Difference Between DNA Replication and Transcription

Main Difference – DNA Replication vs Transcription

Both DNA replication and transcription are involved in binding of complementary nucleotides into DNA, yielding a new DNA and RNA strands respectively. In DNA replication, DNA produces two exact replicas of the whole genome in order to undergo cell division. On the other hand, transcription is the first step of gene expression, where the necessary proteins for the cell functioning are produced. In transcription, only small DNA sequences are transcribed into RNA. The key difference between DNA replication and transcription is that DNA replication is the process of making an exact replica of the genome whereas transcription is the transfer of genetic information of a particular segment of a DNA into RNA.

This article studies,

1. What is DNA Replication
     – Definition, Function, Process, Features
2. What is Transcription
     – Definition, Function, Process, Features
3. What is the difference between DNA Replication and Transcription

What is DNA Replication

DNA replication is referred to as the production of two exact replicas of DNA from an original DNA molecule. The genetic information stored in the DNA is inherited through the progeny by the replication of DNA. During replication, both DNA strands serve as templates. Hence, DNA replication is considered to occur in a semiconservative manner.

DNA replication is initiated at the origin of replication in each chromosome. The process is carried out by the group of enzymes called DNA polymerases. DNA polymerase requires a short strand of RNA known as a primer in order to initiate the replication. The unwinding of the double-helix in the genome produces the replication forks. At the replication fork, various enzymes are associated with the replication. DNA replication occurs bi-directionally at the replication fork. The new DNA strand, which is continuously synthesised is referred to as the leading strand. The other strand, which is synthesised as pieces called Okazaki fragments is referred to as the lagging strand.

DNA polymerase synthesises the new strand by adding nucleotides which are complementary to the template. The addition of nucleotides occurs in the 3′ to 5′ direction, starting from the 3′ end of the existing nucleotide chain. Sugar-phosphate backbone is formed by the phosphodiester bond formation between the proximal phosphate group and the 3′ OH of the pentose ring of incoming nucleotide. Topoisomerase, helicase, DNA primase and DNA ligase are the other enzymes involved in the DNA replication. DNA replication is terminated at the telomeric regions of the chromosome.   

Usually, DNA polymerases maintain high fidelity as the incorporation of a mismatch is less than one into 10⁷ incorporated nucleotides. They also consist of the 3′ to 5′ proofreading activity where they can remove the incorporated mismatches from the end. On the other hand, mismatches can be repaired by the post-replication mismatch repair mechanisms. Final error incorporation rate is less than one into 10⁹ incorporated nucleotides.  

Figure 1: DNA Replication

In vitro DNA replication is carried out by the aid of artificial DNA primers and DNA polymerases, which are isolated from bacteria. Polymerase chain reaction (PCR) is the molecular biological technique used for in vitro replication of DNA. The enzyme used in PCR is Taq polymerase. By using a pair of DNA primers, PCR synthesises DNA fragments from a known sequence.

What is Transcription

Transcription is the process of copying a DNA sequence into RNA by the aid of the enzyme, RNA polymerase. Genes are transcribed into mRNAs in order to initiate the gene expression. RNA polymerase synthesises the mRNA primary transcript by reading the antisense DNA strand from 3′ to 5′ direction. The resulting RNA strand is complementary and antiparallel to the template. It is synthesised from 5′ to 3′ direction. A gene consists of both coding sequence and regulatory sequences. Coding sequence encodes the amino acid sequence of a protein whereas the regulatory sequences regulate the gene expression.  

Figure 2: Transcription at RNA polymerase

Transcription is initiated by the binding of RNA polymerase to the promoter with the aid of transcription factors. The binding forms a transcription bubble, consisting of approximately 14 bases of the unwound double-stranded promoter. After the selection of the transcription initiation site, nucleotides are added by RNA polymerase. At the termination of transcription, polyadenylate tail is added to the 3′ end of the primary transcript. In eukaryotes, polyadenylation, 5′ end capping and the splicing of exons are collectively called post-transcriptional modifications. Genes may also encode for non-coding RNAs, rRNAs and tRNAs which consequently help in synthesising, regulating and the processing of proteins.

Difference Between DNA Replication and Transcription

Definition

DNA Replication: DNA replication produces two exact replicas of the original double-stranded DNA molecule. Each of the new strands consists of one original DNA strand.

Transcription: Transcription produces a single-stranded RNA molecule using the double-stranded DNA.

Function

DNA Replication: It transmits the entire genome to its offspring.

Transcription: It generates RNA copies of a particular gene.

Enzyme Required

DNA Replication: Topoisomerase, Helicase, DNA primase and DNA ligase.  

Transcription: Transcriptase (type of DNA Helicase) and RNA polymerase.

Occurrence in the Cell Cycle

DNA Replication: It occurs in the S phase when the cell is preparing for the division.

Transcription: It occurs in G1 and G2 phases when the cell needs to synthesise proteins.

Nucleotide Precursors

DNA Replication: It utilizes dATP, dGTP, dTTP and dCTP as precursors.

Transcription: It utilises ATP, UTP, GTP and CTP as precursors.

Fidelity

DNA Replication: DNA polymerase maintains high fidelity through its 3′ to 5′ exonuclease activity.

Transcription: RNA polymerase maintains less fidelity compared to DNA polymerase.

Length of the Newer Strand

DNA Replication: It synthesises long DNA strands.

Transcription: It synthesises comparatively short RNA strands.

Bond

DNA Replication: Newly synthesised DNA strand is bound to its template by hydrogen bonds.

Transcription: Transcribed RNA separates from its template.

Primers

DNA Replication: DNA polymerase requires an RNA primer for the initiation of the replication.

Transcription: RNA polymerase requires no primers.

Okazaki Fragment

DNA Replication: The lagging strand generates Okazaki fragments.

Transcription: Transcription occurs only in 5′ to 3′ direction, excluding the Okazaki fragments.

Products

DNA Replication: Two daughter strands are produced.

Transcription: The mRNA, tRNA, rRNA and non-coding RNA such as microRNA are produced.

Fate of the Products

DNA Replication: Replicated DNA remains in the nucleus.

Transcription: Greater part of the product passes into the cytoplasm.

Lifespan of the Products

DNA Replication: Replicated DNA is conserved through progeny.

Transcription: Most of the RNAs are degraded even before functioning.

Processing

DNA Replication: Newly synthesised DNA undergoes no processing.

Transcription: Transcribed RNAs undergo post-transcriptional modifications.

Conclusion

DNA replication occurs when the cell is preparing for the cell division. Thereby, the whole genome of an organism undergoes replication at once. Hence, both of the strands serve as templates for the replication. In the replication fork, leading strand is synthesised continuously, and the lagging strand is synthesised through Okazaki fragments. Finally, DNA polymerases should maintain high levels of fidelity, since the replica is going to be the genome of the offspring. In transcription, genes are copied into RNA in order to synthesise proteins for the cellular functions. Only the antisense strand is transcribed since RNA is a single stranded molecule. RNA polymerases maintain less fidelity compared to DNA polymerases since RNAs are short-lived. Therefore, the key difference between DNA replication and transcription is in their ultimate products.

Reference:
1. “DNA replication”. Wikipedia, the free encyclopedia, 2017. https://en.wikipedia.org/wiki/DNA_replication. Accessed 19 Feb. 2017
2. “Molecular mechanism of DNA replication”. KHANACEDAMY, 2017. https://www.khanacademy.org/science/biology/dna-as-the-genetic-material/dna-replication/a/molecular-mechanism-of-dna-replication. Accessed 19 Feb. 2017
3. “Transcription(biology)”. Wikipedia, the free encyclopedia, 2017. https://en.wikipedia.org/wiki/Transcription_(biology). Accessed 19 Feb. 2017
4. Sagar Aryal, “Difference between Replication and Transcription”. MICROBIOLOGY INFO, Online Microbiology Notes, 2014. http://www.microbiologyinfo.com/difference-replication-transcription/. Accessed 19 Feb. 2017

Image Courtesy:
1. “DNA replication en.svg”. By LadyofHats Mariana Ruiz – Own work (Public Domain) via Commons Wikimedia
2. “RNAP TEC small.jpg”. By Abbondanzieri on the English Wikipedia – Created with the rendering program Protein Explorer using coordinates 1H38 deposited at the RCSB PDB repository (Public Domain) via Commons Wikimedia