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What is Protein Synthesis?
The process of creating polypeptide chains is called protein synthesis. The process completes with an implication of amino acid synthesis, transcription, translation, and post-translational events in a biological system.
The formation of amino acids from carbon sources like glucose is termed amino acid synthesis. It is a set of biochemical reactions. All organisms consist of several essential and non-essential amino acids. Some of the amino acids have been obtained from the diet whereas other amino acids are produced by the body.
Proteins are crucial biomolecules involved in various cellular functioning. These proteins are produced by the process of transcription and translation. The procedure of synthesizing mRNA templates from DNA molecules is called transcription. Later, these mRNA templates are used to translate them into amino acids.
The proteins or polypeptide chains are made by linking together these amino acids in a particular order based on the genetic code. After translation, these polypeptide chains undergo the process of post-translational modification, called protein folding.
Protein Synthesis Definition
The process of the creation of proteins is called protein synthesis. This process completes inside the cell in all organisms. In eukaryotic cells, the transcription takes place in the nucleus.
Later these mRNA templates move into the cytosol where it translated into polypeptides. The prokaryotic cells lack the nucleus since transcription completes in the cytoplasm of the cell.
Protein synthesis is made up of two words, here the word protein is derived from the Greek word protos, meaning “first“. The word synthesis is derived from sunthesis, meaning “to put together“.
Prokaryotic Protein Synthesis vs Eukaryotic Protein Synthesis
All living organisms require several biomolecules to develop and proteins are one of the important biomolecules used by all cellular organisms. Numerous cellular activities in the cell need proteins in prokaryotic and eukaryotic cells.
These proteins are used in different biochemical reactions, act as a catalyst, and used for structural purposes. The mechanism of protein synthesis is different in prokaryotic and eukaryotic organisms.
Eukaryotes have a well-defined nucleus and the transcription completes in the nucleus of the cell whereas transcription in prokaryotes completes in the cytoplasm.
In a eukaryotic cell, the mRNA is transported into the cytoplasm where it is translated by ribosomes. Here the nucleotides are converted into amino acid chains.
Protein Synthesis and Genetic Code
A genetic code or codon is a trinucleotide sequence that identifies a particular amino acid. A codon is a group of three nucleotides so it is called a trinucleotide sequence. For example, a sequence GCC (guanine-cytosine- cytosine) codes for the particular amino acid alanine.
Similarly, 64 codons are present that encode different amino acids. These codons are present in the mRNA template that adjuncts to anticodons present in the tRNA molecule. These anticodons are also trinucleotide sequences.
mRNA, tRNA, and rRNA
There are three types of RNA involved in the process of protein synthesis. The first type is mRNA (messenger RNA) which carries codons that are converted into an amino acid chain. These mRNA molecules are produced by a DNA template in the nucleus of the cell.
However, in prokaryotes, it synthesizes in the cytoplasm. The mRNA is consists of a 5′ cap, 5′ UTR region, coding region, 3′ UTR region, and a poly (A) chain. The coding region of mRNA is responsible for gene expression which has a start codon at 5′ end and a stop codon at 3′ end.
Another type of RNA is the tRNA molecule, also called transfer RNA. The transfer of particular amino acid to the ribosome is completed by a tRNA molecule. It has a cloverleaf resembling structure that has two major sites, named as- anticodon arm and accepter stem.
The anticodons are present in the anticodon arm and the accepter stem specifies the particular amino acid that needs to be attached. Now the ribosome translates the mRNA template into a polypeptide chain.
This ribosome is composed of rRNA (ribosomal RNA) and some proteins. It is a cytoplasmic organelle present in both prokaryotic as well as in eukaryotic cells. However, the structure and composition of the ribosome can differ in both the cell. In prokaryotes, the ribosome is composed of 70S subunit whereas eukaryotic organisms consist of 80S ribosomes.
There are three binding sites present in rRNA, named as- A, P, and E sites. The aminoacyl tRNA docks at the A site and peptidyl- tRNA binds at the P site. The site where the tRNA leaves the ribosome is termed as E (exit) site.
Protein Synthesis Steps
The process of producing an mRNA template from a DNA molecule is termed transcription. The mRNA has trinucleotide sequences (codons) that encode for particular amino acids and provide a template for the process of translation.
Transcription completes in four major steps-
(2) Promoter Escape
In the initiation phase, the RNA polymerase binds to the promoter of DNA with the intervention of any transcription factor. After the binding, the DNA starts unwinding and form a transcription bubble which is also termed as transcription start site.
In the next step, the RNA polymerase escapes the promoter to step into the elongation phase. The RNA travel across the DNA template and base pairs with the nucleotides on the noncoding strand in the elongation phase.
The RNA polymerase forms the sugar-phosphate backbone and thymines are replaced by uracils. The termination step starts with the breaking of hydrogen bonds of RNA- DNA helix.
The mRNA script in eukaryotes goes through polyadenylation, capping, and splicing but the prokaryotic mRNA does not undergo these modifications.
The process of producing amino acids from the mRNA script is called translation. These amino acids are linked together and form a polypeptide chain.
Protein Synthesis Diagram
Translation completes in the cytoplasm of the cell and consist of four phases-
The initiation starts with the binding of the ribosome to 5’end of mRNA. In elongation, the binding of aminoacyl-tRNA to the ribosome occurs. When the A site of ribosome faces a stop codon, the process terminates.
In eukaryotic translation, proteins undergo some post-translational modifications like protein folding, proteolysis. These modifications are termed as several enzymatic actions of a polypeptide chain that occur after the formation of a polypeptide chain.
For example- to ensure proper cellular localization, the ends may be modified. The folding of the polypeptide chain to conclude secondary and tertiary structures is termed as protein folding.
Protein Synthesis Citations
- Mechanism and Regulation of Protein Synthesis in Saccharomyces cerevisiae. Genetics . 2016 May;203(1):65-107.
- Bacterial Protein Synthesis as a Target for Antibiotic Inhibition. Cold Spring Harb Perspect Med . 2016 Sep 1;6(9):a025361.
- Branched-chain amino acids and muscle protein synthesis in humans: myth or reality? J Int Soc Sports Nutr . 2017 Aug 22;14:30.
- Protein synthesis and quality control in aging. Aging (Albany NY) . 2018 Dec 18;10(12):4269-4288.
- Protein Synthesis Initiation in Eukaryotic Cells. Cold Spring Harb Perspect Biol . 2018 Dec 3;10(12):a033092.