"Gene" is considered as the fundamental unit of information in living system. Gene can be defined as a segment of DNA that can encode for the functional biological products which is usually protein. The central importance of this biosynthetic process can be understood from the fact that 90% of chemical energy of cell is devoted to process of protein synthesis. The flow of genetics information from DNA to protein requires three major processes. The first one is replication , the formation of daughter DNA from parental DNA and the second process is transcription, the process by which genetic information present in DNA in form of nucleotide sequence are copied into RNA precisely. The third one is translation or protein biosynthesis, the genetic information in mRNA is converted into polypeptide chain having definite sequence of amino acids.
In early 1950s, Paul Zamecnik and his colleagues suggested that the protein synthesis from aminoacid take place on ribosome. The major breakthrough in this field was contributed by Mahlon Hogland. They found that aminoacid were activated in presence of ATP before the initiation of protein synthesis. A new turn came when Francis crick suggested a hypothesis known as adaptor hypothesis in which he stated that a smaller nucleic acid molecule (known as tRNA presently) act as adaptor molecule; one part of adaptor molecule attached with specific aminoacid and another part recognizing the nucleotide sequence on mRNA encoding that aminoacid.By 1960s, it was discovered that three nucleotide residues in mRNA encode for an aminoacid. The collection of these three nucleotide is termed as Codon. There are sixty four codons for twenty aminoacids.
Before going through the process of biosynthesis, it is necessary to have an overview about ribosomes and tRNA because these both components play vital role in this process.
It is discovered that ribosome is the site where protein synthesis take place. Ribosome is ribonucleoprotein complex not only found in cytosol but also found to be attached with endoplasmic reticulum. This type of endoplasmic reticulum is known as rough endoplasmic reticulum. The bacterial ribosome contain 65% rRNA, 35% protein and they consist of two unequal subunits with sedimentation coefficient of 30s and 50S and combined sedimentation coefficient of 70S. 30S subunit of ribosome is composed of 21 proteins and 16 S rRNA. 50S subunit consists of 5S rRNA, 23S rRNA and thirty three types of proteins are known to be present. Eukaryotic ribosome is large and more complex than bacterial ribosome. It is also composed of large number of proteins and two unequal subunits with sedimentation coefficient of 60S and 40S and combined coefficient of 80S.
Transfer RNA (tRNA):
tRNA serves as an adaptor between incoming aminoacid and its corresponding Codon on mRNA. Cells have at least one kind of tRNA for one aminoacid. tRNA is composed of seventy three to ninety three residues with molecular weight of 24,000 to 31,000.tRNA molecule is mainly stabilized by hydrogen bonding forming clover leaf structure with four arms. Sometime longer tRNA contains an additional short arm or extra arm. tRNA molecule contain guanylate residue at 5' end and two cytosine and one adenine ( CCA) residues at 3'end.
(a) Anticodon arm: This arm carry anticodon for the corresponding codon on mRNA.
(b) D arm: This arm consists of an unusual base dihydrouracil.
(c) TÏˆC arm: This arm of tRNA molecule is composed of an unusual ribothymidine and pseudouridine which have unusual carbon-carbon bonds between base and ribosome.
(d) Aminoacid arms: This arm is site for aminoacid attachment. This arm carry specific aminoacid attached by its carboxyl group of adenine residue at 3'end of tRNA.
Having looked at the structure and function of ribosome and tRNA, we now discussed the detail process of protein biosynthesis. Protein biosynthesis involves following steps:
(a)Activation of aminoacid:
Aminoacyl- tRNA synthetase attach aminoacid to their corresponding tRNA in presence of ATP and Mg++ ion. This process occurs in cytosol of the cell. Aminoacyl- tRNA synthetase is specific for one aminoacid and one or more corresponding tRNA. Amnioacyl-tRNA synthetase of E.coli is divided into two categories according to their difference in reaction mechanism. The reaction catalyzed by Aminoacyl- tRNA synthetase is summarized below:
Aminoacid + tRNA + ATP = aminoacyl-tRNA + AMP + PPi
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