Ribosomes Structure and Function in Animal Cell

In this ribosomes structure and function in animal cell post we have briefly explained about ribosomes in cell, definition, origin, types of ribosomes in cell, structure, and functions.

Ribosomes in Cell

Ribosomes are the cellular components that make proteins from all amino acids. Ribosomes in cell are made from complexes of RNAs and proteins. The number of ribosomes in cell depends on the activity of the cell.

These are freely suspended in the cytoplasm or attached to the endoplasmic reticulum forming the rough endoplasmic reticulum. On an average in a mammalian cell there can be about 10 million ribosomes.

When these are attached to the same mRNA strand, this structure is known as Polysome. The existence of these organelles temporary, after the synthesis of polypeptide the two sub-units separate and is reused or broken up.

Amino acids are joined by the ribosome at a rate of 200 per minute. Therefore small proteins can be made quickly but two or three hours are needed for proteins which are as large as 30,000 amino acids.

It present in the prokaryotes function differently in protein production than the ribosomes of the eukaryote organisms. The ribosomes of bacteria, archaea and eukaryotes differ significantly from each other in structure and RNA sequences.

The differences in the ribosome allows the antibiotic to kill the bacterial ribosome by inhibiting the activity of the bacterial ribosome, the human ribosome remain unaffected. The ribosomes of the eukaryotic cells are similar to bacterial cells, showing the evolutionary origin of the organelle.

Definition

Ribosomes in cell are small particles, present in large numbers in all the living cells. They are sites of protein synthesis. The ribosome word is derived – ‘ribo’ from ribonucleic acid and ‘somes’ from the Greek word ‘soma’ which means ‘body’. The ribosomes in cell link amino acids together in the order that is specified by the messenger RNA molecules.

These are made up of two subunits a small and a large subunit. The small subunit reads the mRNA while the large subunit joins the amino acids to form a chain of polypeptides. Ribosomal subunits are made of one or more rRNA (ribosomal RNA) molecules and various proteins. The ribosomes and associated molecules are also known as the translational apparatus.

It is first observed in the mid-1950s by Romanian-American cell biologist George Emil Palade, using an electron microscope, as dense particles or granules. The term “ribosome” was proposed by scientist Richard B. Roberts in the end of 1950s.

Origin

The ribosome may have first originated in an RNA world, appearing as a self-replicating complex that only later evolved the ability to synthesize proteins when amino acids began to appear.

Studies suggest that ancient ribosomes in cell constructed solely of rRNA could have developed the ability to synthesize peptide bonds.

In addition, evidence strongly points to ancient ribosomes as self-replicating complexes, where the rRNA in the ribosomes in cell had informational, structural, and catalytic purposes because it could have coded for tRNAs and proteins needed for ribosomal self-replication.

In the prokaryotes, the ribosome originates in the cytoplasm as there is no nucleolus, but in eukaryotes, the ribosome is partly nucleolar (rRNA) and partly cytoplasmic (proteins) in origin.

Types of Ribosomes

Ribosomes in cell are of two types on the basis of the size and sedimentation coefficient (S) such as the 70S and 80S. Here, ‘S’ refers to Svedberg unit. This is a sedimentation coefficient which shows how fast a cell organelle sediments in an ultracentrifuge.

Ribosomes

Ribosomes Structure and Function in Animal Cell

70S Ribosome

70S ribosome is relatively small-size with sedimentation coefficient 70s. It contains two subunits, of which, 50s is large and 30s is small and they are linked together. The molecular weight is 2.7×106 Daltons. They are found in all prokaryotic cells, in mitochondria and chloroplast of eukaryotic cells.

80S Ribosome

This type of ribosome is comparatively larger and has sedimentation coefficient 80S which consists of large circular 60S subunit and small elliptical 40S subunit. The molecular weight is 40×106 Daltons. They are found in all eukaryotic cells.

Structure

Ribosomes

Ribosomes Structure and Function in Animal Cell

Ribosomes in cell are tiny particles about 200 Ã. Ribosomes in a cell are located in two regions of the cytoplasm. They are found scattered in the cytoplasm and some are attached to the endoplasmic reticulum.

When the ribosomes in cell are bound to the ER there is known as the Rough Endoplasmic Reticulum (RER). The bound and the free ribosomes are similar in structure and are involved in protein synthesis.

Ribosomes in cell are composed of both RNA and proteins. About 37 – 62% of RNA is made up of RNA and the rest is proteins. Ribosome in cell is made up of two subunits. The subunits of ribosomes are named according to their ability of sedimentation on a special gel which the Svedberg Unit.

Prokaryotes have 70S ribosomes in cell, each subunit consisting of small subunit is of 30S and the large subunit is of 50S. Eukaryotes have 80S ribosomes, each consisting of small (40S) and large (60S) subunit.

The ribosomes found in the chloroplasts of mitochondria of eukaryotes consist of large and small subunits bound together with proteins into one 70S particle. The ribosomes share a core structure which is similar to all ribosomes despite differences in its size.

The RNA is organized in various tertiary structures. The RNA in the larger ribosomes is into several continuous insertions as they form loops out of the core structure without disrupting or changing it.

The catalytic activity of the ribosome is carried out by the RNA, the proteins reside on the surface and stabilize the structure. The differences between the ribosomes of bacterial and eukaryotic are used to create antibiotics that can destroy bacterial infection without harming human cells.

Function

1. Nearly all the proteins required by cells are synthesized by ribosomes in cell. These are found free in the cell cytoplasm and also attached to rough endoplasmic reticulum.

2. Ribosomes in cell receive information from the cell nucleus and construction materials from the cytoplasm.

3. Ribosomes in cell translate information encoded in messenger ribonucleic acid (mRNA). They link together specific amino acids to form polypeptides and they export these to the cytoplasm.

4. A mammalian cell may contain as many as 10 million ribosomes, but each ribosome has only a temporary existence. It can link up amino acids at a rate of 200 per minute.

5. It is formed from the locking of a small sub-unit on to a large sub-unit. The subunits are normally available in the cytoplasm, the larger one being about twice the size of the smaller one.

6. Each of it is a complex of Ribonucleoprotein with two-thirds of its mass is composed of ribosomal RNA and about one-third ribosomal protein.

7. Protein production takes place in three stages: (1) Initiation, (2) elongation, and (3) termination.

8. During peptide production it moves along the mRNA in an intermittent process called translocation.

9. Antibiotic drugs such as streptomycin can be used to attack the translation mechanism in prokaryotes. This is very useful. Unfortunately some bacterial toxins and viruses can also do this.

10. After they leave it most proteins are folded or modified in some way. This is called post translational modification‘.

Further Readings

Reference