Before a cell can enter the active phases of mitosis, however, it must go through a period known as interphase , during which it grows and produces the various proteins necessary for division. Then, at a critical point during interphase called the S phase , the cell duplicates its chromosomes and ensures its systems are ready for cell division.
If all conditions are ideal, the cell is now ready to move into the first phase of mitosis. This page appears in the following eBook. Aa Aa Aa. Walther Flemming's drawing of chromosomes. What happens during mitosis? Figure 1: During prophase, the chromosomes in a cell's nucleus condense to the point that they can be viewed using a light microscope. Prophase is the first phase of mitosis. During this phase, the chromosomes inside the cell's nucleus condense and form tight structures.
In fact, the chromosomes become so dense that they appear as curvy, dark lines when viewed under a microscope Figure 1. Because each chromosome was duplicated during S phase, it now consists of two identical copies called sister chromatids that are attached at a common center point called the centromere. Figure 2: The mitotic spindle white begins to form outside the cell's nucleus. Important changes also take place outside of the nucleus during prophase. In particular, two structures called centrosomes move to opposite sides of the cell during this phase and begin building the mitotic spindle.
The mitotic spindle plays a critical role during the later phases of mitosis as it orchestrates the movement of sister chromatids to opposite poles of the cell Figure 2.
After prophase is complete, the cell enters prometaphase. During prometaphase, the nuclear membrane disintegrates and the mitotic spindle gains access to the chromosomes. During this phase, a protein structure called the kinetochore is associated with the centromere on each sister chromatid.
Stringlike structures called microtubules grow out from the spindle and connect to the sister chromatids at their kinetochores; one microtubule from one side of the spindle attaches to one sister chromatid in each chromosome, and one microtubule from the other side of the spindle attaches to the other sister chromatid Figure 3a.
It is a two-step process that reduces the chromosome number by half—from 46 to 23—to form sperm and egg cells. When the sperm and egg cells unite at conception, each contributes 23 chromosomes so the resulting embryo will have the usual Meiosis also allows genetic variation through a process of gene shuffling while the cells are dividing.
Other chapters in Help Me Understand Genetics. Genetics Home Reference has merged with MedlinePlus. Learn more. Similarities Mitosis Diploid parent cell Consists of interphase, prophase, metaphase, anaphase and telophase In metaphase individual chromosomes pairs of chromatids line up along the equator.
Ends with cytokinesis. Meiosis Diploid parent cell Consists of interphase, prophase, metaphase, anaphase and telophase but twice! In metaphase II individual chromosomes pairs of chromatids line up along the equator. Related Content:. What is mitosis? What is meiosis?
What is a cell? Many single-celled organisms reproduce by cell division and have a single copy of each chromosome. Virtual Genetics Education Centre.
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