In animal cells, during S-phase (a)_____ begins in the the nucleus and (b)_____ duplicates in the cytoplasm

Answer

To answer the question regarding what occurs in animal cells during the S-phase of the cell cycle, we can break it down into the following steps:

### Step-by-Step Solution:

1. **Understanding the S-phase**: The S-phase, or synthesis phase, is a part of interphase in the cell cycle. It is the stage where DNA replication occurs, leading to the duplication of the genetic material.

2. **DNA Replication in the Nucleus**: During the S-phase, the primary event that takes place in the nucleus is the replication of DNA. This process is crucial as it ensures that when the cell divides, each daughter cell receives an identical set of chromosomes.

3. **Centriole Duplication in the Cytoplasm**: In addition to DNA replication, another important event during the S-phase in animal cells is the duplication of centrioles. Centrioles are cylindrical structures that play a key role in cell division by helping in the formation of the spindle fibers that separate chromosomes during mitosis.

4. **Filling in the Blanks**: Based on the above understanding, we can fill in the blanks in the question:
- (a) **DNA replication** begins in the nucleus.
- (b) **Centrioles** duplicate in the cytoplasm.

### Final Answer:
In animal cells, during S-phase (a) **DNA replication** begins in the nucleus and (b) **centrioles** duplicate in the cytoplasm.

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