Chromosomes are packaged by histone proteins into a condensed structure called chromatin. Before a cell can divide, it must first replicate its DNA so that each of the two daughter cells will receive a complete copy of the DNA. The two identical chromosomes that result from DNA replication are referred to as sister chromatids. Sister chromatids are held together by proteins at a region of the chromosome called the centromere.
Chromosomes undergo additional compaction at the beginning of mitosis. When fully condensed, replicated chromosomes appear as thick X-shaped structures that are readily observed under the microscope see figure below. Chromosomes can have 1 or 2 chromatids, depending on whether they have replicated. A chromatid is a condensed DNA subunit of a chromosome. The two chromatids of a duplicated chromosome are held together at a region of DNA called the centromere see figure below.
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.
Figure 3: a Metaphase and b Anaphase. In metaphase a , the microtubules of the spindle white have attached and the chromosomes have lined up on the metaphase plate. During anaphase b , the sister chromatids are pulled apart and move toward opposite poles of the cell. Figure Detail.
After metaphase is complete, the cell enters anaphase. During anaphase, the microtubules attached to the kinetochores contract, which pulls the sister chromatids apart and toward opposite poles of the cell Figure 3c. At this point, each chromatid is considered a separate chromosome.
Figure 4: During telophase, two nuclear membranes form around the chromosomes, and the cytoplasm divides. Finally, once anaphase is complete, the cell enters the last stage of the division process — telophase. During telophase, the newly separated chromosomes reach the mitotic spindle and a nuclear membrane forms around each set of chromosomes, thus creating two separate nuclei inside the same cell. As Figure 4 illustrates, the cytoplasm then divides to produce two identical cells.
Why is mitosis important? As previously mentioned, most eukaryotic cells that are not involved in the production of gametes undergo mitosis.
These cells, known as somatic cells , are important to the survival of eukaryotic organisms, and it is essential that somatic parent and daughter cells do not vary from one another. With few exceptions, the mitotic process ensures that this is the case.
Therefore, mitosis ensures that each successive cellular generation has the same genetic composition as the previous generation, as well as an identical chromosome set. Watch this historic video from to see mitosis in action. Key Questions How do centromeres work? Key Concepts chromosomes replication meiosis. Topic rooms within Genetics Close. No topic rooms are there.
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