Advanced Placement (AP) Biology Practice Exam

During which phase do sister chromatids duplicate?

1. Prophase

2. Metaphase

3. S phase of interphase

4. Anaphase

The correct answer is: S phase of interphase

Sister chromatids duplicate during the S phase of interphase. During this phase, the cell's DNA is replicated with the goal of preparing for cell division. Each chromosome, originally consisting of one chromatid, is duplicated, resulting in two identical sister chromatids linked together at the centromere. This is crucial for ensuring that each daughter cell receives an identical set of chromosomes when the cell eventually divides during mitosis or meiosis. The other phases mentioned do not involve the duplication of sister chromatids. Prophase is characterized by the condensation of chromatin into visible chromosomes and the beginning of spindle formation but does not involve duplication. Metaphase involves the alignment of chromosomes at the cell's equatorial plane and does not include duplication. Anaphase is when the sister chromatids are separated and pulled toward opposite poles of the cell, marking a stage after duplication has already occurred. Thus, understanding the process of replication during the S phase is key to comprehending how genetic information is evenly distributed during cell division.