Cell Cycle Control, Apoptosis, Stem cells and Cancer

I. Control of the Cell Cycle

The central components of the cell-cycle control system are enzymes known as cyclin-dependent kinases or Cdks. Like other protein kinases, Cdks catalyze the covalent attachment of phosphate to protein substrates, resulting in changes in the substrate's enzymatic activity or its interaction with other proteins.

Cdk activities rise and fall as the cell progresses through the cell cycle. These oscillations lead directly to cyclical changes in the phosphorylation of key components of the cell-cycle machinery, resulting in the initiation of cell-cycle events.

Cyclin: any of a family of structurally related proteins that bind to and activate cyclin-dependent kinases (Cdk); originally discovered as proteins whose concentration varies with the phase of the cell cycle.

 G1 cyclins: cyclins whose concentration depends on the rate of cell growth or on growth-promoting signals rather than on the phase of the cell cycle, and that activate Cdks that stimulate entry into a new cell cycle at the end of G1.

G1/S cyclins: cyclins whose concentration rise at the G1/S boundary and that activate Cdks responsible for initiation of DNA synthesis.

M cyclins: cyclins whose concentration rises at the approach to mitosis and that activate Cdks necessary for entry into mitosis.

S cyclins: cyclins whose concentrations rise and remain high during S phase, when they activate Cdks necessary for DNA synthesis, and G2 phase, when they are required for other processes.

The control of how many divisions a cell has undergone and how many remian:

Telomeres:  Lies in the chromosome tips, and function like a cellular fuse that burns down as pieces are lost from the very
                    ends.  At each mitosis, the telomeres gradually shortening the chromosome like a fuse.  After about 50 divisions,
                    acritical amont of telomere DNA is lost, which signals mitosis to stop.

II.  Apoptosis:
 
    A process which dismantels a cell into neat, membrane-bounded pieces and dispose them.
    A continuous process, begins with a signal received by a "death receptor" on the cell membrane,which triggers enzymes
    called caspases to snip apart various cell components and then the cell shatters.  At the end, almost instantly, pieces of
    membrane encapsulate the cell fragments, which prevents inflammation.  Within an hour the cell is gone, eliminated by
    phagocytes ( cells that engulf and destroyed another).

III.  Stem Cells

Stem cells:  Some special cells which are not specialized, less differentiated than others.  They can divide to yield a more
                    specialized daughter cell, as well as another stem cell.  They can be found in nearly every tissue, and can serve as
                    repositories for growth and cell replacement to heal injuries.
 

IV.  Cancer

What is Cancer?

            A diverse class of diseases marked by abnormal cell proliferation, due to the loss of cell
            cycle control.

    A.  Characteristics of Cancer cells.

        1.  Unlimited cell division.

                i.e.  Hela cells...

        2.  Cancer characteristics are passed to progeny.

        Cancer-family syndromes:  Family members seem to inherit a nonspecific predisposition toward
        tumors of various types.

        3.  Cancer cells can metastasize.

            Tumor:  a mass of abnormal cells within an otherwise normal tissue.

            Benign tumor:  The abnormal cells remain at the original site.

            Malignant tumor:  Tumor cells not only continue to proliferate, but also spread beyond their
            original site and invade nearby tissues by a process called metastasis.

            Metastasis:  The spread of cancer cells beyond their original site through blood or lymph
            vessels and start new centers of uncontrolled cell growth wherever they go, bring about unusual
           numbers of chromosomes cease to function in any constructive way, and they also lose their
           attachments to neighboring cells.