Replication and transcription are two different biological processes necessary for the growth and survival of living organisms. Despite the same starting substrate of both processes, DNA (the messenger of genetic information) in both processes yields separate end products. Replication is known to transfer the entire genetic information to its daughter DNA strands to enable cell division while transcription converts the information stored in the DNA into messenger RNA (mRNA) which is intermediate to protein synthesis. This article explains the concept of replication, transcription, and the differences between the two.
Replication is the process of making an exact copy of DNA before cell division. It is mandatory for replication to take place before cell division (through mitosis or meiosis) to ensure that each daughter cell carries its own, complete set of genomes. Replication refers to the process of production of two identical DNA helices from one double-stranded DNA molecule. DNA replication is pivotal for cell growth, repair, and reproduction in living organisms. There are three steps in DNA replication −
Initiation − The unwinding of the DNA helix into two single strands is necessary to initiate the process of replication. Unwinding is a process where the hydrogen bonds between the base pairs AT and GC are broken down by an enzyme called DNA helicase to give rise to a Y-shaped replication fork. This acts as the template for DNA replication. Since replication takes place only from the 5’ to 3’ direction (at the leading strand i.e. 3’ to 5’), it occurs through two different processes to accommodate replication of the 5’ to 3’ strand (the lagging strand). After the separation of the two strands, an RNA primer binds to the leading strand to initiate the replication.
Elongation − Elongation at both the leading and lagging strand is carried out by the enzyme DNA polymerases which bind to a strand and add complementary base pairs. There are 5 variants of DNA polymerases known out of which polymerase III carries out elongation in prokaryotes (e.g. E.coli) while polymerase I, II, IV, and V are necessary for proofreading and repairs. In eukaryotes (e.g. humans) polymerases α, δ and ε play a major role in elongation. Elongation proceeds continuously on the leading strand owing to the favourable 5’ to 3’ direction.
Elongation at the lagging strand takes place in multiple fragments, known as Okazaki fragments, as multiple primers bind to it. Another enzyme, DNA ligase, binds various Okazaki fragments to yield a continuous strand.
Termination − Once both single-strands are formed, an enzyme called Exonuclease removes the RNA primers from the parent strands. The empty spots are filled up with appropriate bases. DNA proofreading is carried out by another exonuclease to repair the errors in replication if any. The ends of parent DNA strands carry special, repeated DNA sequences known as telomeres which act as protective caps at the chromosomal end preventing the fusing of two nearby chromosomes. Another special enzyme, telomerase, then carries out the synthesis of telomere sequences at the ends of newly formed DNA strands. Finally, double-helix DNA is formed from one parent- and other daughters- strand.
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Transcription is the process of transferring the information contained in a DNA strand (that contains a protein-coding gene) into the mRNA (messenger RNA). Information stored in mRNA is not used for long-term storage unlike DNA and exits the nucleus to the cytoplasm. It is called the first stage of gene expression. Like replication, it has three stages namely: initiation, elongation, and termination.
For the transcription to initiate, certain accessory proteins known as transcription factors bind to essential DNA sequences known as enhancer and promoter sequences. This, in turn, helps an enzyme called RNA polymerase to bind at the appropriate transcription site through a transcription initiation complex made up of transcription factors and RNA polymerase. RNA polymerase then begins the mRNA elongation by adding complementary bases to the parent DNA strand. The process is terminated when RNA polymerase encounters the ‘rho’ cofactor. Eukaryotic mRNA needs further chemical processing before it can be transcripted into the protein. It involves the 5’ capping, the addition of a poly-A tail, and splicing.
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The differences between replication and transcription are summarized in the table.
| S. No. | Characteristics | Replication | Transcription |
|---|---|---|---|
| 1 | Definition | It is a process of synthesis of two daughter DNA strands from a double-stranded DNA | It is a process of synthesis of RNA from a DNA template. |
| 2 | Enzymes required | DNA polymerase | RNA polymerase |
| 3 | Occurrence | S phase of the cell cycle | G1 and G2 phases of the cell cycle |
| 4 | DNA template | Double-stranded DNA | Single-stranded DNA |
| 5 | Purpose | It is necessary for cell division | It is necessary for protein synthesis |
| 6 | Bond | The daughter DNA strand remains bound to the parent strand through hydrogen bonds between the complementary base pairs | Transcribed RNA strand separates from its parent DNA strand |
| 7 | RNA primer | Required | Not required |
| 8 | Fate of the End product | They remain in the nucleus | Most of the products are transported into the cytoplasm from the nucleus |
| 9 | Post-processing | Daughter strands do not need any further processing and are fully functional | The product, mRNA, requires further processing to make it functional |
Both the processes, DNA replication and transcription, are pivotal for the sustenance of life on earth. Both processes hold some similarities but differ greatly in terms of their purpose. While replication is a process of synthesis of daughter DNA strands necessary for cell division, transcription is necessary for the synthesis of proteins that carry out important cellular and metabolic functions.
Q1. What do you understand by the term DNA?
Ans. DNA stands for deoxyribonucleic acid and is the molecule that carries hereditary information in almost all living organisms including humans. It has three main components: a deoxyribose sugar molecule, a phosphate, and a nitrogenous base. DNA comprises four bases called adenine (A), thymine (T), cytosine (C), and guanine (G). It is necessary for the proper development and functioning of an organism.
Q2. What do you understand by 5’ and 3’ end?
Ans. The 5’ end of the DNA consists of a phosphate group while the 3’ end consists of a hydroxyl group attached to the DNA backbone. It signifies the direction of DNA replication.
Q3. What are the necessary raw material for the synthesis of daughter DNA during replication?
Ans. Necessary raw materials for the synthesis of daughter DNA during replication are the deoxyribonucleotides (dATP, dGTP, dTTP, and dCTP).
Q4. What is the necessary raw material for the synthesis of mRNA during transcription?
Ans. The necessary raw material for the synthesis of mRNA during transcription are Ribonucleotides (ATP, UTP, GTP, and CTP).
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