Why are chromosomes replicated

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.

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Recovering a Stalled Replication Fork. Aging and Cell Division. Germ Cells and Epigenetics. Citation: Noguchi, E. Nature Education 3 9 The replication fork is more than just a means for DNA duplication. It is connected to a checkpoint system that keeps the genome intact and prevents cancer. Aa Aa Aa. Figure 1: Obstacles on DNA that generate stalled replication forks. Figure Detail. Figure 2: Checkpoint responses. Figure 3: Fork protection. Stabilizing replisome components at the replication fork when the fork stalls, so that the fork can re-start after the problems are solved.

Preventing fork breakage. If the fork is broken, fork protectors may be required to re-assemble replisome components. Protecting the replication forks in a configuration that is recognized by replication checkpoint proteins Figure 2. References and Recommended Reading Abraham, R. Article History Close. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable.

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Evidence for the bidirectional replication of the Escherichia coli chromosome. New Biol. Measelson, M. The replication of DNA in Escherichia coli. Meacock, P. The two chromosomes are called sister chromatids. They divide in two before the cell divides, and each daughter cell gets one chromosome from each of the sister chromatids.

Meiosis is more complicated that mitosis and requires two cell divisions. In the first stage, the chromosomes replicate as in mitosis. However, then the chromatid arms of sister chromatids may overlap with other sister chromatids and cause crossovers — swapping of DNA between chromatids, so that the each chromatid is no longer identical to its sister. The cell then divides twice, so that the sister chromatids separate and the daughter cells have 23 chromosomes each.

Emma Woodhouse is a freelance writer from the UK, and has been writing professionally since She is pursuing a Masters of Science degree in physics at the University of London, and specializes in writing about science and math. Similarities of Mitosis and Meiosis. Stages of Meiosis With a Description.