What is DNA Replication and Why it is Important? 

The mechanism by which deoxyribonucleic acid (DNA) is capable of producing an exact copy of its own is defined as DNA replication. The DNA molecules utilize a semiconservative method for replication. 

Structure of DNA 

The molecule of DNA belongs to the category of nucleic acid which is made up of a phosphate, a nitrogenous base, and five-carbon deoxyribose sugar. The double-stranded DNA is made up of two nucleic acid chains which are spiral and are twisted into a shape known as the double helix. The compact nature of DNA arises due to this twisting of the nucleic acid chains as it is packed into a structure called chromatin that allows it to fit inside the nucleus of the cell. The chromatin is a tightly coiled structure that is usually condensed to create chromosomes during cell division. The chromatin structure must loosen before DNA replication to expose the DNA strands for duplication. 

Mechanism of DNA Replication 

The genetic composition that defines every cell is termed DNA. Before the duplication of cells and its division into new daughter cells via the process of meiosis or mitosis, the biomolecules and organelles present in them must also be copied to get distributed within the cell. Similarly, DNA present in the nucleus of a cell must also be replicated to make sure that each cell that is formed newly is provided with the required number of chromosomes.  This mechanism of duplication of DNA is called DNA replication.

The process of replication is achieved by several steps which include various proteins namely RNA (ribonucleic acid) and certain enzymes. DNA replication specifically in the eukaryotic cell occurs during the interphase’s S phase of the cell cycle. This mechanism of replication in DNA is important for cell growth and repair, and living being’s reproduction. The significant steps involved in the process of DNA replication are initiation, elongation, and termination steps. 

Steps Involved in DNA Replication 

Initiation 

The initiation step in DNA replication mainly involves two steps namely replication fork formation and the binding of primer. To initiate DNA replication, the unzipping of double-stranded DNA molecules must take place. The DNA bases are paired with one another as adenine paired with thymine and guanine paired with cytosine. Topoisomerase enzyme helps in unwinding and prevents the DNA from supercoiling. The bond which interlinks them has to be broken with the help of an enzyme called DNA helicase. DNA helicase helps in disrupting the hydrogen bonds which are present between the bases.

The breaking of hydrogen bonds forms a Y-shaped DNA molecule characterized by the two separated DNA strands. This Y-shaped structure is called a replication fork. This replication fork serves as a region of template for initiating replication. The DNA molecule is directional in both strands which are implied by 3’ and 5’ ends. The DNA replication mechanism takes place only in the 5’ to 3’ direction.

The replication fork formed is bi-directional and is represented as a lagging and leading strand based on the orientation of the 3’ and 5’ ends. When the strand is oriented in a 3’ to 5’ direction, it is named the leading strand and if the direction of orientation is 5’ to 3’, it is a lagging strand. Thus, the replication occurs differently in the two strands for direction difference accommodation.   

Note: Leading strand is oriented in the 3’ to 5’ direction and the lagging strand in the 5’ to 3’ direction. 

After the replication fork formation, the DNA strand is prepared to initiate replication. The primer, which is a short RNA strand binds at the 3’ end and act as an initiation point for the process of replication. DNA primase plays a significant role in generating these primers. 

Elongation 

DNA polymerase plays a key role in the process of elongation. This is an enzyme that is responsible for creating a new strand in the elongation process. DNA polymerase exists in 5 different types in humans and bacterial cells. For example, in Escherichia coli, those 5 types include polymerase I, II, III, IV, and V. Among all, polymerase III is the primary enzyme for replication, and the rest of the types are required for repair and checking errors. DNA polymerase III initiates replication by binding to the primer site in the DNA strand. DNA base pairs that are complementary to the strand get added as a process of elongation in replication. In the case of eukaryotic cells, alpha, epsilon, and beta are the main polymerases involved in the replication.

The newly formed strand is considered continuous as the replication process occurs on the leading strand in the 5’ to 3’ direction. But in the case of lagging strand, replication is initiated by several primers in which each primer is situated after many bases apart. Each segment of DNA replicates and results in DNA pieces named Okazaki fragments which are then added with the help of DNA polymerase. This form of replication is discontinuous as the fragments are created separately. 

Termination 

Exonuclease plays the role of removing RNA primers from the actual strand once the continuous and discontinuous strands are created. The primer’s places are replaced with complementary bases. The process of proofreading also occurs on the newly formed DNA strand with the help of another enzyme of the exonuclease. This helps in replacing, checking, and removing errors, if any. The enzyme named DNA ligase helps in creating a single unified strand by joining the Okazaki fragments together. The ends of the chromosomes are protected with protective cap namely telomeres to prevent them from fusing with the neighbor chromosomes. Thus, to facilitate the synthesis of those telomere sequences, a unique type of DNA polymerase namely telomerase is used to catalyze the synthesis.

After the completion of replication, the parent and the complementary DNA strand coils back to their original double-helical structure. Thus, at the end of the replication process, two DNA molecules are formed. One strand of both the molecules is obtained from the original molecule of DNA and the other strand is completely new. Thus, it is referred to as the semi-conservative method of replication. 

Note: The role of each enzyme involved in DNA replication should be familiarized.  

Modes of DNA Replication 

Though DNA replication is the semi-conservative mode of replication, there are certain other modes of replication that exist. There are conservative and dispersive modes of replication. In the dispersive mode of replication, the parent DNA molecules are broken into several fragments and the newly formed DNA strand contains both old and newly synthesized fragments.

In conservative mode, one molecule contains both the strands from the parent molecule and another molecule consists of completely newly synthesized strands. These replication models are proposed by the scientific community but later DNA replication is concluded as the semiconservative mode of replication as it made more sense. Moreover, the semiconservative model was proved and confirmed by the experiment conducted by Meselson and Stahl. Hence, the semi-conservative mode of replication becomes a universal process that occurs in all organisms.  

Context and Applications  

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelors in Biology
• Masters in Biology