The role of spindle fibres attached to centromeres in the separation of…

The idea

The spindle is a framework of protein microtubules assembled during prophase, running from organising centres at opposite poles of the cell; animal cells use centrioles as those centres, while plant cells build a spindle without them. Each replicated chromosome carries a centromere, and it is at the centromere that spindle fibres attach, one fibre from each pole, so the chromosome is held under tension from both sides. That two-way attachment is what draws the chromosomes into a single line at the equator during metaphase. In anaphase the centromere divides and the microtubules shorten, and because each sister chromatid is now anchored to a different pole the two are pulled apart and dragged centromere first towards opposite ends of the cell, their arms trailing behind to give the familiar V shape. Shortening the fibres requires ATP, supplied by mitochondria clustered near the spindle.

What earns the marks

• Spindle fibres being protein microtubules extending from the poles of the cell.
• Spindle fibres attaching to the centromere of each chromosome, with attachment from both poles.
• The centromere dividing and the fibres shortening in anaphase, pulling sister chromatids to opposite poles.
• ATP being required for the spindle fibres to shorten.
• The consequence that each pole receives one complete and identical set of chromatids.

Common mistakes and how to put them right

• Saying spindle fibres attach to the ends or the arms of a chromosome rather than to the centromere.

• Writing that the spindle pushes the chromatids apart instead of shortening and pulling them.

• Calling the separating structures homologous chromosomes rather than sister chromatids.

• Forgetting that a chromatid is dragged centromere first, which is why it looks V shaped.

• Assuming plant cells cannot form a spindle because they lack centrioles.

Exam tips

• Name the centromere explicitly; answers that only say the spindle pulls the chromosomes apart usually lose the attachment mark.
• Add the ATP requirement when the question mentions mitochondria near the spindle or asks why respiration inhibitors stop mitosis.
• If asked to predict the effect of a spindle inhibitor, say chromatids cannot separate so daughter cells receive an unequal number of chromosomes.