Regulating the Cell Cycle Page 250 & 251

of the cells in your body will be replaced with new cells, all while you read this sentence.

Review of cell cycle In multicellular organisms, cells divide for growth, development, and repair of the organism.

The Cell Cycle

There are three main parts to the cell cycle: interphase mitosis cytokinesis

There are three phases to interphase: G 1 - Growth and normal metabolic processes S - Synthesis (DNA replication) G 2 - Growth and preparation for mitosis

But not all somatic cells undergo mitosis and those that do, do so at different rates (see page 249 in your text).

Cell growth and division are tightly controlled in eukaryotic multicellular organisms.

Cells in a petri dish Cells placed in a nutrient-rich broth in a petri dish will grow and divide until they meet each other or the sides of the dish. (page 250) If some cells are removed, those cells remaining in the petri dish will grow and divide until the dish is full again. Think of your body as a petri dish.

Cell cycle regulators discovered Early 1980s Group of biologists Discovered that cells in mitosis contain a protein that, when injected into a non-dividing cell, caused mitotic spindles to form (remember spindles attach to the chromosomes) Discovered that the amount of this protein in the cell depended on the phase of the cell cycle the cell was in

Cyclins They called this protein cyclin because it seemed to regulate the cell cycle. We now know that there are many closely related proteins involved in regulating the cell cycle in eukaryotic cells. As a group, they are called cyclins.

There are 2 types of cyclins: - internal regulators - external regulators

Internal regulators are: Proteins that respond to events inside the cell. They allow the cell to proceed to the next phase of cell division only when the phase before it has been successfully completed. Ex. All chromosomes must be replicated before cell enters prophase; all chromosomes are attached to spindles before entering anaphase.

External regulators are: Proteins that respond to events outside the cell. They direct the cell to speed up or slow down the cell cycle. Molecules on the surface of the outside of cells signal cells around them to slow down or to stop cell growth (growth regulators). Ex. Healing of wounds; embryonic development

Uncontrolled cell growth Page 252 The consequences of uncontrolled cell growth in a multicellular organism are severe. Cancer cells are cells that are not producing the proper cyclins or that do not respond to signals from the cyclins.

Cancer cells divide uncontrollably and form masses of cells called malignant tumours that damage surrounding tissues. Some of the cells may break off a malignant tumour, enter the blood stream or lymphatic system and travel to other areas of the body where they begin to grow.

Tumours in which cells do not travel to other parts of the body or neighbouring cells are called benign tumours, and are not cancerous. Tumours whose cells do travel to other parts of the body or neighbouring cells are called malignant. The process by which cells from a malignant tumour travel to other parts of the body is called metastasis.

Metastatic cancer has the same name and is the same type of cancer as the original, or primary cancer. For example, breast cancer that spreads to the lung and forms a metastatic tumor is metastatic breast cancer, not lung cancer.

Gene p53 Some studies have shown a link between some cancers and a defect in a protein called p53. In humans, p53 is encoded by the TP53 gene located on the short arm of chromosome 17

Chromosome 17

In its anti-cancer role, p53 works through several mechanisms: It can activate DNA repair proteins when DNA has sustained damage. It can arrest growth by holding the cell cycle at the G 1 /S regulation point on DNA damage recognition (if it holds the cell here for long enough, the DNA repair proteins will have time to fix the damage and the cell will be allowed to continue the cell cycle). It can initiate apoptosis, the programmed cell death, if DNA damage proves to be irreparable.

How Cancer Develops kkCE kkCE

Cancer and the Immune System Killer T-cells hOs

You should know… What chemicals regulate the cell cycle? How do they work? What happens when cells do not respond to the signals that normally regulate their growth? How do cells respond to contact with other cells? Why can cancer be considered a disease of the cell cycle? How do cancer cells differ from noncancerous cells? How are they similar? What is a malignant tumour? A benign tumour; metastasis.