What Happens in S of the Cell Cycle: DNA Replication Explained

The S phase of the cell cycle is when a cell duplicates its DNA, ensuring that each daughter cell receives an identical copy of the genome. This synthesis phase occurs between G1 and G2 in interphase and is essential for growth, repair, and reproduction. Accurate DNA replication during S phase prevents mutations that could lead to developmental disorders or cancer. The process is tightly controlled, involving precise molecular steps that copy the entire genome once—and only once.

What Happens in S Phase

During S phase, the cell synthesizes a complete copy of its DNA. Each chromosome is duplicated into two sister chromatids, held together at the centromere. This duplication begins at thousands of sites along the chromosomes called origins of replication. In human cells, there are an estimated 20,000 to 50,000 origins, allowing the genome to be copied efficiently in parallel.

Initiator proteins bind to these origins and assemble the pre-replication complex (pre-RC), which includes the origin recognition complex (ORC), Cdc6, Cdt1, and the MCM helicase. The pre-RC forms during late mitosis and G1, marking origins as “licensed” for replication. At the start of S phase, cyclin-dependent kinases (CDKs) and Cdc7 kinase activate the MCM helicase, unwinding the DNA and initiating replication.

As the double helix unwinds, replication forks form—Y-shaped structures where DNA strands separate. Helicase breaks hydrogen bonds to open the fork, while single-strand binding proteins stabilize the exposed strands. Topoisomerases relieve torsional stress ahead of the fork, preventing supercoiling. The enzyme primase synthesizes short RNA primers, providing a starting point for DNA polymerase.

DNA polymerase adds nucleotides in the 5’ to 3’ direction, using each original strand as a template. Because the two DNA strands run in opposite directions, replication proceeds differently on each. On the leading strand, synthesis is continuous. On the lagging strand, it occurs in fragments—Okazaki fragments—each requiring its own RNA primer. Afterward, RNase H and FEN1 remove the primers, DNA polymerase fills the gaps, and DNA ligase seals the nicks.

The entire process is highly accurate. DNA polymerase has proofreading ability, correcting most mismatches as they occur. Additional repair systems, like mismatch repair, fix errors that escape detection. These mechanisms reduce error rates to fewer than one mistake per billion nucleotides.

Example: Replication Timing and Chromatin Structure

DNA replication does not occur uniformly across the genome. Instead, it follows a defined temporal order linked to chromatin structure. Euchromatin—loosely packed, gene-rich regions—replicates early in S phase. Heterochromatin—densely packed, repeat-rich regions—replicates later.

This timing is influenced by epigenetic marks. For example, histone acetylation is associated with early replication, while methylation of histone H3 at lysine 9 (H3K9me) marks late-replicating regions. These modifications help regulate access to DNA, ensuring that replication machinery is recruited appropriately.

Replication also occurs in specialized nuclear sites called replication factories. These are clusters of replication forks anchored to the nuclear matrix, allowing efficient use of enzymes and coordination across chromosomes. Early-replicating regions tend to localize to factories in the nuclear interior, while late-replicating regions are often near the periphery.

Regulation and Quality Control

Entry into S phase is tightly controlled. Before replication begins, the cell passes the G1 checkpoint, also known as the restriction point. Here, it assesses nutrient availability, growth signals, and DNA integrity. If conditions are favorable, cyclin E binds CDK2, driving the transition into S phase. Cells that fail to meet these criteria may exit to a resting state called G0.

Once S phase starts, checkpoints monitor replication fidelity. If DNA damage is detected, the ATR kinase activates a response that pauses replication and recruits repair proteins. BRCA1 and BRCA2 help stabilize stalled replication forks, preventing collapse and double-strand breaks. Defects in these pathways increase cancer risk, particularly in breast and ovarian tissues.

Fork progression is also regulated. In mammalian cells, forks move at about 50–100 base pairs per second. Certain sequences, such as repetitive DNA or regions with secondary structure, can slow or stall forks. When this happens, mechanisms like fork reversal allow time for repair before synthesis resumes.

A key safeguard is the prevention of re-replication. The pre-replication complex is disassembled after initiation, ensuring that each origin fires only once per cycle. This licensing system maintains genomic stability and prevents gene amplification.

Telomeres, the protective ends of chromosomes, pose a challenge during replication. DNA polymerase cannot fully copy the ends of linear DNA, leading to gradual shortening with each division. The enzyme telomerase counteracts this in stem cells and immune cells but is less active in most somatic cells. Without telomerase, telomere shortening can trigger senescence or cell death.

S phase typically lasts 6 to 8 hours in mammalian cells, depending on cell type and conditions. By the end, each chromosome consists of two identical sister chromatids, ready for segregation in mitosis.

Key Takeaways

• The S phase is when DNA replication occurs, producing two identical sister chromatids for each chromosome.
• Replication begins at multiple origins across the genome, initiated by the assembly and activation of the pre-replication complex.
• Enzymes including helicase, primase, DNA polymerase, and ligase work together to unwind DNA, synthesize new strands, and seal breaks.
• Proofreading and repair systems ensure high fidelity, minimizing errors during copying.
• Chromatin structure influences replication timing, with euchromatin replicating early and heterochromatin late.
• Checkpoints and regulatory mechanisms prevent re-replication, respond to DNA damage, and maintain genomic stability.