What is Protein Synthesis?
Central Dogma of protein synthesis:
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| Protein Synthesis |
In 1957, Crick proposed that DNA determines the sequence of amino acids in a polypeptide through mRNA, this is the main principle or Central dogma of protein synthesis which involves transcription and translation. In this process, there is need of DNA to provide base sequence, 20 amino acids, non-genetic types of RNA, Ribosomes as site of protein synthesis, enzymes and ATP / GTP as source of energy. So, let's know about What is Transcription and Translation? That is the process DNA to RNA.
Transcription and Translation:
Process of Transcription:
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| Process of transcription |
1) It is the formation of mRNA on DNA template. This process takes place in presence of DNA dependent RNA polymerase.
2) In this process, Genetic information from DNA is copied into RNA. Therefore this process is called as Transcription.
3) The DNA strand which is used for synthesis of RNA is called as Antisense or template strand. This strand is always oriented in 3'---> 5' direction. Other strand which are not involved in the RNA synthesis is called as sense or coding strand,which is oriented in 5'-->3' direction.
4) A small DNA sequence which provides binding site for RNA polymerase is called promoter which is present towards 5'end (upstream), while a small DNA sequence which terminates the transcription process called terminator which is present towards 3' end (downstream).
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| Transcription |
5) The structural gene which is to be transcribed is generally monocistronic in eukaryotes and polycistronic in prokaryotes.
6) In eukaryotes the genes are split, having exons and introns. The DNA sequences which are expressed or appear in the mature or processed RNA are the exons while those, not appearing in mature or processed RNA are the introns.
7) During transcription the enzyme RNA polymerase binds to the promoter site and brings about the initiation of the process. The two strands of DNA separate from each other.
8) According to the base sequence present on the template strand, the complementary RNA nucleotides are selected and joined one after the other to form the mRNA strand. This is called as elongation of the process.
9) A small part of the RNA remains attached to the enzyme. As the enzyme reaches to the terminator region, both the enzyme and newly constructed RNA fall off. This is called as termination of transcription.
10) In the prokaryotes organisms as bacteria, the newly formed RNA do not require further processing, but in eukaryotes the matter is different.
11) In eukaryotes, there are three types of polymerases; 1) RNA polymerase-I (for the formation of rRNA, 2) RNA polymerase-II (for the synthesis of precursor of mRNA i.e. heterogenous nuclear RNA (hnRNA) and 3) RNA polymerase-III (for the formation of tRNA, snRNA (small nuclear RNA).
12) In eukaryotes, the RNA is non-functional when it is formed and undergoes splicing, capping and tailing. The removal of introns from the RNA is called Splicing. Addition of an unusual nucleotide methyl guanosine triphosphate at 5' end of hnRNA is called capping. Addition of adenylate residues at 3' end is called tailing.
13) The hnRNA has undergone capping, splicing and tailing now functions as mRNA.
14) After transcription in prokaryotes, the mRNAs are used in the same compartment as there is no nucleus while in eukaryotes they are produced in the nucleus and then are transported out of the nucleus, to the ribosomes (in cytoplasm) which is a site for protein synthesis i.e. translation.
Process of Translation:
Translation is the process in which sequence of codons on the mRNA strand used and accordingly the amino acids are joined to each other to form a polypeptide chain that makes protein.
Steps in Translation:
1. Initiation:
It begins with the formation of initiation complex which requies the mRNA having codons for a polypeptide, the smaller (30S) and larger (50S) sub-units of ribosome, the initial AA1- tRNA complex and ATP and GTP as source of energy. The process of initiation needs initiation factors. In prokaryotes the first AA1-tRNA complex has amino acid, N-formyl-metheonine while in eukaryotes it is methionine. The process starts with binding of mRNA on the smaller 30S sub-unit of ribosome.
The start codon AUG is positioned properly. The AA1-tRNA complex now gets attached to the start codon AUG. This is done with the help of anticodon UAC of tRNA. Small and large unit of ribosome has three sites i.e. aminoacyl (A) site, peptidyl site (P) and exit (E) site. The empty tRNA leaves from E site. Only the AA1-tRNA complex binds at P site directly while all the other incoming tRNA complexes get attached first at A site and then are shifted to P site. Polypeptide chain is released from P-site. In eukaryotes 40S (smaller sub unit) and 60S (larger sub unit) combine to form 80S type of ribosome.
2. Elongation:
After initiation, the process of elongation starts. This is done by formation of peptide linkage / bonds in between the successive amino acid molecules (AA1, AA2, AA3 and so on). The elongation activity is Catalyzed by the enzyme peptidyl transferase. Each tRNA complex brings a specific amino acid. Due to complementary nature of anticodons and codons the amino acids are placed to their proper positions. The elongation activity involves elongation factors.
During elongation, the ribosome moves along the mRNA in step wise manner from start to stop codon (5'-->3'), one codon ahead each time. This movement is called translocation.
In every step of translocation one amino acid is added in the polypeptide chain causing elongation.
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| Translation |
3. Termination:
Elongation continues until the ribosome adds the last amino acid coded by the mRNA. When the mrna reaches to the last, termination codon, i.e.either UAA, UAG or UGA, termination occurs.
In identifying the stop/ termination codon and in releasing the polypeptide chain from the site, the release or termination factors R1, R2 and S play an important role. After termination, the smaller (30S) and larger ( 50S) sub units of ribosome get separated from each other. The energy required for protein synthesis activity is fulfilled by ATP and GTP.