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Protein Synthesis

Proteins are the end product of many metabolic processes. A typical cell requires thousands of different protein molecules at any given moment. So, the protein requirements of a particular cell has to be satisfied by synthesizing them at site or has to be transferred from where it is being synthesized.

Protein synthesis is one of the most complicated biosynthesis mechanisms. Steps involved in protein synthesis are discussed below.

 

Protein Synthesis Definition

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"Biosynthesis of protein depending upon the cell's current requirement, and transportation of this protein to the targeted location is said to be Protein synthesis."This is in accordance with the central dogma of molecular genetics, which postulates that the genetic information flows from nucleic acid to proteins.

In Eucaryotic cells, protein synthesis requires the participation of over 70 ribosomal proteins; 20 or more enzymes to activate amino acid precursors, etc. And about 100 additional enzymes for the final processing of different kinds of proteins and 40 or more types of transfer and ribosomal RNAs.

In total, 300 different macromolecules are required to synthesize polypeptides or proteins.Many of these macromolecules are organized into the complex 3D structure of the ribosomes to carry out stepwise trans location of the mRNA as the polypeptide is assembled.

Protein Synthesis Steps

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Protein synthesis takes place in the following steps.

1. Transcription


The information in the gene is contained in the base sequence of one of the strands of the double helix of DNA. Messages are transcribed into messenger RNA.

2. Translation


In this step, mRNA is transferred or synthesized into protein sequence.

This can be illustrated with the following diagram


DNA to Protein Process

Protein Synthesis Transcription

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There are three different kinds of RNA, which substantially differ from one to another. All these are involved in the biosynthesis of proteins.
  1. Transcription involves the transcribing of genetic codes or the messages of DNA to a complementary RNA or a messenger RNA. Transcription resembles DNA replication in that a DNA strand serves as the template for construction of a RNA.
  2. Before transcription, the first step in the protein biosynthesis is the replication. In replication, the parental DNA copies the information to the daughter DNA. The daughter DNA formed contains identical nucleotide sequences.
  3. The step after the replication is the transcription process, wherein, the information or the genetic message available in the daughter DNA is copied in the form of RNA. This RNA formed is termed as "Messenger RNA" or simply, mRNA, as it contains the information or the 'message' as to the sequence of nucleotides present in the original DNA molecule.
  4. There is not change of genetic code involved in the transcription process, since the DNA and the mRNA formed are complementary in nature.
  5. Transcription may also be defined as DNA -dependent RNA synthesis.
  6. The entire DNA molecule is not transcribed as a single mRNA. Transcription begins at a prescribed sequence of bases (the promoter sequence) and ends at a termination sequence.
  7. Thus, one DNA molecule can give rise to many different mRNAs and code for many different proteins. There are thousands of mRNAs and they vary in length from about 500 to 6000 nucleotides.
The mRNA is made up of triplets of adjacent nucleotide bases called codons. Since mRNA has only four different bases and 20 amino acids must be coded, codes of some nucleotides may not be accurate. If nucleotides are read in sets of three, the four mRNA bases generate 64 possible 'words', more than sufficient to code for 20 amino acids.

In addition to codons for amino acids, there are start and stop codons. Protein biosynthesis begins at a start codon and ends at a stop codon of mRNA.

Protein Synthesis Translation

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Translation is the third step in the protein synthesis. In translation process, the genetic message carried by the mRNA is translated on the ribosomes, into a protein with a specific sequence of amino acids. Thus, translation process can be defined as the reproduction of primary polypeptide chain according to specification of mRNA.

Thus, the flow of information takes place from DNA to mRNA and then to protein, the final product.
The messenger RNA usually transfers the genetic code to a transfer RNA or tRNA. tRNAs are relatively small nucleic acids, containing only about 70 nucleotides. They get their name because they transfer amino acids to the ribosome for incorporation into a polypeptide. Although 20 amino acids need to be transfered, there are 50-60 tRNAs, some of which transfer the same amino acids.

Ribosome is where the message carried by the mRNA is translated into the amino acid sequence of a protein. In 1968, Barry Commoner suggested a circular flow of information, DNA transcribes RNA, RNA translates into proteins and proteins synthesize RNA and RNA synthesizes DNA. But there is no evidence whatsoever for the synthesize of RNA from protein.

DNA and Protein Synthesis

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A DNA is a genetic material, it carries genetic information from cell to cell and from generation to generation.
A DNA molecule is composed of three kinds of entities.
  1. Phosphoric acid
  2. Deoxyribose sugar
  3. Nitrogen bases
The genetic information may be written in any one of the three moieties to DNA. The sequences of nitrogen bases of a given segment of DNA molecule, actually has been found to be identical to linear sequence of amino acids in a protein molecule.

DNA plays a very important role in the protein synthesis. DNA is where the protein synthesis begins.
The genetic code necessary for protein synthesis is present in the DNA. They carry the inheritable characteristics of an organism and these characteristics are normally expressed at the molecular level via protein synthesis. Genetic expression consists of two stages, transcription and translation, both of which involve RNAs.

Protein Degradation

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The synthesized proteins are usually degraded in all cells so as to prevent the buildup of the abnormal or unwanted proteins and to facilitate the recycling of amino acids. Degradation is a selective process.

The rapidly- degraded proteins include:
  1. The defective proteins because of incorrect amino acid insertion during synthesis, or because of a damage occurring during normal cell functioning.
  2. Many enzymes that act at key regulatory points in metabolic pathways.
More topics in Protein Synthesis
Phosphorylation Glycosylation
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