Main Difference – Transcription vs Translation
Transcription and translation are both involved in the process of gene expression required for cell functioning. Transcription is the copying down of genes in the genome into RNA pieces. Translation is the decoding of the mRNA into proteins. The transcription of DNA into RNA and the translation of RNA into proteins are considered as the central dogma of molecular biology. The main difference between transcription and translation is that transcription involves the production of RNA from DNA whereas translation involves the protein synthesis by decoding the mRNA.
This article looks at,
1. What is Transcription
– Definition, Process, Characteristics
2. What is Translation
– Definition, Process, Characteristics
3. What is the difference between Nucleolus and Nucleus
What is Transcription
Transcription is the first step of the gene expression process. A gene is copied into an RNA piece with the aid of the enzyme, RNA polymerase. This piece of RNA is called the primary transcript. It is complementary and antiparallel to the DNA sequence which it is copied from. Transcription may produce several types of RNAs: messenger RNA (mRNA), transfer RNA (tRNA), ribosomal RNA (rRNA) and non-coding RNA such as microRNA (miRNA). The genes encoded for proteins produce mRNAs. The mRNAs consist of untranslated regions called 5′ UTR and 3′ UTR for the regulation of protein synthesis. Other RNA types are considered to help the synthesis, regulation and processing of proteins.
In viruses, mRNA is synthesised from its RNA genome. Their genome consists of negative-sense, single-stranded RNA. During the RNA replication, a positive-sense, single-stranded RNA which can be utilised in the translation lately, is produced. Some viruses like HIV transcribe RNA genomes into DNA with the aid of the enzyme, reverse transcriptase. Thus, synthesising complementary DNA from RNA is referred to as the reverse transcription.
In prokaryotic and eukaryotic transcription, antisense strand is transcribed into the mRNA in the 5′ to 3′ direction. This excludes the formation of Okazaki fragments as in DNA replication. Moreover, RNA polymerase needs no RNA primers for the initiation of the transcription. The process of transcription occurs in four steps: initiation, promoter escape, elongation and termination. Transcription is initiated by the binding of RNA polymerase into the promoter, with the aid of associated proteins called transcription factors. Six transcription factors which are associated with RNA polymerase II can be identified in eukaryotes: TFIIA, TFIIB, TFIID, TFIIE, TFIIF and TFIIH. Initiation of the transcription is regulated by activators and repressors.
After the formation of transcription initiation complex, a few nucleotides are added, and RNA polymerase escapes from the promoter. Then transcription elongation complex is formed. RNA polymerase traverses the antisense DNA strand and adds nucleotides complementary to the template in order to produce the new RNA strand. The nucleotide precursors used are adenine, uracil, cytosine and guanine. The primary transcript is cleaved from the template in the termination of the process. In eukaryotes, the cleavage is followed by post-transcriptional modifications such as polyadenylation, 5′ end capping and splicing out of introns. A simple diagram illustrating transcription and the processing is shown in figure 1.
Figure 1: Transcription and Processing of RNA
Antibiotics work as transcription inhibitors. Therefore, they can be used to cure bacterial and fungal infections in humans. Rifampicin and 8-Hydroxyquinoline are two antibiotics which inhibit the transcription in bacteria and fungi respectively. On the other hand, transcription can be measured by RT-PCR, DNA microarrays, in situ hybridization, northern blot and RNA-seq like molecular biological techniques.
What is Translation
Translation is the second step in the process of gene expression. The mRNAs, produced by transcription are translated into proteins in the cytoplasm by ribosomes. During translation, mRNA is decoded by ribosomes in order to produce an amino acid chain or a polypeptide chain. Complementary tRNA anticodon sequences, carrying a specific amino acid bind to the mRNA. This type of tRNAs is called amino acyl tRNAs. The binding is facilitated by ribosomes. The amino acids carried by tRNA from the polypeptide chain by peptide bond formation between two amino acids. This amino acid chain undergoes post-translational modifications and then folds into 3-D structure in order to become an active protein.
Translation is occurred in three steps: initiation, elongation and termination. In order to initiate the translation, the ribosomes are assembled around the target mRNA. The first tRNA added is the methionine carrying tRNA which matches the start codon, AUG on the 5′ end of the mRNA. A codon is a sequence of three nucleotides on the mRNA, which code for a specific amino acid. After the first tRNA is attached to the start codon, the tRNA corresponding to the second codon gets attached to mRNA. Then the ribosome translocates to the second tRNA. The first and the second amino acids, which are carried by the tRNA, form a peptide bond between them. Likewise, the decoding proceeds as the ribosome are translocated into 5′ to 3′ direction on the mRNA. The amino acid is added to the C-teminus of the polypeptide chain. Therefore, translation is considered as amino-carboxyl directed. When the ribosome reaches the stop codon (UAG, UAA, UGA), it releases the polypeptide chain. A simple diagram of translation is shown in figure 2.
Figure 2: Ribosome mRNA translation
Prokaryotes contain small ribosomes called 70S ribosomes whereas eukaryotic ribosomes are comparatively large and called 80S ribosomes. A ribosome is composed of two subunits which are called large subunit and small subunit. In eukaryotes, a small subunit of 80S ribosome binds to the 5′ end of the mRNA. But, in prokaryotes, small subunit, of 70S ribosome binds to Shine-Dalgarno sequences in the mRNA. A Shine-Dalgarno sequence marks the starting of each coding sequence of the prokaryotic operon.
Numerous antibiotics capable of inhibiting translation are chloramphenicol, tetracycline, anisomycine, cycloheximide, streptomycin, etc. Translation can be measured by spectrometry methods, biochemical assays and antibody-based methods such as ELISA and Western blot.
Difference Between Transcription and Translation
Purpose
Transcription: Synthesis of RNA copies of the genetic instructions written in the genome is the main purpose.
Translation: The main purpose is the synthesis of proteins from RNA which are copied from genes.
Template
Transcription: Template is the genes in the genome.
Translation: Template is the mRNA.
Location
Transcription: This occurs in the nucleus.
Translation: This occurs in the cytoplasm.
Enzymes
Transcription: RNA polymerase are the enzymes.
Translation: Ribosomes are enzymes.
Initiation
Transcription: Binding of RNA polymerase into the promoter of the gene initiates the formation of transcription initiation complex. RNA polymerase is directed by the promoter to the transcription initiation site.
Translation: The binding methionine carrying tRNA to the AUG start codon initiate the translation.
Precursors
Transcription: The four nitrogenous bases: Adenine, guanine, cytosine and uracil are the precursors.
Translation: The 20 different amino acids carried by tRNAs are the precursors.
Elongation
Transcription: RNA polymerase elongates from 5′ to 3′ direction.
Translation: Incoming aminoacyl tRNA binds to the codon at A-site. The new amino acid binds with the growing chain. Ribosome moves to the next codon position from 5′ to 3′ direction.
Type of Bond Forms
Transcription: A phosphodiester bond between two nucleotides can be observed.
Translation: A peptide bond between two amino acids can be observed.
Termination
Transcription: Transcript is released, the enzyme detaches and DNA rewinds.
Translation: Ribosome dissembles by encountering into one of the three stop codons, and polypeptide chain is detached.
Product
Transcription: Several functional forms of RNA is produced in transcription: mRNA, tRNA, rRNA and non-coding RNA.
Translation: Proteins are the products.
Product Processing
Transcription: Post-transcriptional modifications occur such as addition of 5′ cap, the 3′ poly A tail and splicing out of introns occur.
Translation: Numerous post-translational modifications such as formation of disulfide bridges, phosporylation, farnesylation, etc. occur.
Inhibition by Antibiotics
Transcription: They are inhibited by rifampicin and 8-Hydroxyquinoline.
Translation: They are inhibited by tetracycline, chloramphenicol, streptomycin, erythromycin, anisomycin, cyclohexamide, etc.
Localization
Transcription: They are localised into prokaryotes’ cytoplasm and eukaryotes’ nucleus.
Translation: They are localised into prokaryotes’ cytoplasm and eukaryotes’ ribosomes on the endoplasmic reticulum.
Conclusion
Transcription and the translation are collectively called gene expression. During transcription, nucleotides are utilised to produce a new RNA strand by the RNA polymerase and other associated proteins. On the other hand, amino acids are utilised to produce a polypeptide chain in translation. In eukaryotes, transcription and translation both add modifications into their end products which are referred to as post-transcriptional and the post-translational modifications, respectively. Post-transcriptional modifications involve the addition of 5′ cap, 3′ poly A tail and splicing out of introns. During post-translational modifications, protein maturation is acquired through phosphorylation, formation of the disulfide bridges and carboxylation like reactions. Therefore the key difference between transcription and translation is in their role in the process of gene expression.
Reference:
1. “Transcription (biology)”. Wikipedia, the free encyclopedia, 2017. Accessed 26 Feb 2017
2. “Translation (biology)”. Wikipedia, the free encyclopedia, 2017. Accessed 26 Feb 2017
3. Clancy, S. and Brown, W. “Translation: DNA to mRNA to Protein”. Nature Education, 2008. 1(1):10 . Accessed 26 Feb. 2017
4. “Stages of translation”. KHANACEDAMY. 2017. Accessed 26 Feb. 2017
Image Courtesy:
1. “Process of transcription (13080846733)” By Genomics Education Programme – Process of transcription (CC BY 2.0) via Commons Wikimedia
2. “Ribosome mRNA translation en”By LadyofHats – Own work (Public Domain) via Commons Wikimedia
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