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Divide and SplitCan 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. Figure \(\PageIndex{1}\): Dividing cell stained with fluorescent dyes. You can see chromosomes in blue and spindles in green.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. Figure \(\PageIndex{2}\): Chromosome. After DNA replicates, it forms X-shaped chromosomes like the one shown here. 1. Chromatid, 2. Centromere, 3. short arm, 4. long arm. Centromere contains proteins called kinetochores (not shown) where spindles attach during mitosis.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. Figure \(\PageIndex{3}\): Mitosis is the phase of the eukaryotic cell cycle that occurs between DNA replication and the formation of two daughter cells. Mitosis has four substages, prophase, metaphase, anaphase, and telophase.ProphaseFigure \(\PageIndex{4}\): Prophase in later stage is called prometaphase. The spindle starts to form during the prophase of mitosis. The spindles start to attach to the Kinetochores of centromeres of sister chromatids during Prometaphase.The 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 \(\PageIndex{5}\). Figure \(\PageIndex{5}\): Chromosomes, consisting of sister chromatids, line up at the equator or middle of the cell during metaphase. The blue lines are spindles, and the orange rectangles at the cell poles are centrioles. Some spindles from the opposing centrioles attach with each other, and some spindles attach to the kinetochores of the sister chromosomes from their respective sides. Each chromosome is attached to two spindles.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 Figure \(\PageIndex{6}\): Anaphase: Sister chromatids break apart and move to the opposite pole with the help of spindles. The newly separated sister chromatids are called chromosomes now.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 \(\PageIndex{7}\). Figure \(\PageIndex{7}\): Telophase: The chromosomes decondense, spindles start to disappear, two nuclei form in a cell.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. Figure \(\PageIndex{8}\): Cytokinesis is the final stage of eukaryotic cell division. It occurs differently in animal (left) and plant (right) cells. You can see a microfilament ring forming at the center of the elongated animal cell. This creates a depression called cleavage furrow. This invagination ultimately separates the cell cytoplasm into two cells. A cell plate forms at the center of the elongated plant cell. Then a new plasma membrane and cell wall form along each side of the cell plate.Review
Explore Morehttps://bio.libretexts.org/link?16755#Explore_More Watch the video below to visualize mitosis. Attributions
This page titled 7.3: Mitotic Phase - Mitosis and Cytokinesis is shared under a CK-12 license and was authored, remixed, and/or curated by Suzanne Wakim & Mandeep Grewal via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request. What happens to DNA in prophase 1?Prophase I
Figure 1: Recombination is the exchange of genetic material between homologous chromosomes. During prophase I, the chromosomes condense and become visible inside the nucleus.
Is DNA replicated in the prophase?DNA replication occurs during interphase, not prophase.
A common misconception is that DNA copies itself during prophase, but this is not true. In prophase, the DNA has already been copied while the cell was in interphase.
What happens to the DNA between interphase and prophase?The key difference between interphase and prophase is that a cell spends a lot of time in interphase undergoing protein synthesis, DNA replication, and growth while the cell spends a short time in prophase by undergoing condensation of chromatin, pairing of homologous chromosomes and spindle fiber formation.
What happens to DNA during mitosis?Mitosis is a type of cell division in which one cell (the mother) divides to produce two new cells (the daughters) that are genetically identical to itself. In the context of the cell cycle, mitosis is the part of the division process in which the DNA of the cell's nucleus is split into two equal sets of chromosomes.
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