Mechanisms Of Cell Division

THE MECHANISMS OF CELL DIVISION





Stefan Andrei Anghel
January 8, 2008















An Overview of the M Phase
– The central problem for a mitotic cell in M phase is how to accurately separate and distribute (segregate) its chromosomes, which were replicated(COMPLETELY!) in the preceding S phase, so that each new daughter cell receives an identical copy of the genome– The M Phase(a.k.a. cell division) consists of six phases: the first five(prophase, prometaphase, metaphase, anaphase and telophase) are part of mitosis(the division of the nucleus) and the sixth phase is cytokinesis.

The Five Stages of Mitosis
Prophase
– The interphase array of microtubules us replaced as the duplicated centrosomes become more active in microtubule nucleation. Thisprovides two sites of assembly of dynamic microtubules known as asters.
– The asters move to opposite sides of the nucleus(they will become the two poles of the mitotic spindle)
– Because it is dependent on the interphase array of microtubules, the internal order of membranes is disassembled. The nuclear envelope retracts into the endoplasmic reticulum in most cells from higher EK, and Golgimembranes break down into vesicles.
– The microfilament organization is generally rearranged to give rise to a rounded cell.
– The nucleolus breaks down and chromosomes begin to condense.
– Cohesins(holding the sister chromatids together) are degraded except at the centromeric region.

Prometaphase
– Initiated by the breakdown of the nuclear envelope and the nuclear pores and disassembly ofthe lamin-based nuclear lamina.
– Microtubules capture chromosomes at kinetochores
– Because each chromatid has a kinetochore, each replicated chromosome will be attached to both spindle poles and will eventually align equidistantly from the spindle poles(the process is known as congression)
– Continues until all chromosomes have congressed

Metaphase
– The stage when all the chromosomesare aligned at the metaphase plate.

Anaphase
– Induced by activation of the anaphase-promoting complex/cyclosome(APC/C). The activated APC/C leads to the destruction of the cohesins, so now each separated chromosome can be pulled to its respective pole by the microtubules attached to its kinetochore(this movement is known as anaphase A).
– A separate and distinct movement also occurs: themovement of the spindle poles farther apart in a process known as anaphase B.

Telophase
– The nuclear envelope reforms.
– The chromosomes decondense.
– Cytokinesis closely follows or has begun by the end of telophase.

The following pictures were obtained by in vivo florescence microscopy(From “Biology” by Neil Campbell and Jane Reece, 7th edition):
[pic]

– Before the duplicatedchromosomes can be separated and distributed equally to the two daughter cells during mitosis, however, they must be appropriately configured, and this process begins in S phase.

Condensins and Cohesins
– The sister chromatids formed in the S phase are glued together by multisubunit protein complexes called cohesins, which are deposited along the length of each sister chromatid as the DNA isreplicated(actually cohesin associates with DNA in late G1 and like the daughter chromosomes as they replicate.
– The first readily visible sign that a cell is about to enter M phase is the progressive compaction of the replicated chromosomes, which become visible as threadlike structures—a process called chromosome condensation. Proteins called condensins do the work of chromosome condensation.Activated M-Cdk phosphorylates some of the condensin subunits, triggering the assembly of condensin complexes on DNA and, thereby, the progressive condensation of the chromosomes. The condensins can use the energy of ATP hydrolysis to promote DNA coiling in a test tube, and they are thought to do the same in cells during chromosome condensation.
– The cohesins and condensins are structurally...