Final answer:
During the S phase, a single unreplicated DNA molecule is the starting point for replication. Origins of replication initiate this process, which is semiconservative and results in the formation of sister chromatids held together by cohesin proteins at the centromere.
Step-by-step explanation:
A single unreplicated DNA molecule at the beginning of the S phase is a template for duplication. During the S phase of the cell cycle, each chromosome's DNA is replicated to produce two identical copies known as sister chromatids, which are held together at the centromere by cohesin proteins. This process is essential to ensure that during cell division, each new cell receives an identical set of chromosomes.
Multiple origins of replication are present along the DNA molecule, allowing the formation of multiple replicons that work concurrently to ensure complete replication during the S phase. The replication of DNA is a semiconservative process, meaning that each new DNA molecule consists of one parental strand and one newly synthesized strand.
Before replication can begin, the cell must successfully pass the G₁ checkpoint, which ensures all necessary replication proteins are prepared. The origins of replication are specific nucleotide sequences recognized by proteins that initiate the unwinding of DNA, permitting the formation of Y-shaped structures known as replication forks. Replication proceeds bidirectionally from each origin of replication, producing new DNA strands according to the base pairing rules.