. This process continues until enough RNA has been produced so that it can be translated into proteins.

Transcription occurs in the nucleus. Therefore, the nucleus plays a key role in determining what proteins will be expressed in a given tissue at any given time.

The nucleus is also responsible for processing genetic material before it’s exported out of the cell. For example, some genes encode enzymes that act on other genes.

Enzymes are proteins that catalyze chemical reactions. These enzymes must first undergo a series of steps before they can function.

First, these enzymes must fold themselves correctly. Then, they must bind to their substrates.

Finally, they must perform the reaction that converts one compound into another. All of these steps occur within the nucleus.

Additionally, the nucleus performs many other functions. For example, it stores chromosomes.

Chromosomes are structures found inside cells that carry the genetic instructions needed to make all the different kinds of proteins that constitute the body.

Each chromosome is made up of two long threads called telomeres. Telomeres protect the ends of each chromosome by preventing them from being recognized as damaged DNA.

When DNA is damaged, repair mechanisms attempt to fix the damage.

If the damage cannot be repaired, however, the broken piece of DNA may become separated from the rest of the chromosome.

This separation is called an end-to-end fusion or breakage.

The result is that the chromosome is no longer complete. A chromosome without its telomere is called a dicentric chromosome. Dicentrics are unstable and can cause cancer.

In addition to storing chromosomes, the nucleus contains ribonucleoprotein particles called nucleoli. Ribonucleoproteins are complexes of RNA and protein.

They play important roles in regulating gene expression. Specifically, nucleoli help regulate the production of rRNAs, which are necessary for protein synthesis.

Cellular Nuclei

A single human cell typically contains about 100,000 nuclei. Most of the nuclei in a human cell are not actively involved in protein synthesis.

Instead, most nuclei contain inactive copies of genes. Inactive genes are genes that have lost the ability to produce proteins because their promoters are mutated.

Mutated promoters prevent the genes from producing proteins.

The number of active genes varies depending on the type of cell. Cells that divide rapidly, such as blood cells, tend to have large numbers of active genes.

Cells that do not divide often, such as neurons, tend to have fewer active genes.

The number of active genes per cell depends on how much energy the cell needs to express proteins. For example, muscle cells need lots of energy to synthesize proteins.

Thus, they have lots of active genes. On the other hand, nerve cells only need a small amount of energy to synthesize protein. As a result, nerve cells have few active genes.

There are several types of nuclei. The main types are macronucleus (large), micronucleus (small), and karyocyte (nucleus).

Macronucleus refers to the nucleus of a diploid organism, while micronucleus refers to a haploid organism.

Karyocytes refer to both male gametes (spermatozoa) and female gametes (oocytes).


Macronuclei are the largest nuclei in eukaryotic organisms. They are usually spherical with a diameter ranging from 5 to 15 μm. Macronuclei are also referred to as polyploid nuclei.

Polyploidy means having more than 2 sets of chromosomes. Macronuclei are composed of chromatin, which is the material that makes up chromosomes.

Macronuclei stores the bulk of the genome. However, some genes stored in macronuclei are inactive. These genes are known as silent genes.

Silent genes are genes whose promoters are mutated so that the genes cannot produce proteins.


Micronuclei are smaller than macronuclei. Their size ranges from 0.5 to 1 μm. Micronuclei are sometimes referred to as mononuclear or binuclear nuclei.

Binuclei are nuclei that contain two separate masses of chromatin. Mononuclear nuclei are nuclei that have one mass of chromatin.

This distinction between binuclei and mononuclear nuclei is very important when studying micronuclei.


Karyocytes are nuclei that occur in spermatozoa and oocytes. Spermatozoa are haploid cells that carry half the normal complement of chromosomes.

Oocytes are haploid cells that have been fertilized by spermatozoa. Spermatozoan nuclei are larger than oocyte nuclei.

Spermatozoal nuclei can be distinguished from oocyte nuclei based on their shapes. Spermatozoa have an elongated shape, while oocytes are rounder.

Atomic Nuclei

Atomic Nuclei

An atomic nucleus is the smallest unit of matter that has the properties of a nucleus. An atom consists of protons, neutrons, and electrons.

Protons and neutrons make up the nucleus of an atom. Electrons orbit around the nucleus like planets orbiting around the sun.

Each electron orbits at a different distance from the nucleus. The closer an electron is to the nucleus, the higher its energy level will be.


A proton is a positively charged particle found inside atoms. A neutron is a neutral particle found inside atoms. Neutrons do not have any charge.

The number of protons determines what element an atom belongs to. There are 92 naturally occurring elements.

All of these elements except hydrogen are made up of protons and neutrons. Hydrogen is the simplest element. It contains no protons or neutrons.


Neutrons are often used to study nuclear reactions. Nuclear reactions involve the transformation of one type of particle into another.

In order for this reaction to take place, there must be a source of particles and a target where the particles can collide.

For example, if you were to drop a neutron bomb on a city it would cause damage because the bomb would release neutrons.

When a neutron collides with something else, it may change into a proton, an alpha particle, a beta particle, or even a gamma-ray.

Alpha Particles

Alpha particles are helium nuclei. Alpha particles are the most abundant form of radiation.

Alpha particles are emitted during radioactive decay. Alpha particles are released when uranium decays. Uranium is a heavy metal found in rocks.

Beta Particles

Beta particles are electrically charged subatomic particles. Beta particles are emitted during radioactive disintegration.

Beta particles are produced when radium decays. Radium is also a heavy metal found in rock.

Gamma Rays

Gamma rays are high-energy electromagnetic waves. Gamma rays are emitted during radioactive decay and nuclear fission.

They are also emitted during nuclear fusion. Gamma rays are extremely dangerous. If they enter your body, they can cause cancer.


Electrons are negatively charged particles. They are emitted during radioactive decay, nuclear fission, and nuclear fusion. Electrons are also emitted during chemical reactions.

During chemical reactions, electrons move back and forth through molecules. Chemicals react together when electrons flow back and forth through them.

Nuclear Fission

When a large amount of energy is applied to a nucleus, it splits apart into smaller pieces called nucleons (protons and neutrons).

This process is known as nuclear fission. Nuclear fission releases large amounts of energy.

Nuclear Fusion

When two nuclei come close enough to each other, their strong force pulls them together. As the nuclei get closer, the strong force gets weaker.

Eventually, the strong force becomes so weak that the nuclei fuse. The result is a new heavier nucleus. This process is known as fusion. Fusion produces more energy than fission.

Atomic Number

An atom’s atomic number is the total number of protons plus neutrons in the atom. Each element has its unique atomic number.

Cellular Vs Atomic Nuclei

The nucleus of a cell is much larger than the nucleus of an atom. A cell is about 10,000 times bigger than an atom. The nucleus of a cell contains all the genetic material of the cell.

The nucleus of an atom only contains the genetic material of one kind of atom.

The cellular nuclei contain DNA which controls how cells grow and divide. Most of the DNA in a cell is located in the nucleus.

The nucleus is surrounded by a membrane made up of proteins. Inside the nucleus is a liquid called cytoplasm.

Cytoplasm holds the chromosomes. Chromosomes hold genes. Genes tell the cell what to do.

The nucleus of an atom contains protons, neutrons, and electrons. Protons have positive charges. Neutrons have no charge. Electrons have negative charges.

When atoms split, the positively charged protons go out of the nucleus. The neutrons stay inside the nucleus. The electron goes with the proton.

A nucleus usually consists of between 200 and 1000 protons. Some nuclei consist of more or fewer protons. For example, uranium has 92 protons.

Uranium is very unstable because it has too many neutrons. It will break down into lighter elements.

A nucleus typically consists of between 80 and 140 neutrons. Some nuclei consist mostly of neutrons. For example, plutonium has 93 neutrons.

Plutonium is very stable because it has too few protons. It won’t break down into lighter elements as uranium does.

The main difference between the two types of nuclei (cellular and atomic) is that they are different types of matter. The matter is anything that takes up space.

An atom is a tiny piece of matter. Atoms make up everything we see around us. Cells are groups of billions of atoms.

This means that the size of the nucleus in an atom is much, much smaller than that of a cell.

Final Thoughts

This article explains the structure of the nucleus, including the differences between cellular and atomic nuclei, which is important for understanding ourselves and the world around us.

You can think of the nucleus as a small box. In this box is all the genetic information for the cell.

If you want to know what your body looks like (its scientific make-up), look at the nucleus. You would need to take a microscope to see individual atoms.

The nucleus is one of the most essential parts of a cell. It makes up 99% of the mass of a cell and contains the genetic information for the cell.

Jennifer Dawkins