Ribosomes
Proteins are necessary for the cells to perform cellular functions. Ribosomes are the cellular component that make proteins from all amino acids. Ribosomes are made from complexes of RNAs and proteins. The number of ribosomes in a cell depends on the activity of the cell. Ribosomes 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 the ribosomes are attached to the same mRNA strand, this structure is known as polysome. The existence of ribosomes is temporary, after the synthesis of polypeptide the two sub-units separate and is reused or broken up. Amino acids are joined by the ribosomes 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.
The ribosomes present in the prokaryotes function differently in protein production than the ribosomes of the eukaryote organisms. The ribosomes of bacteria, archea and eukaryotes differ significantly from each other in structure and RNA sequences. The differences in the ribosomes allows the antibiotc to kill the bacterial ribosome by inhibiting the activity of the bacterial ribosomes, the human ribosome sramin unaffected. The ribosomes of the eukaryotic cellas are similar to the ribosomes of the bacterial cells, showing the evolutionary origin of the organelle.
Ribosomes Defintion
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
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….
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.
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.
Bacterial Ribosome
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
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.
