Regulating the Cell cycle

Life Span of Cells No cell lives forever. 2. The number of cell divisions is what causes cells to die. Once they reach a certain number of cell divisions, they die. 3. When human cells are grown in the laboratory, they divide about 50 times and then die.

Cell Cycle Regulation Three major checkpoints – G1, G2, M G1 – most important If the cell receives the go-ahead signal at the G1 checkpoint, it will usually complete G1, S, G2 and M phases and divide. If the cell does not receive a go-ahead signal, it will exit the cycle, switching into a nondividing state called G0 phase.

G0 Phase

Adoptosis :Programmed Cell death The number of cells in a multicellular organism is tightly regulated—not simply by controlling the rate of cell division, but also by controlling the rate of cell death. If cells are no longer needed, they commit suicide by activating an intracellular death program.

Adoptosis :Programmed Cell death In the developing vertebrate nervous system, for example, up to half or more of the nerve cells normally die soon after they are formed. In a healthy adult human, billions of cells die in the bone marrow and intestine every hour.

Apoptosis during the metamorphosis of a tadpole into a frog As a tadpole changes into a frog, the cells in the tadpole tail are induced to undergo apoptosis; as a consequence, the tail is lost. All the changes that occur during metamorphosis, including the induction of apoptosis in the tail, are stimulated by an increase in thyroid hormone in the blood.

Sculpting the digits in the developing mouse paw by apoptosis The paw in this mouse embryo has been stained with a dye that specifically labels cells that have undergone apoptosis. The apoptotic cells appear as bright green dots between the developing digits. This interdigital cell death eliminates the tissue between the developing digits, as seen one day later, when few, if any, apoptotic cells can be seen.

Cell Cycle Regulation vocabulary Protein kinases – are enzymes that give the go ahead signals at G1 and G2 checkpoints. Cyclins – a protein that attaches to the kinases to activate them. Internal Regulators – proteins that respond to events inside the cell. External Regulators – proteins that respond to events outside the cell. They direct the cell to speed up or slow down the cell cycle. Growth factors are one of the most important external regulators.

Cyclins Regulate the timing of the cell cycle in a eukaryotic cell. Early 1980’s: Scientists such as Tim Hunt and Mark Kirshner made this discovery. For example: when cytoplasm from a cell in mitosis is injected into another cell, the second cell enters mitosis.

Internal regulators Examples include: Making sure that the a cell does not enter mitosis until all of its chromosomes have been replicated Preventing a cell from entering Anaphase until all of its chromosomes are attached to the mitotic spindle fibers

External regulators Examples include: Stimulating the growth and division of cells Providing signals to prevent excessive cell growth Keeping the tissues of the body from disrupting each other, such as a wound healing with healthy skin nearby

What happens when things go wrong? Click here

Uncontrolled Cell Growth Cancer: a disorder in which some of the body’s own cells lose the ability to control growth Cancer cells – do not respond to signals that regulate the growth of cells.

Contribution of scientists click here

Cancer specifics Transformation – the process that converts a normal cell into a cancer cell. Benign tumor – abnormal cells remain at original site, usually do not cause serious problem and can be removed by surgery. Malignant tumor – invasive – impairs functions of one or more organs. Metastasis – cancer cells proliferate and form new tumors. Cancer cells spread to distant locations from original site.

From mitosis to meiosis…Coming up! Click here for review clip