Final answer:
The structure represented by the waist in a duplicated chromosome is called the centromere. It holds the sister chromatids together and plays a role in cell division.
Step-by-step explanation:
The structure represented by your waist, where you are joined to your twin in a duplicated chromosome, is called the centromere.
The centromere is a region of the chromosome that holds the sister chromatids together. It plays a crucial role in cell division, ensuring that each daughter cell receives one copy of the duplicated chromosome.
For example, in a duplicated chromosome, like the one shown in Figure 3.31, the centromere is the region where the two identical sister chromatids are joined together, giving the chromosome its familiar 'X' shape.