LECTURE 22

 

Regulation of the Cell Cycle:

 

Cyclin-dependent protein kinases (Cdks) control these events (Figure 9.5).  Cdks are heterodimeric proteins consisting of a regulatory subunit called a cyclin and a catalytic subunit, the protein kinase, that is inactive unless complexed with a cyclin.   (Cyclin as a name has dual meaning: cyclins control the cell cycle and the concentration of cyclins cycles, as cyclins are synthesized and degraded.)

 

Protein Phosphorylation and Protein Degradation Are the 2 Mechanisms Underlying Cell Cycle Control

1.     Cdks act to phosphorylate target proteins and thereby activate (or inactivate) processes appropriate to a particular phase of the cell cycle.  The Cdk-catalytic subunit is also regulated by reversible phosphorylation.

2.     Cyclin subunits are destroyed by selective protein degradation at the right moments in the cell cycle: an irreversible mechanism – there’s no going back!

 

The Steps in the Cell Cycle – M Phase:

Mitosis is the eukaryotic solution to the problem of distributing a large amount of genetic material.

Very reliable: a error in chromosome distribution occurs only once in about every 100,000 cell divisions (in yeast).

Scientists describe 4 stages in mitosis, but the process occurs as a continuum.

 

The 4 stages of mitosis (Figure 9.9):

Prophase:

The boundary between interphase and prophase is not discrete; no sharp transition occurs.

Late interphase nucleus: well defined, bounded by nuclear envelope; nucleoli obvious; chromosomes have duplicated but remain dispersed and cannot be distinguished as distinct entities; in animal cells, 2 pair of centrioles just outside.

At prophase, changes take place in both the nucleus and the cytoplasm: the nucleoli disappear; chromatin fibers are now evident as sister chromatids joined at the centromere.  In the cytoplasm, the mitotic spindle (made of microtubules) forms.  The cytoskeleton has disassembled into a,b-tubulin dimers, forming a large pool of building blocks  for construction of the mitotic spindle.  The spindle is centered on the 2 centrosomes (in animal cells, containing the centriole pairs).  The centrosomes function as mitotic centers that organize the microtubules.  Spindle microtubules that run between the 2 mitotic centers  (so-called polar microtubules) preferentially elongate and begin to overlap with one another in the center of the spindle.  This elongation moves the 2 centrosomes apart along the surface of the nucleus, forming the bipolar spindle.

 

Late prophase (Prometaphase):

The nuclear envelope fragments, allowing the spindle to interact with the chromosomes, which have become even more condensed.  Polar microtubules from each pole extend toward the equator of the spindle.  At the centromeres of the chromosomes, kinetochores appear.  (Kinetochores, one per chromatid,  are 3-layered structures composed of proteins.)  The kinetochores ‘capture’  microtubules (so-called kinetochore microtubules).  The kinetochore of one chromatid is attached to microtubules from one spindle pole, while the kinetochore of its sister chromatid is attached to microtubules from the other spindle pole.  The kinetochores contain ATP-dependent motor proteins that move the chromosomes along the microtubules towards the poles. 

 

Metaphase:

The chromosomes are arranged on the metaphase plate (equatorial plate),  midway between the 2 spindle poles.   The centromeres are aligned with each other and the chromosomes lie ^ to the spindle axis.  The chromosomes are under tension as the microtubules attached to the kinetochores ‘pull’ against each other; this is an important centering force.  Chromosomes are like the “corpse at a funeral”: they are the reason for the proceedings, but they don’t have an active role in it.

 

Anaphase:                  (Web video – McGill University)

Anaphase begins when the centromeres of sister chromatids suddenly separate.  (A protein that inhibits separation has been targeted for rapid degradation by the APC (the anaphase-promoting complex; also called the cyclosome).) Each chromatid is now a full-fledged chromosome, and each moves toward opposite poles.  The movement is achieved by kinetochore motor proteins and shortening of kinetochore microtubules.  At the same time, the interdigitated polar microtubules are continuing to slide past one another, driving the poles further apart.  By the end of anaphase, each pole has a full complement of chromosomes.

 

Telophase:

Kinetochore microtubules have disappeared and the nuclear envelope reforms around each daughter nucleus from fragments of the original nuclear envelope.  The spindle begins to break down.  The chromatin begins to uncoil and disperse.  Mitosis has ended.

 

Cytokinesis:

In animal cells, cytokinesis occurs by formation of a cleavage furrow that begins as a shallow groove in the cell surface, near the position of the old metaphase plate.  Threads of actin and myosin form a contractile ring (like a purse string) just beneath the plasma membrane that pinches the cell in two.

 

In plant cells, membrane vesicles derived from the Golgi converge at the old metaphase plate and these vesicles fuse to form a dividing region consisting of a double membrane (the new plasma membranes defining 2 cells); the contents of these vesicles are cell wall precursors are used to make a cell plate that separates the 2 daughter cells.

 

Centrioles

 

Animal cells have a pair of centrioles composed of microtubules.  These centrioles are located near the nucleus and they duplicate prior to chromosome replication.  (Centrioles are not absolutely required for cell division; plant cells lack them.)  Following duplication, the pairs of centrioles separate and move to opposite sides of the nucleus.  The centriole pairs serve as organizing centers (centrosomes) around which the mitotic spindle forms.  Each daughter cell gets a pair.

 

APOPTOSIS – PROGRAMMED CELL DEATH

 

Cells die in 2 ways:

1. Necrosis: From damage or nutrient limitation.
2. Apoptosis: (“apo-toe- sis” - the 2^nd “p” is silent), also called programmed cell death (Figure 9.20). 

 

Apoptosis occurs when cells must be killed during embryogenesis (as in Figure 15.12 – development of digits in the hand) or during normal cell turnover.

 

Apoptosis is triggered by death factors, proteins of the cytokine class that bind to plasma membrane receptors and initiate an intracellular cascade that causes, among other things,  the activation of a class of intracellular proteases, the caspases, that begin cell destruction.  (TNF = tumor necrosis factor is an example of a death factor.)