Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
Figure 5.9 Longitudinal section of onion root tip showing stages of mitosis and cell division typical of plant cells (× 400). Try to
identify the stages based on information given in Figure 5.7.
100
Centromeres, centrosomes and
centrioles
The centromere is needed for the separation of
chromatids during mitosis. It is visible as a constriction
(Figures 5.2 and 5.3) and is the site of attachment of
spindle microtubules. Each metaphase chromosome has
two kinetochores at its centromere, one on each chromatid
(Figure 5.10). These are made of protein molecules which
bind specifically to the DNA in the centromere and also
bind to microtubules. Bundles of microtubules called
spindle fibres extend from the kinetochores to the poles of
the spindle during mitosis. Construction of kinetochores
begins before nuclear division starts (during the S phase of
the cell cycle) and they are lost again afterwards.
The microtubules attached to a given kinetochore pull
the kinetochore, with the rest of its chromatid dragging
behind, towards the pole. This is achieved by shortening
of the microtubules, both from the pole end and from the
kinetochore end.
The poles of the spindle are where the centrosomes
are located, one at each pole. As noted in Chapter 1, the
centrosome is an organelle found in animal cells that
acts as the microtubule organising centre (MTOC) for
construction of the spindle. Each centrosome consists
of a pair of centrioles surrounded by a large number of
proteins. It is these proteins that control production of
the microtubules, not the centrioles. Plant mitosis occurs
without centrosomes.
microtubules
kinetochore
chromatid
microtubules
shorten
chromosome
kinetochore
centromere
Figure 5.10 Role of the centromere, kinetochores and
microtubules during mitosis.
QUESTION
5.1 How can the microtubules be shortened?
(See Chapter 1.)