The cell cycle is a series of organized events culminating in cell division. This process allows for growth, tissue repair, and reproduction. It is broadly divided into two main phases: interphase and the mitotic (M) phase. Interphase is a period of cellular activity and growth, preparing for cell division. Within interphase, the S phase (synthesis phase) is a crucial segment, preparing for the accurate distribution of genetic material to new daughter cells.
DNA Replication: The Core Activity
The defining event of the S phase is DNA replication, where a cell precisely duplicates its entire genetic blueprint, ensuring each new daughter cell receives a complete and identical set of chromosomes. DNA replication begins at specific sites called origins of replication. Enzymes like helicases unwind the double helix, creating replication forks where DNA strands separate.
Each separated strand then serves as a template for synthesizing a new complementary strand. DNA polymerase adds new nucleotides, following base-pairing rules, to build new DNA strands. This mechanism is known as semi-conservative replication because each newly formed DNA molecule consists of one original (template) strand and one newly synthesized strand. As a result of this meticulous duplication, each chromosome, previously a single structure, now consists of two identical sister chromatids joined at a centromere.
Maintaining Integrity: S Phase Checkpoints
To safeguard the accuracy of genetic information, cells employ sophisticated quality control mechanisms known as checkpoints throughout the cell cycle. During the S phase, specific checkpoints monitor the progression of DNA replication and detect any damage or errors that may occur. These S-phase checkpoints ensure that DNA replication is completed accurately and without damage before the cell advances to the next division stage.
If DNA damage or replication issues are detected, these checkpoints temporarily halt cell cycle progression. This pause allows time for DNA repair mechanisms to correct errors or for replication to complete successfully. The failure of these checkpoints can lead to genomic instability, where cells accumulate genetic mutations or incorrect chromosome numbers. Such instability is often associated with diseases, including cancer.
Beyond DNA: Other Preparations
While DNA replication is the central activity, the S phase also involves other preparations that contribute to successful cell division. In animal cells, for instance, the centrosome, the primary microtubule-organizing center, undergoes duplication. This duplication involves new centrioles forming adjacent to existing ones, ensuring each daughter cell inherits a complete centrosome.
The cell continues to grow during the S phase, accumulating proteins and other essential molecules. This includes histone protein synthesis, crucial for packaging newly replicated DNA into compact chromatin structures. These additional processes, alongside DNA replication, prepare the cell for the subsequent M phase, ensuring all necessary components are ready for precise genetic material segregation and division into two daughter cells.