S-Phase Calculators

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S-phase (synthesis phase) is the stage of the eukaryotic cell cycle during which DNA replication occurs — the entire genome is duplicated so each daughter cell receives a complete copy of the genetic information. S-phase follows G1 and precedes G2 in interphase, and is initiated at thousands of replication origins scattered across the chromosomes. The proportion of cells in S-phase (S-phase index or S-phase fraction) is a measure of proliferation and is used in cancer biology as a prognostic indicator and to assess treatment response.

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S-Phase in the Cell Cycle

The eukaryotic cell cycle consists of: G1 (growth and preparation) → S (DNA synthesis) → G2 (growth and DNA repair) → M (mitosis). After M, cells either re-enter G1 or exit to G0 (quiescence).

During S-phase:

  • DNA replication initiates simultaneously at thousands of replication origins (ori) across the genome
  • Replisomes (DNA polymerase complexes) extend bidirectionally from each origin
  • Histones are synthesized and newly replicated DNA is immediately packaged into nucleosomes
  • Each chromosome is duplicated from 2C to 4C DNA content (in diploid cells, genome goes from 2N to 4N)

Duration of S-Phase

In rapidly proliferating human cells: S-phase lasts approximately 6–8 hours (total cell cycle ~24 hours). G1 duration is the most variable phase — cells that receive mitogenic signals in G1 can pass through the restriction point and commit to S-phase.

Measuring S-Phase: Methods

Flow Cytometry (DNA Content Analysis)

Cells are stained with a DNA-intercalating fluorescent dye (propidium iodide, DAPI). G1 cells (2C DNA) and G2/M cells (4C) produce distinct peaks; S-phase cells with intermediate DNA content appear between the peaks. S-phase fraction (SPF) = percentage of cells between G1 and G2/M peaks.

BrdU / EdU Incorporation

BrdU (bromodeoxyuridine) or EdU (5-ethynyl-2'-deoxyuridine) is a thymidine analog incorporated into newly synthesized DNA during S-phase. After a defined pulse period, cells are stained and those in S-phase are labeled. EdU is detected by click chemistry (no denaturing needed); BrdU requires antibody staining after DNA denaturation.

S-Phase Index in Cancer

High S-phase fraction in tumor biopsies correlates with high proliferative activity. Elevated SPF is associated with aggressive behavior and poor prognosis in breast cancer and other solid tumors. It is used alongside Ki-67 immunostaining as a proliferation marker.

Glossary

S-Phase (Synthesis Phase)
The stage of the eukaryotic cell cycle during which DNA replication occurs. Follows G1 and precedes G2. Initiates at thousands of replication origins; DNA content increases from 2C to 4C (diploid cells).
S-Phase Fraction (SPF)
The percentage of cells in S-phase in a population, measured by flow cytometry (DNA content analysis) or BrdU/EdU incorporation. A marker of proliferative activity used in cancer prognosis.
BrdU (Bromodeoxyuridine)
A synthetic thymidine analog incorporated into newly synthesized DNA during S-phase. Used to label and identify replicating cells by immunofluorescence or immunohistochemistry. Requires anti-BrdU antibody after DNA denaturation.

Frequently Asked Questions

During S-phase (synthesis phase), the entire genome is replicated. DNA replication initiates simultaneously at thousands of origins of replication distributed across all chromosomes. DNA polymerase complexes (replisomes) extend bidirectionally from each origin, copying both strands of DNA. Histone proteins are concurrently synthesized and assembled onto newly replicated DNA. By the end of S-phase, the cell contains twice as much DNA (4C content in diploid cells) as at the start.

Cells are stained with a DNA-intercalating fluorescent dye (propidium iodide or DAPI) and analyzed by flow cytometry. G1 cells have 2C (diploid) DNA content and form the first fluorescence peak; G2/M cells have 4C content (second peak). S-phase cells, which are in the process of replicating DNA, have intermediate DNA content and appear as a broad shoulder between the two peaks. The S-phase fraction = percentage of cells in this intermediate region.

BrdU (bromodeoxyuridine) is a thymidine analog that is incorporated into DNA only during active replication (S-phase). Adding BrdU to cell culture medium for a defined pulse period labels all cells actively synthesizing DNA. After fixation, anti-BrdU antibody (following DNA denaturation to expose BrdU) identifies S-phase cells by fluorescence or DAB staining. EdU, a newer analog detected by click chemistry, is more convenient as it doesn't require DNA denaturation.

A high S-phase fraction (SPF) indicates that a large proportion of tumor cells are actively replicating DNA — reflecting high proliferative activity. In breast cancer and other solid tumors, high SPF correlates with more aggressive tumor behavior, higher grade, and poorer prognosis. Elevated SPF is associated with increased chemotherapy sensitivity (rapidly dividing cells are more vulnerable to DNA-targeting agents) but also faster tumor regrowth between treatment cycles.