Prophase

A stage in both types of cell division (meiosis and mitosis) during which the chromosomes become visible under a microscope. The nuclear membrane begins to dissolve at this point, too. This is the first major step of mitosis and of the first step of meiosis. It's the first major step of the second meiotic division.

Previous step: Interphase
Next step: Metaphase

Prophase I (meiosis):

Chromatin condenses and chromosomes come into view in pairs. Once contact is made at any point between the two homologues, paring extends in a zipperlike fashion along the length of the chromatids in a process called synapsis. Pairing of homologous chromosomes actually involves four chromatids, a complex of chromatids called a tetrad.

• Crossing over: exchange of segments of one chromosome with corresponding segments from its homologous chromosome.
• As prophase progresses, the homologues begin to pull away from each other, except at the crossover points (chiasmata.) At chiasmata, the homologues remain in close association until the end of prophase. Then, the chiasmata seem to slip off the ends of the chromosomes.
• Although the homologues chromosomes have moved slightly apart, they are still paired.
• The nuclei and nuclear envelope disappear towards the end of this stage.

Prophase II (meiosis):

Nuclear envelopes (if present) disappear and new spindle fibers begin to form.

1. Prophase is the first stage of mitosis. During prophase, the chromosomes, the chromosomal material, or chromatin, gradualling appears as shortened, distinct, rods. This shortening of the chromatin is one of the first observable signs that mitosis has begun. In animal cells the centrioles begin to move to opposite sides of the cell.

Each chromosome is made of two distinct strands that are called chromatids. Each pair of chromatids is held together by a centromere.

As the chromosomes become visible, other events take place within the cell. The nuclear membrane and the nucleolus gradually disintegrate. A new structure, the spindle, appears. The spindle is a three-dimensional structure shaped somewhat like a football. It consists of microtubules that extend across the cell. The fibers of the spindle appear to guide the movements of the chromosomes during mitosis. Most plant cells do not have centrioles, although they have a spindle. Animal cells also have a structure that plant and other cells do not have. This structure is the aster. The aster is made of microtubules that radiate out from the centrioles.