8/12/2019 Cell Div Part 2

1/2

Ridge Cell Dynamics Lecture Notes September 8, 2011, 8:25 AM PAGE 1

Cell division, part 2.

The mechanical events of cell division (M phase)

Chapter 18.

1. This phase includes the various stages of nuclear division (mitosis) andcytoplasmic division (cytokinesis).

2. M phase is initiated by large numbers of protein phosphorylations, triggered by

the activation of MPF (see lecture notes, cell division, part 1), and is terminated

by the dephosphorylations after the inactivation of MPF (by proteolysis of its

subunits).

3. The protein phosphorylations during M phase are responsible for many

morphological changes that accompany mitosis. These are:

condensation of the chromosomes

breakdown of the nuclear envelope fragmentation of the ER and GA into small vesicles

loosening of adhesion to other cells and the extracellular matrix

transformation of the cytoskeleton

Because M phase involves a complete re-organisation of the cell interior, the

number of proteins involved must be very large, and every part of the cell is

affected.

4. Three features are unique to M phase: chromosome condensation, the mitotic

spindle, the contractile ring.

5. Chromosome condensation is required for segregation of the chromosomes into

daughter cells and is accompanied by extensive phosphorylation of histone by

MPF.

6. Chromosomes are separated by a bipolar mitotic spindle, which is composed of

MTs and MAPs, and aligns the chromosomes in a plane that bisects the cell. The

spindle moves each set of daughter chromosomes to the opposite spindle poles

(the mechanism to be discussed later).

7. A second cytoskeletal structure is the contractile ring, which is composed of actin

and myosin-II, and forms just beneath the plasma membrane in a plane

perpendicular to the axis of the MT spindle. The ring contracts after separation

of the chromosomes, dividing the cell into two, separating not only the two sets

of chromosomes but also half the parent cell contents.

8. Cell division also depends on the duplication of the centrosome, which is an MT

organising centre (MTOC). The centrosome is a fuzzy, poorly defined area of the

cell (centrosome matrix) that is associated with a pair of centrioles. Before

division, the centrosome is duplicated, and each then forms the two poles of the

mitotic spindle. In animal cells the centrosome closely associates with a pair of

centrioles, which are thought to be involved in the duplication of the centrosome.

Centrioles are not necessary for division, and are lacking in plant cells. The

centrosome matrix is the important and fundamental part of the centrosome.9. During interphase the centrosome is duplicated but the two centrosomes remain

together on one side of the cell until mitosis. The complex then splits into two

8/12/2019 Cell Div Part 2

2/2

Ridge Cell Dynamics Lecture Notes September 8, 2011, 8:25 AM PAGE 2

and each pair becomes a part of a MTOC that nucleates a radial array of MTs

called an aster. The two asters move to opposite sides of the nucleus to form

the poles of the spindle. Each daughter cell thus receives a single centrosome.

10. Centrosomes and centrioles remain an enigma, and still nothing is known about

their substance, replication or evolutionary origin.

11. M phase is traditionally divided into six stages. They form a dynamic sequence.Duration of each stage varies with cell type, and they are much shorter in

embryonic cell cycles. Cytokinesis begins before mitosis ends. The stages are:

prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis.

The first five stages constitute mitosis.

12. The beginning of prophase (and thus M phase as a whole) is defined as the point

in which condensed chromosomes first become visible. But this is somewhat

arbitrary, since chromosome condensation increases gradually and continuously

during late G2. The beginning of prometaphase is defined as the moment the

nuclear membrane breaks down.13. A summary of the stages

by early prophase the centrosome contains two centriole pairs;

chromosomes are visible

at late prophase the centrosome divides and the two asters move apart

at prometaphase the nuclear envelope breaks down (do you remember

nuclear lamins?), and the spindle MTs interact with the chromosomes

at metaphase the bipolar structure of the spindle is clear and all

chromosomes align across the middle of the spindle

at early anaphase, the paired daughter chromosomes (chromatids) separatesynchronously and move towards the poles of the spindle

by late anaphase the spindle poles have moved apart, increasing the

separation of the two groups of chromosomes

at telophase the daughter nuclei re-form

by late telophase cytokinesis is almost complete, but a midbody (remains of

the spindle) persists between the two daughter cells

14. These stages occur in strict sequential order. Cytokinesis begins at anaphase

and continues through to the end of M phase of the cell cycle.

15. Metaphase takes the longest time, which is not surprising considering this is the

stage at which all the chromosomes are gathered at the centre. Anaphase is the

quickest.

16. During M phase cytoplasmic organelles are fragmented or are present in large

numbers to ensure they are inherited in both daughter cells. Organelles such as

chloroplasts and mitochondria cannot assemble spontaneously (we have already

discussed this) and it also may not be possible for ER and the GA to arise de

novo. By fragmentation or by the presence of large numbers, the organelles are

safely inherited in both daughter cells. ER apparently associates with MTs of the

mitotic spindle, which may help with an even distribution.

17. As in cytokinesis, mitochondria and chloroplasts separate into daughter

organelles by an actin-based contractile ring.