What is a Ribosome | Definition, Types, Function, Structure

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Occurrence and location of the ribosome?

Ribosomes are some microscopic granules about 150 to 200 angstroms in diameter and found in all living cells, either attached to the outer membrane of the Endoplasmic Reticulum or scattered in the cytoplasm. These are described as protein factories of the cell because they synthesize the protein of the cell.

Who discovered the ribosome?

Ribosomes were first observed by Palade in 1955 under an electron microscope as dense particles.

How many types of ribosomes?

Ribosomes found in prokaryotes and eukaryotes are of different sizes and have different sedimentation rates. Therefore, ribosomes are of the following two types.

Ribosomes are generally oblate spheroidal, measuring 18 nm in bacteria and 20-22 nm in Eukaryotic cells. These occur in 2 categories.

  1. 70S ribosomes are found in bacteria (prokaryotic cells), mitochondria, and chloroplast.
  2. 80S Ribosomes found in Eukaryotic cells.

Ultrastructure of Ribosomes

The basic design of both prokaryotic and Eukaryotic chromosomes is the same. A small and a large subunit. The small subunit forms a cap on the large subunit.

What are prokaryotic ribosomes?

Prokaryotic ribosomes (70S Ribosomes) –

The ribosome of prokaryotes has about 18 nm diameter and 2.8 million Dalton’s Particle weight. Two subunits are 50S and 30S. Lake (1985) gave the structure model for prokaryotic ribosomes.

Prokaryotic ribosome

Lake’s model of 70S prokaryotic ribosome –

(i) 30S subunit –  The small subunit of prokaryotic ribosomes is the 30S. It is asymmetrical and rod-like. It is partially divided into two lobes by a Deep transversal cleft or groove.

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The smaller segment is called the head and the larger one is the base. A small outgrowth arises from the base segment and is called a platform.

Lake’s model of the prokaryotic ribosome

The particle weight of a 30S subunit is about 1.0 million Dalton. It is formed of one molecule of 16S rRNA and 21 proteins.

(ii) 50S subunit – It is the large subunit. It is more or less Spherical and forms the body. At its flat Anterior end are present three projections, one Central projection and two lateral projections.

The particle weight of the 50s subunit is 1.8 million Dalton’s.  It is formed of one molecule of 23s rRNA, One molecule of 5S rRNA, and 34 proteins.

In a ribosome, the two subunits are fitted together in such a way that a tunnel is formed between them. During Protein synthesis, mRNA is threaded through this tunnel.

What is a Eukaryotic ribosome?

Eukaryotic ribosome ( 80S ribosome) –

The 80s ribosome has a diameter of about 20- 22 nm and a particle weight of 4.2 million Dalton’s. The two subunits are the 60S and 40S.

eukaryotic ribosome
Eukaryotic ribosome

The 60S Subunit has a weight of about 2.7 million Dalton’s. It contains 28S, 5S and 5.8S rRNA Find about forty different proteins. It has a broad anterior flat surface and three projections.

The 40S subunit is about 1.5-1.8 million Dalton in weight. It has 1.8 rRNA and 30 different polypeptides.

Attachment of the ribosome to the Endoplasmic Reticulum –

The large subunit gets attached to the membrane of the Endoplasmic reticulum and bears two slots for two molecules of transfer RNA. These are called P-site (peptidyl or donor site) and A-site (aminoacyl or acceptor site).

The aminoacyl tRNA complex (AA-tRNA) is attached to the acceptor site and the tRNA-carrying peptide chain is attached to the peptidyl or Donor site. The smaller subunit receives the messenger RNA. The cleft separating the two subunits lies parallel and remains attached to the membrane of the Endoplasmic reticulum.

Chemical composition of Ribosome –

Ribosomes are formed of approximately equal amounts of RNA and proteins. Proteins form the Periphery and RNA lies in the Interior, remaining intertwined within the two subunits. 

  1. Ribosomal RNA –
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(i) In prokaryotic ribosome –

 Ribosomal RNA (rRNA) Occurs in three e different forms, 23S rRNA, 16S rRNA, and 5S rRNA.

  • 23S rRNA occurs in the 50S subunit. The molecular weight of 1.2 million (i.e. 1.2×106) Dalton’s and contains about 3200 nucleotides.
  • 16S rRNA Line in 30s subunit. It has a molecular weight of .6×106 Dalton’s and contains about 1600 nucleotides.
  • 5S rRNA also lies in a large subunit. It has a molecular weight of 3.2X104 Dalton having 120 nucleotides.

 (ii) In Eukaryotic ribosome –

The ribosomal RNA occurs in four different forms 28S, 18S, 5.8S, and 5S.

  • 28S rRNA lies in 60s subunit. It has a molecular Weight of 1.5 – 1.8 million Dalton.
  • The 18S ribosomal RNA is found in 40S subunits and weighs .8×106 daltons.
  • 5S and 5.8S ribosomal RNA are also found in large subunits with molecular weights of 3.2x 104 and 5×104 Dalton.

2. Ribosomal proteins –

About 55 different proteins are found in ribosomes. At the time of dissociation of subunits into inactive core particles, some proteins are released from each particle. These are known as split proteins (SP). The split proteins are of two types: acidic and basic.

About 28 proteins bind specifically and directly to sRNAs (About 14 to 16S RNA). These are called primary binding proteins. Split proteins do not bind directly to rRNA, but interact with the primary binding proteins. These are also called secondary binding proteins

Biogenesis of ribosome –

The ribosome in bacteria is found inside the cytoplasm. Because of the absence of nucleolus. The ribosomal RNA is coded from the specific cistrons of the genome.

But in eukaryotes the process is complicated, and ribosomal RNA is synthesized in the nucleolus. In the beginning, 45S RNA is formed from the nucleolar organizer region of the chromosome.

This 45S RNA is a Precursor of both 28S and 18S sRNAs. The process of conversion of 45S RNA into 28S and 18S ribosomal RNA is illustrated in the figure.

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The protein syntheses in the cytoplasm, a symbol in the nucleolus and gets associated with RNA to form ribonucleoprotein particles (RNP).

Difference between ribosome and lysosome –

Ribosomes are the granule of protein and RNA present in the cytoplasm. These are often attached to the endoplasmic reticulum. These are the sites of protein synthesis.

Lysosomes are cytoplasmic organelles of cells that contain digestive enzymes for intracellular digestion of bacteria and other foreign bodies, which enter the cells by the process of phagocytosis or PinOcytosis. They may cause cell destruction.

What are Mitochondrial ribosomes (mt, ribosome)?

Mitochondria ribosomes occur either freely in the mitochondrial matrix or attached to the membranes of cristae. The sedimentation coefficient of mitochondrial ribosomes in yeast fungi Protozoan and higher plants varies from the 70S to 80S.

In the mitochondria of animal cells, the mitoribosomes have a 50-80S Sedimentation coefficient. These also have ochondial Subunits. These contain only two types of ribosomal RNA.

In animal mitochondria, the large subunit contains 16S to 18S rRNA and small subunits have 12S to 13S RNA.

Mitochondrial ribosome synthesis of mitochondrial proteins, which form respiratory or oxidative enzymes.

Chloroplast ribosomes (chl. ribosome) –

Chloroplast ribosomes are similar to prokaryotic ribosomes. These have a sedimentation coefficient of the 70S and are formed of two subunits, 50S and 30s. The large subunit contains 5S and 23S RNA, and the small subunits have 16S RNA.

These are similar in all groups of plants.

Polysome or polyribosome –

During protein synthesis, when a number of ribosomes are aligned In line with a molecule of mRNA, This structure is called a polysome or polyribosome. The ribosome dissociates from the mRNA After synthesizing protein molecules.

What are the Functions of ribosomes?

Ribosomes are involved in protein synthesis in the cell, but ribosomes alone cannot synthesize proteins. Ribosomes can do protein synthesis only then.

When many ribosomes are arranged in a linear order on the mRNA. This state of the ribosome is called polysome or polyribosome state.

This is the stage when ribosomes take part in protein synthesis. Initially, the subunits of the ribosome are scattered separately in the cytoplasm. The integration of these two subunits of the ribosome is done by the magnesium ion Mg2+ as soon as the concentration of Mg2+ reaches 0.001M.

Both the ribosome subunits join together to form a complete ribosome. As this concentration increases 10- fold to 0.01 M, the two ribosomes begin to associate, eventually forming the polysome state.

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