DNA Replication

DNA is the hereditary material present in every cell. Before a cell duplicates, it divides into new daughter cells either by mitosis or meiosis. The nucleus and organelles have to be duplicated before being distributed among the cells. To ensure that each new cell receives the correct number of chromosomes, DNA within the nucleus must be duplicated. Several proteins in enzymes and RNA are involved in the replication process. DNA replication occurs in interphase during the cell division cycle in eukaryotic cells like animals and plants.

DNA (deoxyribonucleic acid) is a nucleic acid that has three main components: A phosphate, nitrogenase base and a phosphate group. Being genetic material, it is crucial that when a cell divides, it has to be copied within the daughter cells. There is a production of identical helices from one parent double-stranded DNA in this process. Enzymes hold high significance in each and every step of the replication, and they catalyse vital reactions. 

DNA replication plays a major role in cell repair, growth, and reproduction.

THE STRUCTURE OF DNA

Deoxyribonucleic acid (DNA) is a nucleic acid consisting of a five-carbon sugar (deoxyribose), a phosphate group, and a nitrogenous base. DNA consists of two spiral chains of nucleic acid twisted into the shape of a double helix. This twisting makes the DNA more compact and complex. DNA, being stretched, is a very long chain but to fit inside the small nucleus, DNA is highly packed into a tight structure called chromatin. Chromatin is highly coiled. 

These structures condense to form a more complex system called a chromosome. Just before DNA replication, the structure of chromatin loosens up, providing access to the strands of DNA. There are two bases: purines (adenine, guanine) and pyrimidines (cytosine, thymine, uracil).

Adenine is attached to thymine with two hydrogen bonds, and cytosine is attached to guanine with three hydrogen bonds. The double helix DNA structure turns after every ten base pairs with 3.4nm, and the distance between two base pairs is 0.34nm.

STEPS OF DNA REPLICATION

FORMATION OF REPLICATION FORK

The unwinding of DNA takes place in this step. The interaction between the base pairs is broken. The above step is catalysed by an enzyme called DNA helicase, and it breaks the hydrogen bonding between the base pairs and creates a Y shape known as Replication Fork.

These regions act as a template region to start the replication. The 5’end of the strand has a phosphate(P) group attached to it, and the 3’end of the strand has a hydroxyl(OH) group attached to it. The replication fork is bidirectional. One of the strands runs through the 3’ to 5’ direction that is the leading strand. And the other strand runs from 5’ to 3’ end, which is called the lagging strand. Once the replication fork is formed, a short piece of RNA, called a primer, gets attached to the 3’ end of the strand, remarking initiation.

ELONGATION

In this process, a new strand is created with the help of an enzyme DNA polymerase, and the process is called Elongation. This enzyme binds at the primer site, and the addition of new base pairs takes place complementary to the other strand. The replication generally proceeds in the direction 5’-3’ concerning the leading strand. The newly formed strands are discontinuous, which are Okazaki fragments, and later they are joined by DNA ligase.

TERMINATION

When both the new strands are formed as a result of replication, the removal of RNA primers takes place from the original strand, done by an enzyme named Exonuclease. Telomeres are the repetitive DNA strands present on the parent strands. Telomerase is a particular type of DNA polymerase enzyme that helps synthesise Telomeres. Once these telomeres are synthesised, one parent strand and one daughter strand form, complementing each other. Two DNA molecules are produced at the end of the replication. 

CONCLUSION:

DNA replication is essential for living because this process ensures the distribution of the genome into the daughter cells and provides the inheritance of the genetic traits. It is necessary for cell growth, repair, and cell reproduction