The process of conversion of RNA (Ribonucleic acid) from the DNA (Deoxyribonucleic acid) template is known as Transcription. However, when the RNA (mRNA) template is converted into protein (amino acid sequence) it is known as translation.
We can also say that transcription is the initial step of gene expression, which gives rise to the primary transcript in the form of mRNA with the help of enzyme RNA polymerase. While translation is the formation of the polypeptide chain, where the mRNA templates get attached to the ribosomes and decode the mRNA template for the formation of the amino-acid, having 3 base segment code of each and these amino acids together constitute polypeptide chain known as protein.
The above lines are the simple way to explain the process of the central dogma of life. But the actual process is tedious and is done with high fidelity.
We all know, that every living cell on Earth stores their genetic information in the form of DNA, which is double-stranded nucleic acids. It is long polymer chains and is composed of four monomers and are named as adenine, guanine, cytosine, and thymine or simple as A, G, C, T respectively. These monomers are attached together in a linear sequence and encode the genetic information.
These processes are common in all living organisms. In order to transfer the genetic information, the DNA should make more than one copy of itself, and than it gets expresses by the mechanism of RNA synthesis and finally protein. So in this content, with the way of highlighting the difference between the two main process (transcription and translation), we will also summarise them.
Content: Transcription Vs Translation
|Basis for Comparison||Transcription||Translation|
|Meaning||It is the process of formation of RNA from DNA.||It is the process of formation of polypeptide over ribosome.|
|It contains ||DNA, RNA, enzymes, sigma subunit.||mRNA, small and large ribosomal, initiation factors, elongation factors, tRNA.|
|Location ||In eukaryotes, it is in the nucleus and prokaryotes it is in the cytoplasm.||It occurs in cytoplasm.|
|Process||RNA polymerase (enzyme) act along the DNA template strand. It also involves modifications like cutting, splicing, folding, modification of nitrogenous bases and addition of the specific groups at the ends.||Ribosome complex interacts with the mRNA strand. It also involves the modification of amino acids chain like sorting, packing, glycosylation, acetylation.|
|Raw materials||Four types of ribonucleoside triphosphates-ATP, CTP, GTP, UTP.||20 types of amino acids.|
|Template ||DNA (antisense strand).||mRNA.|
|Controlling factor ||RNA polymerase.||Ribosomes.|
|Splicing ||It occurs in transcription.||Absent.|
|Types of bond||Phosphodiester bonds are there between the nucleotides.||The peptide bond is present between the amino acids of the polypeptide chains.|
| Inhibitors/ Antibiotics||Rifampicin, Actinomycin D, and 8-Hydroxyquinoline.||Streptomycin, Chloramphenicol, Tetracycline, Cycloheximide, etc.|
Definition of Transcription
Likewise the DNA structure, RNA also consists of the chain of nucleotides joined by the phosphodiester bonds. But there are many differences between both of these, and among them, one of the main difference is the strand, where DNA is double helical or have two polynucleotide strand, while RNA is single stranded. As we know all RNA is synthesized from the DNA strand only from the process of transcription. It is considered as the way of expressing the genes.
The process of transcription is mediated by the enzyme, RNA polymerase and produces messenger RNA (mRNA), tRNA and rRNA. This process occurs in both types of organisms, that is in prokaryotes and in eukaryotes, but with the slight difference.
However in eukaryotes, mRNA produced in transcription is translated after processing but in prokaryotes, it is directly translated. Secondly, in eukaryotes, the process of transcription and translation is separated by the nuclear envelope, that is transcription occurs in the nuclear envelope and translation in the cytoplasm, while in prokaryotes both the process occurs in the cytoplasm.
Transcription does not take place of the whole DNA template, it is very particularly done in some of the regions only. The exact reason is still not known, but it is said that gene expressions take place of the required part of the DNA template. The process of transcription takes place in three place steps, in prokaryotes as well as in eukaryotes: Initiation, Elongation, and Termination.
Transcription in prokaryotes
Initiation: For starting the process, there is an enzyme called an RNA polymerase which gets attached to the DNA template to the specified area known as promoter region, which enhances or give the signal for the RNA synthesis. But as there is two coding DNA strand, and it is the work of sigma factor (one of the subunits of RNA polymerase present in E. coli) to recognize and initiate the process.
Now together the RNA polymerase and other factors are called as transcription initiation factors recognize to the promoter regions which are TATA box or Pribnow box, this box consist of the six nucleotides bases (TATAAT) and the ‘-35’ sequence and has TTGACA nucleotides for the recognition. These regions are situated left side and 10 and 35 bases respectively away from the starting point of transcription.
Elongation: As soon as the promoter regions are known, the process of transcription starts from the 5′ to 3′ end, which is antiparallel to the DNA strand. The RNA polymerase unwinds the DNA double helix and proceeds along bases 10-20 times.
Termination: To stop the process there are two types of factors, called as Rho-dependent and Rho-independent termination factors. The Rho factors are the protein, it provides or acts by binding to the growing RNA, and hence the mRNA synthesis will stop, and DNA polymerase will also dissociate. The Rho-independent factors work by the formation of hairpin kind of structure by RNA and cause termination of the process of transcription.
Transcription in eukaryotes
The process of eukaryotes is similar to that of the prokaryotes but is more complicated, in this type, there is three RNA polymerase, which is RNA polymerase I, II, and III. These polymerase play different roles in the transcription process, RNA polymerase II is responsible for the synthesis of mRNA and snRNA.
Here the sequence, present in the upstream region of the DNA sequence and is almost similar to the prokaryote Pribnow box is present. This sequence is called the TATA box or Hogness box. Another sequence called the CAAT box is another site which supports the initiation of the transcription.
There are other molecular events required for the initiation, which are assisted by transcription factors (TFs), namely TFIID, TFIIA, TFIIB, TFIIF, TFIIE, TFIIH, these factors bind with the promoter regions and helps in the transcription. The enhancer can stimulate the process by 100 fold, they work by binding to transcription factors and together known as activators.
Post Transcriptional Modifications
The RNA produced by RNA polymerase II after the process of transcription is the primary mRNA transcript in eukaryotes. These mRNA undergoes many types of modifications which are referred to as post-transcriptional modifications. These modifications are done on both the sides of the transcript and include splicing, base additions, base modifications, etc. These changes in the mRNA are done with high fidelity.
The alterations in each of the pre mRNA are done as at 5′ end capping is done and at the 3′ end, the poly-A tail is added.
These alterations are helpful to mRNA in many ways as it makes ribosome to recognize the 5′ end and gets attached, it also inhibits the action of the hydrolytic enzyme and supports the transportation of the mRNA.
Definition of Translation
Translation is the process of expressing the genes in the form of protein in a living cell. In the eukaryotes as well as in prokaryotes it occurs in the cytoplasm of the cell. Here the Ribosomes play the most critical role, as the mRNA comes out of the nucleus and bind to this cellular machine. The reason that only mRNA is only used for the process, is that mRNA is messenger RNA and carry messages or genetic information from DNA till the protein.
The process initiates when mRNA binds to ribosomes and ribosomes attracts another RNA called as tRNA (transfer RNA). This tRNA carry specific amino acids, which brings the matched mRNA code carrying the three-base sequence.
The time tRNA and mRNA three base sequence are matched, they start pairing with their complementary three-base sequence and tRNA starts depositing the amino acids and the process goes on. The polypeptide chain gets from when one amino acid is attached with another and result in a final protein.
This process is not so easy going, there are certain precursors which initiate the process, firstly ribosome decodes the mRNA chain, the aminoacyl tRNA or tRNA containing the specific amino acids binds to the complementary three-base sequences. This process also completes in three steps, initiation, elongation and termination.
Initiation: The ribosomes gets attached to the target mRNA chain. Methionine is the first tRNA added, which paired with the AUG codon (start codon) of the mRNA. The process starts from the 5′ end of the mRNA chain. One amino acid is added in complementary with the three nucleotides sequence of the mRNA; these three nucleotides are called codon.
Elongation: After the first amino acids get paired, the tRNA moves to the second codon to pair another amino acids, likewise the process goes on and forms the chain from 5′ to 3′ end. The peptide bond is formed between the two amino acids.
Termination: There are three termination codon or stop codon, UAG, UGA, and UAA. Whenever the ribosomes find any of these codons, it will stop moving further and will release the polypeptide chain.
The distinction between the prokaryotic and eukaryotic translation is the size of the ribosomes, as in prokaryotes the ribosome is the 70s, while in eukaryotes it is of 80s. Even the prokaryotes have the Shine-Dalgarno sequence, which is the starting point of the coding sequence and the ribosomes bind here.
The genetic information is translated from the four alphabet letters of polynucleotides to the 20 alphabet letters of proteins. There are 64 possibilities of the combinations of the nucleotides, and these genetic codes are universal among all species, but the exception is the mitochondria, which creates its proteins.
Post Transcriptional Modification
The post-transcriptional modification is for the newly synthesized polypeptide chain, which is directed to form the proper structure by the interacting with ionic bonds, hydrophobic bonds, Vander Waals, etc. and other modifications like glycosylation, acetylation, amino-terminal modifications, carboxyl-terminal modifications.
Key Differences Between Transcription and Translation
Following points highlight the main difference between the process of gene expression which is transcription and translation:
- After the process of replication, the next step is the transcription, where RNA is synthesized from the duplicated DNA. The next phase of the gene expression is the protein synthesis called as translation, here the protein or polypeptide is synthesized from the mRNA.
- Transcription occurs in the nucleus in eukaryotes, and in prokaryotes, it is in the cytoplasm, while translation happens in the cytoplasm in both.
- The leading role in transcription is of polymerase, while in translation ribosomes play the essential character.
- Transcription proceeds when RNA polymerase (enzyme) act along the DNA template strand. Post-transcriptional modifications involve cutting, splicing, folding, modification of nitrogenous bases and addition of the specific groups at the ends. On the other hand, translation starts when a ribosome complex interacts with the mRNA strand. The post-transcriptional modification involves the modification of amino acids chain like sorting, packing, glycosylation, acetylation.
- DNA is the template strand in transcription, while in translation mRNA act as the template strand.
- Splicing is required in the transcription, where the introns are spliced off from the primary transcript, it is also kind of modification in the chain of mRNA, although it is not required in translation.
- Phosphodiester bonds are there between the nucleotides of RNA formed, while peptide bond is present between the amino acids of the polypeptide chains in the process of translation.
- Inhibitors or Antibiotics to stop the process of transcription are Rifampicin, Actinomycin D, and 8-Hydroxyquinoline, while to inhibit the translation Streptomycin, Chloramphenicol, Tetracycline, Cycloheximide, etc. are used.
- Both process requires template.
- Both are the chemical mechanism.
- Nucleotides requires are Adenine, Guanine, Cytosine, Uracil.
- The process completes in three steps – Initiation, Elongation, and Termination.
In this content, we discussed the transcription and translation, which are regarded as the main steps of the protein synthesis, in our earlier post we have already addressed the replication. However, we can say that these studies have been useful for us not only to gain knowledge but to know the science profoundly and can do more research on them. It is also helpful from the medical point of view.