Detailed Instructions: Given a question and a context passage, generate the answer having the word or phrase from the context passage. Here, the answer should be a shortest continous span from the passage.
Q: Context: The genetic information of the cell, or DNA, is stored in the nucleus. During mitosis, two nuclei (plural for nucleus) must form, so that one nucleus can be in each of the new cells after the cell divides. In order to create two genetically identical nuclei, DNA inside of the nucleus must be copied or replicated. This occurs during the S phase of the cell cycle. During mitosis, the copied DNA is divided into two complete sets, so that after cytokinesis, each cell has a complete set of genetic instructions. To begin mitosis, the DNA in the nucleus wraps around proteins to form chromosomes. Each organism has a unique number of chromosomes. In human cells, our DNA is divided up into 23 pairs of chromosomes. Replicated DNA forms a chromosome made from two identical sister chromatids, forming an "X" shaped molecule ( Figure 1.1). The two chromatids are held together on the chromosome by the centromere. The centromere is also where spindle fiber microtubules attach during mitosis. The spindles separate sister chromatids from each other. During mitosis, the two sister chromatids must be divided. This is a precise process that has four individual phases to it. After the sister chromatids separate, each separate chromatid is now known as a chromosome. Each resulting chromosome is made of DNA from just one chromatid. So, each chromosome after this separation is made of "1/2 of the X." Through this process, each daughter cell receives one copy of each chromosome. The four phases of mitosis are prophase, metaphase, anaphase and telophase ( Figure 1.2). 1. Prophase: The chromatin, which is unwound DNA, condenses forming chromosomes. The DNA becomes so tightly wound that you can see them under a microscope. The membrane around the nucleus, called the nuclear envelope, disappears. Spindles also form and attach to chromosomes to help them move. 2. Metaphase: The chromosomes line up in the center, or the equator, of the cell. The chromosomes line up in a row, one on top of the next. 3. Anaphase: The two sister chromatids of each chromosome separate as the spindles pull the chromatids apart, resulting in two sets of identical chromosomes. 4. Telophase: The spindle dissolves and nuclear envelopes form around the chromosomes in both cells. An overview of the cell cycle and mito- sis: during prophase the chromosomes condense, during metaphase the chromo- somes line up, during anaphase the sister chromatids are pulled to opposite sides of the cell, and during telophase the nuclear envelope forms. This is a representation of dividing plant cells. Cell division in plant cells differs slightly from animal cells as a cell wall must form. Note that most of the cells are in interphase. Can you find examples of the different stages of mitosis?
Question: when do the two sister chromatids of each chromosome separate?
A:
anaphase