process of translation
definition:
this is the process in which genetic information in the form of genetic codes that are present on the mRNA is decoded in to the amino acid molecules of polypeptide chain leads to protein synthesis formation.
like the process of transcription it also has three stages which are:
- initiation stage
- elongation stage
- termination stage
Role of genetic code and tRNA:
genetic codes that are present on the mRNA specifies that what will be the sequence of the protein. For each protein that is synthesized the start codon ever is AUG which code for methionine amino acid. And the protein synthesis terminate at the reach of codon UAA, UAG, and UGA.as we know genetic codes are universal and degenerative too so like serine amino acid is attached by the codes: AGC, AGU, UCU, UCG, UCC, UCA
Translational machinery:
actually the protein synthesis take place in the part of the cell that we call cytoplasm. but only those protein which are encoded by the genes of the mitochondrial and chloroplast DNA are synthesized there. the triple set of the codon that are present on mRNA are decoded by only ribosome organelle into the polypeptide chain.
the basic machinery that is required for translation is including:
- template strand for translation which is mRNA
- tRNA as an adaptor molecule for amino acid
- protein manufacturing organelle which is ribosome
- many necessary proteins
- and GTP hydrolysis produce energy which is used in this process
prokaryotic Ribosome & eukaryotic organelle’s ribosome:
- this type of ribosome is composed of 70S.
- which is further composed of 50S which is larger sub-unit and 30S the smaller sub-unit
- the larger sub-unit which is of 50S is further composed of 5 S rRNA (which is composed of 120 nucleotides), 23 S rRNA( which is composed of 2,900 nucleotide) and -34 protein
- and the smaller sub-unit which is 30S is composed of 16S rRNA (which is composed of 1,540 nucleotides) and 21 protein
Eukaryotic Ribosome:
- eukaryotic ribosome is made up of 80S
- it is further composed of 60S and 40S sub-units
- 60S sub-unit which is larger one is further composed of 5.8S rRNA( which is composed of 160 nucleotides), 5S rRNA( which is composed of 120 nucleotides), 28s rRNA ( which is composed of 4,700 nucleotides) and 49 protein.
- and the smaller sub-unit which is composed of 40S is further composed of -33 protein and 18S rRNA ( which is made up of 1,900 nucleotides)
process of initiation stage of translation in prokaryotes and eukaryotes:
- presence of 5’cap:
for the ribosome to bind with the 5′ end of the mRNA after synthesis of 20-30 nucleotides the 5′ end of mRNA is capped with the nucleotide guanine.
- presence of poly A tail:
To prevent mRNA from any reaction in the process of travelling from site of synthesis to the site of translation which is ribosome,50-250 adenine nucleotides are added at the 3′ end of mRNA. this process also help in the termination process of transcription.
Translation process in prokaryotes:
- it is simple process in prokaryotes and the include involvement of initiation factors that are:IF-1, IF-2, and IF-3.
- than the mRNA will bind to the 30S sub-unit of the ribosome in such a way that the initiation codon AUG is located in in the precursor to p-site of the ribosome.
- in prokaryotic translation process there is no involment of 5’end mRNA like there is no capping.
- rather than cap there is Shine-Dalgarno sequence is present and AUG that is start codon these are present at the 5′ non translated end of the mRNA this sequence can also occur in the internal site within the mRNA.
ability of the the ribosome to bind and start the process of translation from the site internal to the prokaryotic mRNA makes it to allow:
- the genes are now organized into the operons
- this operon have to be transcribed into(polycistronic) single mRNA.
- then the expression of a lot of genes that are related each other in function is controlled by only one promotor.
Translation in Eukaryotes:
- Shine-Dalgarno and ribosomal binding site that is present in prokaryotes is not present here.
- here 5’end cap is present on the mRNA.
- cap binding protein complex produce 43S complex.
- than the process of scanning of mRNA by presence of 43 complex that is produced by the 5’cap for the initiation after recognizing AUG start codon of translation.
- elongation process of translation start at the assembly of 80S larger sub-unit of the ribosome.
- then the stage of termination reached( which is proximal to the site of initiation)
- translation of mRNA takes place as a circular complexes.
- basically no mechanism exist for internal initiation process and mRNA in eukaryotic organisms are mono-cistronics.
Major difference that will make eukaryotic translation different from prokaryotic one:
- 5’cap structure is present in eukaryotic mRNA.
- mRNA of eukaryotes is polyadenylated.
- bases that are present near around the initiation codon AUG will influence and enhance the efficiency of the process of initiation.
- the function of the eukaryotic initiation factor is to scan along the mRNA from the 5;cap to recognize the AUG start codon
mRNA in eukaryotes translated as circular complex this is by polysomes:
- many ribosomes that are attached to the same mRNA string is called polysomes.
- every single ribosome will be in contact with approximately 30 nucleotides of the mRNA.
- if the size of mRNA is large it will allow only 80 nucleotides per ribosomes
- this process happen as when the one ribosome that was attach covers approximately distance of 80 nucleotides that is sufficient one it will leave the next ribosome comes ,binds and than followed by the next one ribosome. in this many many polypeptides are synthesized from the same mRNA string
Other steps of translation:
- proof reading (which is checking of aa-tRNA against codon) take place when the repeating cycle of codon for the addition of aa-tRNA take place
- elongation take place by the eukaryotic elongation factor-1 which first bound to the GTP and than bring aa-tRNA toward the ribosome to add it to the growing polypeptide chain.
- GTP hydrolysis take place and the eukaryotic elongation factor-1 detach from the ribosome.
- eukaryotic TF bind to the stop codon then the process of translation stops.
- the newly synthesized polypeptide undergo into the further modification like S-S bond formation and than folding take place an convert into the tertiary structure of the protein than goes to the appropriate compartment of the cell for storage and utilization where it will be functional.
- many protein can go for transient post translational modification this is because these protein than will e able to perform their tightly regulated function.
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