A closer look at what makes up the human genome...
How is DNA passed on to new cells?
In earlier sections, we have seen that DNA is a molecule found in living cells that contains the chemical code of heredity. Because all cells, whether they are nerve cells, muscle cells, skin cells, etc., have the same DNA, all of this DNA must be passed on when cells replicate during the process of cell division. Cells start dividing from the time a zygote is formed (the single cell that results from the fusion of egg and sperm in animals, or from pollen and ova in plants), allowing it to develop and grow. You, too, began as a zygote and much of your early development took place through cell division. Cell division slows as you age but continues throughout life, keeping you healthy and replacing cells that are damaged or have died. Most of the cells in your body undergo a type of division called mitosis, in which one cell fully replicates its DNA and then divides into two identical daughter cells.
Mitosis consists of several programmed stages. The stage in which the cell spends the most time, while it is between divisions, is aptly named interphase. During interphase, DNA replicates and the cell synthesizes proteins that it will need for the other stages. You can think of interphase as the preparatory stage of mitosis.
The first stage of division is called prophase. During prophase, DNA condenses into tightly coiled chromosomes. (For more information on chromosomes, click here). Because DNA has already replicated, each chromosome appears as a joined pair of identical sister chromatids, forming the X shape that you can see in Figure B-9. Another important part of this stage is the formation of the mitotic spindle. The mitotic spindle is a structure that will be responsible for physically separating the sister chromatids into the two daughter cells. It consists of two organizing structures called centrosomes (one of which was replicated during interphase) and a set of microtubules. Think of the microtubules as tiny fibrous strings that will be used to pull the sister chromatids apart.
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In the next stage, prometaphase, each centrosome arrives at opposite poles of the cell. Each centrosome has its own set of microtubules that extend out across the cell. Also during prometaphase, the nuclear membrane (which separates the nucleus from the rest of the cell) breaks down, allowing the chromosomes to move freely. Microtubules then attach to the centers of the chromosomes, where the sister chromatids are joined, and guide them toward the equator of the cell.
The cell has reached metaphase when each of the chromosomes is attached to two microtubules, one on each side, and are lined up in one long row across the middle of the cell.
In anaphase, the two centrosomes move further apart and the microtubules shorten. These changes pull the pairs of sister chromatids apart. The newly separated chromosomes can then be sorted into two groups.
During telophase, the chromosomes arrive at the centrosomes, and a new nuclear membrane forms around each group, creating two complete nuclei. Inside, the chromosomes begin to decondense. Also during telophase, the equator of the cell begins to be pinched by a contractile ring.
The final step of mitosis is cytokinesis (pronounced CY-TOH-KIN-EE-SIS). During cytokinesis, the contractile ring divides the cytoplasm, or the contents of the cell, in two. The result is two complete daughter cells, each containing DNA that is identical to the original parent cell.
Last Modified: 05/22/2009
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