Which constriction is essentially present in every chromosome?

Answer

### Step-by-Step Solution:

1. **Understanding Chromosome Structure**:
- A chromosome consists of two arms, which are typically referred to as the 'p' arm (shorter) and the 'q' arm (longer).

2. **Identifying the Primary Constriction**:
- The primary constriction in a chromosome is where the centromere is located. This is a crucial part of the chromosome as it plays a vital role during cell division.

3. **Defining the Primary Constriction**:
- The primary constriction is defined as the region of the chromosome where the centromere is found. This constriction is essential and is present in every chromosome.

4. **Understanding Secondary Constriction**:
- In addition to the primary constriction, chromosomes may also have secondary constrictions, which are found at other locations along the arms of the chromosome. These are not essential and can vary between different chromosomes.

5. **Conclusion**:
- The constriction that is essentially present in every chromosome is the **primary constriction**, where the centromere is located.