Mitosis: Prophase

Prophase is the beginning of mitosis (Fig. 3).

Figure 3. Prophase of mitosis. Chromosomes are unreplicated during interphase as represented by the four chromosomes with the D/B, E, d/b, and e genes. After going through the S phase, the chromosomes enter prophase where the chromosomes are replicated and have two identical chromatids. Notice the identical nature of the chromatids by looking at how the genes on one chromatid exactly match the genes on the attached chromatid. Image by M. Hanneman, 2021. CC BY-NC-SA 4.0.  

During interphase, the chromosomes look like a plate of spaghetti in the nucleus. It is difficult to pick out an individual chromosome because they are each so spread out. The chromosomes in the nucleus change from being loosely dispersed to becoming more condensed. This change in chromosome structure makes them easier to move around the cell, an important issue for what is about to happen. As the chromosomes condense, they get shorter and thicker and can be seen through the microscope as individual structures (Fig. 4). The chromosomes at prophase will consist of two identical parts called sister chromatids that stay connected at the centromere. It is now clear that the chromosomes have been replicated. Chromosome replication occurred during the S-phase of interphase (Fig. 1). Next, the nucleus dissolves. At the end of prophase, the spindle apparatus moves the replicated chromosomes to the center of the cell. 

Figure 1. The cell cycle depicts the stages in the life of a cell. Those phases are interphase (with the G1, S phase, and G2 phase) and mitosis (prophase, metaphase, anaphase, telophase). Cells are in interphase the majority of the time as depicted with interphase being about 4/5 of the circle. Cell Cycle by NIH-NHGRI, n.d. Public Domain.

Figure 4. Chromosomes of the human genome. Humans have 22 somatic chromosomes and two sex chromosomes. Genome by NIH-NHGRI, n.d. Public Domain.