Main Difference – Transcription vs Reverse Transcription
Transcription and reverse transcription are two cellular mechanisms that involve encoding genetic material into other nucleic acids. The main difference between transcription and reverse transcription is that transcription is the encoding of the DNA genome into RNA molecules whereas reverse transcription is the encoding of the RNA genome into DNA molecules. mRNA, tRNA or rRNA can be produced during transcription. cDNA is produced during reverse transcription. RNA polymerase is involved in transcription while reverse transcriptase is involved in reverse transcription as enzymes. Transcription occurs in both prokaryotes and eukaryotes while reverse transcription mainly occurs in RNA viruses.
Key Areas Covered
1. What is Transcription
– Definition, Mechanism, Significance
2. What is Reverse Transcription
– Definition, Mechanism, Significance
3. What are the Similarities Between Transcription and Reverse Transcription
– Outline of Common Features
4. What is the Difference Between Transcription and Reverse Transcription
– Comparison of Key Differences
Key Terms: DNA, Host Genome, mRNA, Reverse Transcription, RNA, RNA Polymerase, Transcription
What is Transcription
Transcription involves making a copy of genetic information stored in DNA genomes into a complementary strand of RNA. RNA polymerase is the enzyme involved in transcription. The transcript is complementary and antiparallel to the template. The main type of RNA produced is mRNA. mRNAs are produced by the transcription of genes that are encoded for proteins. This mRNA is decoded during protein synthesis to produce an amino acid sequence of a functional protein. Furthermore, tRNA and rRNA, which aid in protein synthesis, are also transcribed. Transcription is shown in figure 1.
Figure 1: Transcription
Transcription Mechanism
In prokaryotic and eukaryotic transcription, the antisense strand is transcribed into the mRNA in the 5′ to 3′ direction. RNA polymerase needs no RNA primers for the initiation of the transcription. The four steps involved in the transcription process are initiation, promoter escape, elongation and termination.
Initiation
Transcription is initiated by the binding of RNA polymerase into the promoter of the gene with the aid of associated proteins called transcription factors. The six transcription factors associated with RNA polymerase in eukaryotes are TFIIA, TFIIB, TFIID, TFIIE, TFIIF and TFIIH. Initiation of the transcription is regulated by activators and repressors.
Promoter Escape
A few nucleotides are added to the new strand after the formation of transcription initiation complex and RNA polymerase escapes from the promoter.
Elongation
The transcription elongation complex is formed immediately after the promoter escape. RNA polymerase traverses the antisense DNA strand and adds nucleotides complementary to the template, producing the new RNA strand. The nucleotide precursors used are adenine, uracil, cytosine and guanine.
Termination
The primary transcript is cleaved off from the template at the transcription termination site for 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.
What is Reverse Transcription
Reverse Transcription refers to the reverse process of the normal transcription in which the RNA template is copied to form a cDNA molecule in retroviruses. It begins with the entrance of viral particles to the host. Hence, reverse transcription occurs in the cytoplasm of the host. The produced cDNA is integrated into the host genome for the replication and protein synthesis. The main type of enzyme involved in the process is reverse transcriptase. The mechanism of reverse transcription is shown in figure 2.
Figure 2: Reverse Transcription
Reverse Transcription Mechanism
- The lysyl tRNA that acts as the primer for the reverse transcription binds with the primer binding site (PBS) of the RNA genome.
- Reverse transcriptase adds nucleotides to the 3’ end of the primer, synthesizing non-coding (U5) and R regions of the viral RNA.
- The RNAse H domain of the reverse transcriptase degrades the U5 and R domains from the RNA genome.
- The primer ‘jumps’ into the 3’ end of the RNA genome, hybridizing the newly-synthesized DNA with the R region of the 3’ end of the genome.
- All the sequences except the PP region are cleaved off by RNAse H activity.
- The cDNA is synthesized by the reverse transcriptase enzyme, adding complementary nucleotides to the new strand.
- The tRNA is degraded.
- During the synthesis of the second strand, another “jump” occurs, hybridizing PBS from the second strand with the complementary PBS on the first strand.
- Synthesis of both strands is completed by the DNAP function of reverse transcriptase.
Similarities Between Transcription and Reverse Transcription
- Transcription and reverse transcription are two mechanisms of encoding genetic material into other nucleic acids.
- Both transcription and reverse transcription are mediated by enzymes.
- The nucleotide precursors used are adenine, uracil, cytosine and guanine in both transcription and reverse transcription.
- The product of both transcription and reverse transcription is complementary and antiparallel to the template.
Difference Between Transcription and Reverse Transcription
Definition
Transcription: Transcription is the process of copying the genetic information stored in a DNA genome into a complementary strand of RNA.
Reverse Transcription: Reverse Transcription refers to the reverse process of the normal transcription in which the RNA template is copied to form a cDNA molecule in retroviruses.
Type of Encoding
Transcription: Transcription is the encoding of DNA genome into RNA.
Reverse Transcription: Reverse Transcription is the encoding of RNA genome into cDNA.
Occurrence
Transcription: Transcription occurs in both prokaryotes and eukaryotes.
Reverse Transcription: Reverse transcription occurs in retroviruses.
Location
Transcription: Transcription occurs in the cytoplasm of prokaryotes and nucleus in eukaryotes.
Reverse Transcription: Reverse transcription occurs in the cytoplasm of the host.
Types of Enzymes
Transcription: RNA polymerase is involved in the transcription.
Reverse Transcription: Reverse transcriptase is involved in the reverse transcription.
Primer
Transcription: No primer is used by RNA polymerase in transcription.
Reverse Transcription: Lysyl tRNA acts as the primer for reverse transcriptase.
Template
Transcription: The transcript is flanked by 5′ UTR, 3′ UTR, and a poly-A tail.
Reverse Transcription: The transcript is flanked by long terminal repeats (LTRs).
Significance
Transcription: The products of transcription are used in the protein synthesis.
Reverse Transcription: The products of reverse transcription are integrated into the host genome.
Conclusion
Transcription and reverse transcription are two methods involved in the encoding of genetic material in other forms of nucleic acids. Transcription encodes DNA genome into RNA molecules that can be used in the protein synthesis. Reverse transcription encodes RNA genomes into cDNA that can be integrated into the host genome. Transcription occurs in both prokaryotes and eukaryotes while reverse transcription mainly occurs in retroviruses with RNA genomes.
Reference:
1. Coffin, John M. “Transcription.” Retroviruses., U.S. National Library of Medicine, 1 Jan. 1997, Available here.
2. Coffin, John M. “Overview of Reverse Transcription.” Retroviruses., U.S. National Library of Medicine, 1 Jan. 1997, Available here.
Image Courtesy:
1. “DNA transcription” By reworked and vectorized by myself – National Human Genome Research Institute, (Public Domain) via Commons Wikimedia
2. “Reverse transcription” By Filip emThis vector image was created with Inkscape. – Own work (CC BY 3.0) via Commons Wikimedia
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