Select the incorrect sentence for heterochromatin.

Options

A. It is densely packed
B. It stains dark
C. It is transcriptionally active
D. It is late replicating

Answer

### Step-by-Step Solution:

1. **Understanding Heterochromatin**:
- Heterochromatin is a type of chromatin that is densely packed and generally transcriptionally inactive. This means that the genes within heterochromatin are usually not expressed.

2. **Characteristics of Heterochromatin**:
- **Densely Packed**: Heterochromatin is known for its tightly packed structure, which makes it less accessible for transcription machinery.
- **Staining Properties**: Heterochromatin appears dark when stained due to its compact structure.
- **Transcriptional Activity**: Heterochromatin is typically transcriptionally inactive, meaning that it does not actively participate in gene expression.
- **Replication Timing**: Heterochromatin replicates later in the S phase of the cell cycle compared to euchromatin.

3. **Evaluating the Statements**:
- **Statement 1**: "It is densely packed." - This is correct.
- **Statement 2**: "It is stained dark." - This is also correct.
- **Statement 3**: "It is transcriptionally active." - This is incorrect, as heterochromatin is transcriptionally inactive.
- **Statement 4**: "It is late replicating." - This is correct.

4. **Identifying the Incorrect Statement**:
- The statement that heterochromatin is "transcriptionally active" is the incorrect one. Therefore, this is the answer to the question.

### Final Answer:
The incorrect sentence for heterochromatin is: "It is transcriptionally active."

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