During the S phase of the cell cycle, DNA replication takes place through a mechanism known as semi-conservative replication.

In the S phase, which stands for synthesis phase, the DNA within a cell’s nucleus is duplicated. This step is vital for cell division, as it guarantees that each daughter cell receives an exact copy of the DNA from the parent cell. The term “semi-conservative” refers to the fact that each of the two resulting DNA molecules is composed of one original (conserved) strand and one newly synthesized strand.

The replication process begins with the unwinding of the double helix structure of the DNA molecule. This unwinding is facilitated by an enzyme called DNA helicase, which breaks the hydrogen bonds between the base pairs, resulting in the separation of the two strands. This separation forms a structure known as the ‘replication fork,’ which resembles a Y shape where the DNA is being unwound and replicated.

Following this, another enzyme, DNA polymerase, attaches to each of the separated strands. DNA polymerase moves along the strand, reading the sequence of nucleotides (the fundamental building blocks of DNA) and adding complementary nucleotides to create a new strand. This synthesis occurs in a 5’ to 3’ direction, meaning that the new strand is formed from the 5’ end toward the 3’ end.

Due to the antiparallel nature of the two DNA strands (they run in opposite directions), the replication process differs for each strand. The strand that is synthesized continuously is referred to as the leading strand, while the other strand, known as the lagging strand, is synthesized in short segments. These segments, called Okazaki fragments, are subsequently joined together by an enzyme known as DNA ligase to form a continuous strand.

Throughout this replication process, the original DNA strands act as templates for the newly formed strands, ensuring that the genetic information is accurately replicated. This accuracy is crucial for preserving the integrity of the genetic code and preventing mutations.

In conclusion, DNA replication during the S phase of the cell cycle is a complex yet highly regulated process that guarantees the precise duplication of the cell’s genetic material. It involves several key enzymes and occurs in a semi-conservative manner, resulting in each new DNA molecule being composed of one old strand and one new strand.