How does DNA replicate in the S phase of the cell cycle?

During the S phase of the cell cycle, DNA replication occurs through a process called semi-conservative replication.

In the S phase, or synthesis phase, of the cell cycle, the DNA within a cell's nucleus is replicated. This is a crucial step in cell division as it ensures that each new cell will have an exact copy of the DNA from the parent cell. The process of DNA replication is known as semi-conservative replication because each of the two new DNA molecules consists of one old (conserved) strand and one new strand.

The process begins with the unwinding of the double helix structure of the DNA molecule. This is facilitated by an enzyme called DNA helicase, which breaks the hydrogen bonds between the base pairs, causing the two strands to separate. This creates a 'replication fork', a Y-shaped structure where the DNA is being unwound and replicated.

Next, another enzyme called DNA polymerase attaches to each of the separated strands. It moves along the strand, reading the sequence of nucleotides (the building blocks of DNA) and adding complementary nucleotides to form a new strand. This is done in a 5' to 3' direction, meaning that the new strand is synthesised from the 5' end to the 3' end.

However, because the two strands of DNA are antiparallel (run in opposite directions), one strand, known as the leading strand, is replicated continuously, while the other, the lagging strand, is replicated in fragments. These fragments, called Okazaki fragments, are later joined together by an enzyme called DNA ligase to form a continuous strand.

Throughout this process, the original DNA strands serve as templates for the new strands, ensuring that the genetic information is accurately copied. This is crucial for maintaining the integrity of the genetic code and preventing mutations.

In summary, DNA replication in the S phase of the cell cycle is a complex but highly regulated process that ensures the accurate duplication of the cell's genetic material. It involves several key enzymes and occurs in a semi-conservative manner, with each new DNA molecule consisting of one old and one new strand.