A cell's lifespan is broken up into stages that we refer to as the cell cycle. The cell cycle begins with a three stage growth and duplication phase called "interphase". Next, it moves into mitosis and cytokinesis, both of which are considered the "dividing" phase.

The centromere is a crucial component that allows for mitosis to happen. 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/DNA replicates. This happens during the S (synthesis) phase; S phase is the part of interphase when DNA duplication takes place.

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. In this post, we're going over the S phase definition, the cell cycle, and how centromeres fit into both.

Interphase is the first stage of a cell's life. It has three distinct parts: G1 phase, S phase and G2 phase. G1 and G2 are growth phases, and the period of interphase when DNA is replicated is the S phase.

It's during interphase that the cell grows, functions, and prepares for eventual division.

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.

The cell cycle has two general phases. Interphase is the preparation phase before cell division, and mitosis the phase in which the dividing happens. As we said earlier, interphase can be further divided into three phases.

G1 phase is for cell growth. The cell does its job, grows, makes copies of organelles, and function as normal. S phase is the part of interphase when DNA duplication takes place. G2 phase is for more cell growth, further duplication of cytoplasmic organelles, and general preparation for mitosis.

Since the period of interphase when DNA is replicated is the S phase, it's also the time during which centromeres are replicated. This makes sense since centromeres are part of chromosomes and chromosomes are S phase is the part of interphase when DNA duplication takes place.

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.

Humans have 46 chromosomes and each of them has a centromere. The period of interphase when DNA is replicated is the S phase, and thus is when all centromeres replicate by S phase definition. But, not all of them replicate at the same time during that phase. 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.

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.