The structural and functional unit of all the organisms or living beings is called the cell.

The shape and size of the cell are different.

It varies from one organism to another organism

This smaller organism has limited cells. while larger organisms like elephant, whale, jack tree etc.. Have unlimited cells

Usually, in the human body, nearly 1014 cells exist.

History of the cell:-

It was first invented by an English scientist Robert Hook in 1665 and he asserted that cell has been made from a life-saving substance.

Later in 1939 J.E.Pirkinje pointed out that this life-saving substance is protoplasm which is the physical base o the life and in protoplasm 80% water is to be found.

Thus protoplasm which exists, inside the nucleus of the cell is called nucleoplasm.

Cell theory:-

The cell theory was jointly propounded by a botanist Schleiden and zoologist Schwann in 1833-1839.

The main points regarding the cell theory were

  1. Every organism originates from a cell.
  2. The body of every organism is made from one or more cells
  3. A cell is a self-independent unit, although it takes part in the metabolic activities and with other cells form an organism.
  4. The process through which the cell is formed has a complex mechanism but the core of the cell is the nucleus.

Cell structure:-

It as various types of structures like cylindrical, elliptical, spherical etc..

Cells have three main components

  1. Cell membrane
  2. Nucleons
  3. Nucleoplasm

Cellular components:-

Cell membrane: –  It is an outer layer of the cell and it is basically a semi-permeable membrane. The main function of the cell membrane is to control the molecular activities of the cell and its outer medium with the cell.

Cell wall: – It was only found in plants in animals cell wall are absent.

These cell walls are made up of cellulose, hemicelluloses, pectin main function is to protect the nucleoplasm and cell membrane from the external attack.

Nucleus: –  it was invented by Robert brown in 1831. It is spherical or elliptical. It is confined to the cellular center of the cell.

It consists of various components of the nucleus of cell-like nuclear membrane, nucleoplasm, nucleolus, nuclear network.

Nuclear membrane: –  it is made from the two layer’s membrane through which nucleus of the cell is surrounded. It is made up of protein and fat. The outer layer of the nuclear membrane is connected through the endoplasmic reticulum of the cell.

Nucleoplasm: –  it is the protoplasm of the inside nucleons of the cell. It is basically made from protein, phosphorous and nucleonic acids which are abundantly found in it.

Nucleolus: –  There is one or more spherical structures inside the nucleons of the cell is called nucleons.

The main functions are

  1. To synthesize r-RNA in the nucleons.
  2. To assist in the synthesis of r-RNA and intro separation of it from nucleons to another place
  3. To collect ribosome inside the nucleons.

Nuclear network: – The main body of the nucleons is like a network structure in which thick particles are found which is called chromatin.

According to modern Geneticists, chromatin is a genetic substance found in the cell and it is basically compassed of histone-DNA and RNA

DNA: Deoxy Ribonucleic Acid

RNA: Ribonucleic acid

In chromatin, the main component is DNA which causes the genetic characters in the organisms.

In the cells of almost all species of various organisms, the nucleus has a definite number of chromosomes.

1 Human 23(=46)
2 Housefly 6(=12)
3 Mosquito 3(=6)
4 Bee 8(=16),16(=32)
5 Dog 39(=78)
6 Monkey 21(=42)
7 Chimpanzee 24(=48)
8 Horse 32(=64)
9 Wheat 21(=42)
10 Onion 8(=16)


Chromosomes: – It is the basic constituents of a genetic substance chromatin and in every chromosome, there exists a dense jelly-like substance called the matrix.

Thus chromosome is called the vehicles of the heredity.

The structure of DNA and RNA:-

DNA is a polynucleotide and almost all amount of it is confined in the nucleons of the cell but a very small amount is also present in the mitochondria and green plastids.

In 1953 Wasson and Crick propounded a comprehensive double helix model. Both plans of this double helix are in the opposite parallel direction and each helix gas a diameter of nearly 20 A0. Thus the structure of DNA is a double helix.



DNA controls all the genetic activities and its basic unit is a gene. It also regulates the protein synthesis.

RNA is single stranded but in some viruses, it has the double-stranded structure like in rhinovirus. The main function is to assist in protein synthesis. Some virus genetic substances are either.

DNA or RNA but both never

Usually, RNA is Three types:-

  1. Ribosomal RNA(r-RNA)
  2. Transfer RNA( t-RNA)
  3. Messenger RNA (m-RNA)

Ribosomal RNA( r-RNA): – it is attached on the ribosome and helps in protein synthesis and it forms from DNA in the nucleus of the cell. The r-RNA is nearly 80% of the total RNA.

Transfer RNA( t-RNA): – It brings all types of amino acids on the ribosome where the protein is formed. It is 10-15% of the total RNA and it is the smallest among all. It has two-dimensional structures like a cloverleaf.

Messenger RNA(m-RNA): – it carries the information of the protein synthesis from DNA OF the nucleus to the ribosome. It is 305: of all RNA. It also helps in selecting amino acids.