prophase

Meaning: Prophase is the first phase of M phase. It follows DNA replication in S phase and growth in G2. Chromatin condenses into compact chromosomes, each made of two identical sister chromatids joined at a centromere. The nucleolus disappears. The centrosomes move toward opposite poles while spindle microtubules assemble, and kinetochores form at the centromeres. Some texts place nuclear envelope breakdown at the end of prophase. Others treat it as the start of a separate stage called prometaphase.

On the MCAT: You may be asked to put the phases in order (prophase, metaphase, anaphase, telophase), to identify a phase from a micrograph or description, or to compare mitosis with meiosis. In prophase I of meiosis, homologous chromosomes pair (synapsis) to form tetrads, and crossing over at chiasmata exchanges segments between nonsister chromatids. This recombination adds genetic variation. Mitotic prophase has no pairing of homologs and no crossing over.

Common confusion: Students think chromosomes are copied during prophase. They are copied in S phase, and prophase only condenses them. A cell in prophase has the same chromosome number as in G1 (count centromeres), but twice the DNA content, because each chromosome has two chromatids.