Ribosomes

Ribosomes Defintion

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Ribosomes 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 link amino acids together in the order

that is specified by the messenger RNA

molecules. The ribosomes 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. Ribosmal subunits are made of one or

more rRNA (ribosomal RNA) molecules and various

proteins.

Characteristics of Ribosomes

Characteristics of ribosomes:

Typically ribosomes are composed of two

subunits: a large subunit and a small subunit.

The subunits of the ribosome are synthesized by

the nucleolus.

The subunits of ribosomes join together when

the ribosomes attaches to the messenger RNA

during the process of protein synthesis.

Ribosomes along with a transfer RNA molecule

(tRNA), helps to translate the protein-coding

genes in mRNA to proteins.

Ribosome Structure

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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 are bound to the ER there

are known as the rough endoplasmic reticulum.

The bound and the free ribosomes are similar in

structure and are invloved in protein synthesis.

Ribosomes are tiny particles about 200 Ã….

Ribosomes are composed of both RNA and

proteins.

About 37 - 62% of RNA are made up of RNA

and the rest is proteins.

Ribosome is made up of two subunits. The

subunits of ribosomes are named according to

their ability of sedimentation on a special gel

which the Sevdberg Unit.

Prokarytotes have 70S ribosomes each subunit

consisting of small subunit is of 30S and the

large subunit is of 50S. Eukarytotes have 80S

ribosomes each consisting of small (40S) and

large (60S) subunit.

The ribosomes found in the chloroplasts of

mitochondria of eukaryotes consists 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 are

into several continuous insertion 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.

Ribosome Function

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The main functions of ribosmes are :

They assemble amino acids to form specific proteins,

proteins are essential to carry out cellular activities.

The process of production of proteins, the

deoxyribonucleic acid produces mRNA by the process

of DNA transcription.

The genetic message from the mRNA is translated

into proteins during DNA translation.

The sequences of protein assembly during protein

synthesis are specified in the mRNA.

The mRNA is synthesized in the nucleus and is

transported to the cytoplasm for further process of

protein synthesis.

In the cytoplasm, the two subunits of ribosomes are

bound around the polymers of mRNA; proteins are

then synthesized with the help of transfer RNA.

The proteins that are synthesized by the ribosomes

present in the cytoplasm are used in the cytoplasm

itself. The proteins produced by the bound ribosomes

are transported outside the cell.

ULTRA STRUCTURE

Bacterial Ribosome

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Prokaryotic ribosomes

In a bacterial cell there are about 10,000 ribosomes which

make upto 30% of the weight of the cell. The bacterial

ribosomes are present free in the cytoplasm. The bacterial

ribosome sediments as 70S particle which is composed of

30S and a large subunit is of 50S. The small subunit of the

prokarytoic ribosome functions in the association with

messenger RNA during translation and decoding. The large

subunits of the ribosomes function as peptidyl transferase

center and it is the site of peptide bond formation. The

structure of bacterial ribosome is made up of over 50

proteins and three large domains of RNA molecule. They are

the site of protein synthesis.

Plant Cell Ribosome

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Plant cell do have ribosomes and they are composed of

proteins and ribosomal RNA. The ribosomes in a plant cell

are found in the cytoplasm, the surface of the rough

endoplasmic reticulum, the mitochondria and on

chloroplasts. There are two types of ribosomes - free

ribsomes and attached ribosomes. The attached ribosomes

are bound to the surface of the endoplasmic reticulum and

they are the site for protein synthesis. Synthesis of proteins

also occurs in the free ribosomes.

Animal Cell Ribosome

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Ribosomes are proein builders of the cll. They are found in

many place in the cytoplasm. They might be sseen freely

floatin gin the cytoplasm and they are seen attached to the

endoplasmic reticulum. Ribsomes of animal cells are also

made of two subunits large (60S) and small (40S. When

there is need for proteins ina cell, the mRNA is produced in

the nucleus and is sent to the cell and the ribosomes. The

two subunits of the ribsomes come together at the time of

protein formation and combine with the mRNA molecule

and hence the proteins are synthesized.