Back To CourseAP Biology: Help and Review
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Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.
In eukaryotes, a centromere is a region of DNA that is responsible for the movement of the replicated chromosomes into the two daughter cells during mitosis and meiosis. There is one centromere on each chromosome, and centromeres are responsible for two major functions.
One major function of a centromere is joining the sister chromatids. The two copies of a replicated chromosome are called sister chromatids, and they must stay joined together until it is time for them to be physically pulled into the two future daughter cells. This ensures that each daughter cell will get exactly one copy of each chromosome.
Cohesins are proteins that keep the chromatids stuck together. At the beginning of mitosis, the cohesins are distributed evenly along the chromatids, so they are stuck together along their whole lengths. By metaphase, when all the chromosomes are lined up at the middle of the cell just before they separate, the cohesins are only located at the centromere regions, so the sister chromatids are only connected there. In this diagram, you can see what this looks like.
The other major function of the centromere is attaching the microtubules in the mitotic spindle. In this function, the centromere directs the formation of the kinetochore, which is a special protein structure that attaches to the microtubules in the mitotic spindle. On each chromatid, the kinetochore forms at the centromere region of the DNA. Once all of the chromatids are attached to the mitotic spindle, the microtubules pull the sister chromatids apart into the two future daughter cells.
With this information, you can see why it's so important that each chromosome has exactly one centromere. If a chromosome had two centromeres, it could be broken apart by being pulled in two different directions during mitosis. If it had no centromeres, it would assort randomly into the daughter cells and would eventually be lost.
On a condensed, duplicated chromosome it is easy to see where the centromere is located. It's the part where the two chromatids are connected and form an X shape. Even on a single condensed chromosome, the centromere forms a constriction that can be seen in the microscope. 'Centro' means center and 'mere' means part, but centromeres are not always located right at the center of a chromosome. They can have various different positions, as shown in this diagram.
When the centromere is approximately in the center of a chromosome, it is called metacentric. 'Meta' means middle, so this makes sense. Submetacentric centromeres are closer to one end of the chromosome than the other. Their name means that they are not quite at the middle. Centromeres that are very close to one end of the chromosome are called acrocentric. 'Acro' means top or extremity. Telocentric centromeres are positioned at the very end of a chromosome. 'Telo' means last or end. You can compare this word to 'telomeres,' which are specialized regions of DNA at the very ends of chromosomes.
The different centromere positions are useful for biologists. First, they provide one way to help recognize which chromosome is which under the microscope. Also, centromeres are good landmarks for describing the locations of genes along the length of a chromosome. Often, the centromeres are not exactly in the center, so they divide chromosomes into long arms and short arms. Traditionally, the short arms are called 'p' for petite, and the long arms are called 'q' (simply because it comes after p in the alphabet). So, you can say that a gene is located on the q arm of chromosome 13, for example.
Centromeres are in heterochromatin areas. In higher eukaryotes, centromeres are found in long stretches of heterochromatin, or highly condensed DNA where gene transcription is not active. Centromeres have special nucleosomes. Centromere nucleosomes contain a special kind of histone H3 that is not found anywhere else in the chromosome; it is called CENP-A. Scientists don't yet know what this special histone does, but they know it is essential for centromere function because cells that don't have CENP-A don't divide properly.
Centromeres are made up of repeating DNA sequences. In higher eukaryotes, like humans, centromeres are made up of long stretches of repeating DNA sequences called alpha satellite sequences. In humans, alpha satellite sequences are 171 base pairs long, are rich in the nucleotides A and T, and have slight variations from each other. These sequences are repeated thousands of times in the centromere. A protein called CENP-B binds to the alpha satellite sequences and forms the basis of the kinetochore.
In lower eukaryotes, like yeast, the centromere is much simpler. It is a particular 125-base pair sequence that is present only once on the chromosome. Instead of recognizing repeating alpha satellite sequences, yeast kinetochore proteins recognize and assemble on this one specific centromere sequence.
In this lesson, you've learned that a centromere is a region of DNA on each eukaryotic chromosome that has two major functions during mitosis. One function is to hold the two sister chromatids together until it's time for them to separate during anaphase. The second function is to recruit the proteins that form the kinetochore, which attaches each chromatid to the microtubules in the mitotic spindle. Centromeres can be located in different positions on chromosomes, and biologists use this characteristic to help distinguish the chromosomes and describe gene locations. Finally, the distinctive centromere DNA sequences are found in highly condensed heterochromatin areas of the chromosome, where there are also specialized nucleosomes.
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Back To CourseAP Biology: Help and Review
28 chapters | 382 lessons