Can you guess what this colorful image represents? It shows a eukaryotic cell during the process of cell division. In particular, the image shows the nucleus of the cell dividing. In eukaryotic cells, the nucleus divides before the cell itself splits in two; and before the
nucleus divides, the cell’s DNA is replicated, or copied. There must be two copies of the DNA so that each daughter cell will have a complete copy of the genetic material from the parent cell. How is the replicated DNA sorted and separated so that each daughter cell gets a complete set of genetic material? To answer that question, you first need to know more about DNA and the forms it takes. The Forms of DNAExcept when a eukaryotic cell divides, its nuclear DNA exists as a grainy material called chromatin. Only when a cell is about to divide and its DNA has replicated does DNA condense and coil into the familiar X-shaped form of a chromosome, like the one shown in Figure \(\PageIndex{2}\). Because DNA has already replicated, each chromosome actually consists of two identical copies. The two copies of a chromosome are called sister chromatids. Sister chromatids are joined together at a region called a centromere. The process in which the nucleus of a eukaryotic cell divides is called mitosis. During mitosis, the two sister chromatids that make up each chromosome separate from each other and move to opposite poles of the cell. Mitosis occurs in four phases. The phases are called prophase, metaphase, anaphase, and telophase. They are shown in Figure \(\PageIndex{3}\) and described in detail below. ProphaseThe first and longest phase of mitosis is prophase. During prophase, chromatin condenses into chromosomes, and the nuclear envelope (the membrane surrounding the nucleus) breaks down. In animal cells, the centrioles near the nucleus begin to separate and move to opposite poles of the cell. Centrioles are small organelles found only in eukaryotic cells that help ensure the new cells that form after cell division each contain a complete set of chromosomes. As the centrioles move apart, a spindle starts to form between them. The blue spindle, shown in Figure \(\PageIndex{4}\), consists of fibers made of microtubules. MetaphaseDuring metaphase, spindle fibers fully attach to the centromere of each pair of sister chromatids. As you can see in Figure \(\PageIndex{5}\), the sister chromatids line up at the equator, or center, of the cell. The spindle fibers ensure that sister chromatids will separate and go to different daughter cells when the cell divides. Some spindles do not attach to the kinetochore protein of the centromeres. These spindles are called non-kinetochore spindles that help in the elongation of the cell. This is visible in Figure \(\PageIndex{5}\). AnaphaseDuring anaphase, sister chromatids separate and the centromeres divide. The sister chromatids are pulled apart by the shortening of the spindle fibers. This is a little like reeling in a fish by shortening the fishing line. One sister chromatid moves to one pole of the cell, and the other sister chromatid moves to the opposite pole (see Figure \(\PageIndex{6}\)). At the end of anaphase, each pole of the cell has a complete set of chromosomes TelophaseThe chromosomes reach the opposite poles and begin to decondense (unravel), relaxing once again into a stretched-out chromatin configuration. The mitotic spindles are depolymerized into tubulin monomers that will be used to assemble cytoskeletal components for each daughter cell. Nuclear envelopes form around the chromosomes, and nucleosomes appear within the nuclear area (see Figure \(\PageIndex{7}\). CytokinesisCytokinesis is the final stage of cell division in eukaryotes as well as prokaryotes. During cytokinesis, the cytoplasm splits in two and the cell divides. The process is different in plant and animal cells, as you can see in Figure \(\PageIndex{8}\). In animal cells, the plasma membrane of the parent cell pinches inward along the cell’s equator until two daughter cells form. In the plant cells, a cell plate forms along the equator of the parent cell. Then, a new plasma membrane and cell wall form along each side of the cell plate. Review
Explore MoreWatch the video below to visualize mitosis. Attributions
What stage of mitosis do chromosomes get pulled to opposite poles?Metaphase leads to anaphase, during which each chromosome's sister chromatids separate and move to opposite poles of the cell.
During which stage of mitosis are the chromatids are pulled apart?During prophase, the chromosomes condense and the nuclear envelope dissolves. During metaphase, the chromosomes align at the center of the cell. During anaphase, the sister chromatids are separated and pulled to opposite ends of the cell.
During which phase of mitosis are the chromosomes actively being separated and pulled to opposite poles of the cell by the motors that are on the spindle fibers?The phase of mitosis in which separation and movement of chromosomes to opposite ends (poles) of the cells is called anaphase.
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