How is DNA replicated during the S phase?

DNA replication during the S phase occurs through a semi-conservative process involving unwinding, base pairing and joining.

In more detail, the S phase, or synthesis phase, is a part of the cell cycle where DNA replication takes place. This process is semi-conservative, meaning that each of the two resulting DNA molecules will have one old strand, derived from the old molecule, and one newly synthesised 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 forms a 'replication fork', a Y-shaped structure where the DNA is being unwound and replicated.

Next, another enzyme called DNA polymerase attaches to the separated strands. It moves along each strand, reading the sequence of bases (adenine, thymine, cytosine, guanine) and adding complementary bases to form a new strand. This is the base pairing stage. It's important to note that DNA polymerase can only add bases in one direction, from 5' to 3'. This means that one strand, known as the leading strand, is synthesised continuously, while the other, the lagging strand, is synthesised in fragments called Okazaki fragments.

The final stage is the joining of these fragments on the lagging strand. This is done by an enzyme called DNA ligase, which seals the gaps between the Okazaki fragments to form a continuous DNA strand.

Throughout this process, various other proteins and enzymes play crucial roles in ensuring the accuracy and efficiency of DNA replication. For example, single-strand binding proteins bind to the separated strands to prevent them from re-forming the double helix before replication is complete, and DNA primase creates a short RNA primer that acts as a starting point for DNA polymerase.

In summary, DNA replication during the S phase is a complex, highly regulated process that ensures the accurate duplication of the cell's genetic material. It involves several stages and a variety of enzymes and proteins, each with a specific role in the process.