centromere
Summary
The centromere is the constricted region of a chromosome where sister chromatids are joined and where the kinetochore forms for spindle attachment. Chromosomes are counted by centromeres, so sisters become separate chromosomes only when cohesin is cleaved in anaphase.
Detail
Meaning: The centromere is a specialized region of repetitive, tightly packed DNA that holds sister chromatids together after S phase. The kinetochore, a protein complex, assembles on it and binds spindle microtubules. In anaphase, cohesin linking the sisters is cleaved and microtubules pull each chromatid, now a separate chromosome, to opposite poles. Its position along the chromosome gives metacentric, submetacentric or acrocentric shapes.
On the MCAT: Expect chromosome-versus-chromatid counting. A cell with 46 chromosomes in G2 has 46 centromeres but 92 chromatids, because chromosomes are counted by centromeres. A fragment without a centromere cannot attach to the spindle and is usually lost. In meiosis I, homologs separate while sister centromeres stay joined; sisters separate in meiosis II.
Common confusion: Centromere, kinetochore, centrosome and telomere are different structures. The centromere is a DNA region; the kinetochore is the protein complex built on it; the centrosome organizes microtubules in the cytoplasm; telomeres cap the chromosome ends.
Sources
- Campbell Biology
- Molecular Biology of the Cell
- Cell and Molecular Biology
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.