A centromere is like belt on chromosomes that can be pulled on when chromosomes are moved within an cell. Since centromeres are part of the chromosome, they replicate when the rest of the chromosome replicates. This happens during the S (synthesis) phase of interphase. Different organisms replicate their centromeres at different times during S phase, some at the beginning and others at the end, but all centromeres need to be replicated before S phase is over.
Centromeres are parts of a chromosome that can be pulled on when a chromosome is moved inside the cell. This happens during mitosis, or cell division, when chromosomes are pulled apart into different cells. Centromeres aren’t always in the middle of a chromosome, like a belt worn on the waist. Centromeres can be at the ends of chromosomes, in the middle or between the middle and the end. They are made of many proteins, including ones called cohesins, centromere proteins and kinetochore proteins.
S phase, or synthesis phase, is the time during which centromeres are replicated. This makes sense, since centromeres are part of chromosomes and chromosomes are replicated during S phase. The cell cycle has two general phases. Interphase is the preparation phase before cell division, and mitosis the phase in which the dividing happens. Interphase can be further divided into three phases. G1 phase is for cell growth, S phase is for DNA synthesis and G2 phase is for more cell growth. Without new centromeres for the new copy of DNA that is replicated during S phase, the cell would not be able to pull the replicated DNA copies apart.
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Humans have 46 chromosomes and each of them has a centromere. All centromeres replicate during S phase, but not all of them replicate at the same time. The journal “Molecular and Cell Biology” reports that the X chromosome, chromosome 7 and chromosome 17 all replicate in S phase, but do so at different times. Centromeres contain regions of DNA called alpha satellite sequences. These are segments of DNA that are repeated, like rows of print in a book. These regions are thousands of nucleotides long -- a nucleotide is a building block of DNA -- and are replicated during S phase to make new centromeres.
Fruit Flies and Yeast
The centromeres on the chromosomes of fruit flies and yeast also replicate during S phase. In certain fruit flies, centromeres replicate early in S phase. The journal “PLOS Genetics” reports that in a type of disease-causing yeast, the DNA in centromeres is the first region of DNA that is copied during S phase. A chromosome is a very long string of DNA that is packed by proteins into a neat and compact structure, which looks like fingers. A long string of DNA is copied in multiple places at once, to make the copying process faster. Centromeres are the first places where this copying happens.