**Step-by-Step Solution:**
1. **Understanding the Cell Cycle:**
- The cell cycle consists of several phases: G1 (Gap 1), S (Synthesis), G2 (Gap 2), and M (Mitosis).
- Each phase has specific functions that contribute to cell growth and division.
2. **Defining the S-Phase:**
- The S-phase is the part of the cell cycle where DNA synthesis occurs.
- During this phase, each chromosome is replicated, resulting in the duplication of the genetic material.
3. **Changes in DNA Content:**
- Before the S-phase, the DNA content of the cell is referred to as 2C (where C represents the amount of DNA in a haploid set).
- After the S-phase, the DNA content doubles to 4C, indicating that each chromosome has been replicated.
4. **Chromosome Number:**
- It is important to note that while the DNA content doubles, the number of chromosomes remains the same.
- For example, if a cell starts with 2N (where N is the number of chromosomes in a haploid set), it remains 2N after the S-phase, but each chromosome now consists of two sister chromatids.
5. **Duplication of Centrioles:**
- In addition to DNA replication, the S-phase also involves the duplication of centrioles, which are essential for cell division.
- Centrioles help in organizing the mitotic spindle during mitosis.
6. **Conclusion:**
- In summary, during the S-phase, DNA replication occurs, resulting in the doubling of DNA content from 2C to 4C, while the chromosome number remains unchanged at 2N. Additionally, centrioles are also duplicated.
**Final Answer:**
In the S-phase of the cell cycle, DNA replication occurs, doubling the DNA content from 2C to 4C while maintaining the same chromosome number (2N). Centrioles also duplicate during this phase.
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