The spindle fibre is attached to which of the following specific regions of the chromosomes?

Correct — A, Kinetochore. The centromere is the constricted region that holds sister chromatids together, and sitting on it on each chromatid is a disc-shaped protein plate called the kinetochore. Spindle microtubules do not attach to the DNA itself; they bind this protein plate, and it is the kinetochore that converts microtubule shortening into the poleward pull that separates chromatids at anaphase. The distinction the examiner wants is precisely this one: the centromere is the chromosomal region, the kinetochore is the protein structure assembled on it to which the fibre actually attaches.

• (b)Chromonema — The chromonema is the thread-like coiled filament of chromatin visible in the early prophase chromosome. It is the material of the chromosome arm, not an attachment plate, and no spindle fibre binds to it.
• (c)Telomere — Telomeres are the repetitive DNA caps at the two ends of a chromosome. Their job is to stop the ends being read as broken DNA and to absorb the shortening that occurs at each replication — they are deliberately the part of the chromosome that nothing attaches to.
• (d)Centriole — Centrioles are cylindrical organelles in the centrosome of an animal cell, and they organise the spindle from the poles. They are the source of the fibres, not the target of them, and they are not part of a chromosome at all.

During cell division each chromosome must be gripped and pulled to a pole. The grip point is built in two layers. The centromere is the primary constriction of the chromosome, a DNA region rich in repetitive sequence; on the surface of the centromere, on each of the two sister chromatids, cells assemble a multi-protein plate called the kinetochore. Spindle microtubules growing from the poles capture these plates, and a checkpoint holds the cell in metaphase until every kinetochore is correctly attached.

The trap in this item is that three of the four options are genuine chromosome or spindle vocabulary, so a candidate who half-remembers the topic can be pulled anywhere. Sort them by what each one is: chromonema is chromosome material, telomere is a chromosome end, centriole is a cytoplasmic organelle. Only kinetochore is a protein assembly built for attachment, and the question says 'specific regions of the chromosomes' — which rules the centriole out on the wording alone.

• The centromere is the chromosomal region; the kinetochore is the protein complex built on it.
• Each chromosome carries two kinetochores at metaphase, one per sister chromatid, so the two chromatids can be pulled to opposite poles.
• Telomeres are the repeat-sequence caps at chromosome ends and are not attachment sites.
• Centrioles lie in the centrosome and organise the spindle from the poles; higher plants form a spindle without them.
• The spindle assembly checkpoint delays anaphase until every kinetochore has captured microtubules.

• Answering 'centromere' when the question asks what the fibre attaches to — the fibre binds the kinetochore built on the centromere.
• Picking centriole because it is associated with the spindle; it organises the spindle but is not part of a chromosome.
• Assuming plant cells must have centrioles because they divide by mitosis; higher plants form spindles without them.

A one-word recall item that separates candidates who learned the chromosome's parts by name from those who only learned the stages of mitosis.

• practice — not a real PYQ

  The repetitive DNA sequences that cap the two ends of a eukaryotic chromosome are called

  • (a)centromeres
  • (b)telomeres
  • (c)kinetochores
  • (d)chromomeres

  Answer(b) telomeres — they protect the chromosome ends from being treated as double-strand breaks and shorten a little at every round of replication.

• practice — not a real PYQ

  In which stage of mitosis do the sister chromatids separate and begin moving to opposite poles?

  • (a)Prophase
  • (b)Metaphase
  • (c)Anaphase
  • (d)Telophase

  Answer(c) Anaphase — the cohesion holding the chromatids together is cleaved and the kinetochore microtubules shorten, drawing one chromatid to each pole.