In animal cells, during the S-phase -

Options

A. DNA replication occurs in nucleus and centrioles duplicate in the cytoplasm
B. Only DNA replication occurs in nucleus
C. Only Centrioles duplicate in the cytoplasm
D. Centrioles duplicate in the nucleus and DNA replication occurs in the cytoplasm.

Answer

**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). The S-phase is specifically where DNA synthesis occurs.

2. **Identifying the S-phase**: During the S-phase, the primary event is the replication of DNA. This is crucial for cell division because each daughter cell needs a complete set of DNA.

3. **DNA Replication Process**: In the S-phase, the DNA in the nucleus is duplicated. This involves unwinding the double helix and synthesizing new strands complementary to the original strands.

4. **Centrioles Duplication**: Along with DNA replication, the centrioles in the cytoplasm also duplicate during the S-phase. Centrioles are important for forming spindle fibers, which are necessary for chromosome segregation during mitosis.

5. **Consequences of Errors**: If centriole duplication fails, it can lead to improper chromosome segregation, which may result in conditions like aneuploidy (an abnormal number of chromosomes).

6. **Conclusion**: Therefore, in animal cells during the S-phase, both DNA replication in the nucleus and centriole duplication in the cytoplasm occur.

**Final Answer**: In animal cells during the S-phase, DNA replication occurs in the nucleus and centriole duplication occurs in the cytoplasm.

---