Mitosis: Prophase and Prometaphase

• Prophase, prometaphase, and metaphase are sequential stages in mitosis, a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus.
• These phases occur after interphase and before anaphase and telophase in eukaryotic cells.

Prophase

• Prophase marks the beginning of mitosis, characterized by several key events in the nucleus and cytoplasm.
• Chromatin condenses, causing the chromosomes to become tightly packed and visible under a microscope.
• Each duplicated chromosome appears as two identical sister chromatids joined at the centromere.
• The nucleoli disappear during prophase.
• The mitotic spindle begins to form in the cytoplasm.
• The mitotic spindle is composed of the centrosomes and the microtubules that extend from them.
• The centrosomes move away from each other, propelled by the lengthening microtubules between them.

Prometaphase

• Prometaphase follows prophase once the nuclear envelope fragments.
• Microtubules extending from each centrosome can now invade the nuclear area.
• The chromosomes become even more condensed.
• Each of the two chromatids of a chromosome now has a kinetochore.
• A kinetochore is a specialized protein structure located at the centromere.
• Some microtubules attach to the kinetochores, becoming "kinetochore microtubules."
• These microtubules interact with the kinetochore, linking the chromosome to the spindle.
• Nonkinetochore microtubules interact with those from the opposite pole of the spindle.
• Prometaphase involves chromosomes becoming more condensed, nuclear envelope breakdown, and the attachment of spindle microtubules to kinetochores.

Metaphase

• Metaphase is the third stage of mitosis.
• The centrosomes are now at opposite poles of the cell.
• The chromosomes convene at the metaphase plate, an imaginary plane that is equidistant between the spindle's two poles.
• The chromosomes’ centromeres lie on the metaphase plate.
• For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles.
• The entire apparatus of microtubules is called the spindle.
• Metaphase concludes with all chromosomes aligned at the metaphase plate, attached to the spindle apparatus via kinetochore microtubules.

Mitosis is a type of cell division. Prophase is the first stage of mitosis, and is characterized by the condensing of chromatin into chromosomes. Prometaphase follows prophase after the nuclear envelope fragments.