STAGES OF DNA REPLICATION

INTRODUCTION

There are two steps to starting DNA replication. An initiator protein unwinds a small section of the double helix of DNA first. The helicase protein subsequently binds to the hydrogen bonds between the bases on the DNA strands and breaks them, causing the two strands to separate. The helicase continues to break hydrogen bonds and separate the two polynucleotide chains as it advances along the DNA molecule.

Meanwhile, when the helicase divides the strands, another enzyme known as primase attaches itself to each strand for a brief amount of time and lays the groundwork for replication to begin. 

Why replicate DNA?

Every cell is defined by its genetic substance, DNA. Biomolecules and organelles must be replicated and dispersed across the cells before a cell replicates and divides into new daughter cells, whether through mitosis or meiosis. To ensure that each new cell receives the necessary number of chromosomes, DNA located within the nucleus must be reproduced. The process of duplicating DNA is referred to as DNA replication. The replication process involves several proteins known as replication enzymes as well as RNA. In eukaryotic cells, such as animal cells and plant cells, DNA replication occurs during the S phase of interphase during the cell cycle. DNA replication is required for cell growth, repair, and reproduction in all living species.

Stages of DNA replication

DNA replication is divided into three stages: initiation, elongation, and termination.

Initiation

DNA synthesis begins at specified locations along the DNA strand known as ‘origins,’ which include specific coding sequences. These origins are targeted by initiator proteins, which attract other proteins to speed up the replication process, resulting in a replication complex encircling the DNA origin. The DNA structure contains several origin sites, which are referred to as replication forks when DNA replication begins.

DNA helicase is found within the replication complex. This enzyme unravels the double helix and exposes each of the two strands, allowing them to be used as replication templates. It accomplishes this by hydrolyzing the ATP needed to construct the nucleobase-to-nucleobase connections, causing the bond between the two strands to be broken.

Another enzyme involved in DNA replication is DNA primase. It makes a tiny RNA primer that works as a DNA polymerase ‘kick-starter.’ This enzyme is ultimately in charge of the formation and growth of new DNA strands.

Elongation

DNA Polymerase can begin synthesising new strands of DNA to match the templates once it has linked to the two unzipped strands of DNA. Only free nucleotides can be added to the 3′ end of the primer by DNA polymerase.

The new strand will be generated in a 5′ to 3′ direction since one of the template strands is read in a 3′ to 5′ direction. The leading strand refers to the newly created strand. To initiate DNA polymerase along the leading strand, DNA primase only has to synthesise an RNA primer once, at the start. Because DNA polymerase can read the template 3′ to 5′ and synthesise in a 5′ to 3′ manner, the new DNA strand can be extended.

The lagging strand, on the other hand, is antiparallel and is read in a 5′ to 3′ direction. Continuous DNA synthesis in the 3′ to 5′ direction, as in the leading strand, would be impossible due to DNA polymerase’s inability to add nucleotides to the 5′ end. Instead, RNA primers are added to the newly exposed bases on the lagging strand as the helix unwinds, and DNA synthesis occurs in fragments, but still in the 5′ to 3′ orientation as before. Okazaki fragments are the name given to these pieces of art.

Termination

The process of extending new DNA strands continues until either no more DNA template strands can be replicated (i.e. at the chromosome’s end) or two replication forks meet and terminate. The meeting of two replication forks is uncontrolled and occurs at random throughout the chromosome’s length.

The newly synthesised strands are bound and stabilised when DNA synthesis is completed. Two enzymes are required to stabilise the lagging strand: RNAase H removes the RNA primer at the start of each Okazaki fragment, and DNA ligase binds the fragments together to form a single strand.

CONCLUSION

The synthesis of identical DNA helices from a single double-stranded DNA molecule is referred to as DNA replication. Each molecule is made up of a strand from the original molecule and a strand that has just been produced. The DNA uncoils and the strands separate before replication. A replication template is constructed in the form of a replication fork. DNA polymerases create new nucleotide sequences in the 5′ to 3′ orientation, and primers bind to the DNA.

In the leading strand, the addition is continuous, while in the lagging strand, it is fragmented. Once the DNA strands have been elongated, they are verified for mistakes, repaired, and telomere sequences are inserted into the ends of the DN.