In eukaryotes cell division occurs in two main stages1. Mitosis2. Cytokinesis
Chromosomes• In eukaryotic cells, the genetic information that is passed on from one generation to the next is carried on Chromosomes ___________________________
• Chromosomes are made of DNA – which carries the cells coded information – and proteins• The cells of every organism have a specific number of chromosomes• You can’t see chromosomes except during cell division, when they condense and compact
• Before cell division, each chromosome is replicated, or copied• Because of this, each chromosome consists of two identical _______________________________ “sister chromatids”• When the cell divides, the sister chromatids separate and one chromatid goes to each of the two new cells• Each pair of chromatids are attached at an area called the centromere
Interphase• “In-between” period of growth between division
• During the cell cycle, a cell grows, prepares for division, and divides to form two daughter cells, each of which then begins the cycle again• The cell cycle consists of 4 phases1. M phase2. G1 phase3. S phase4. G2 phase
• G1 – Cells do most of their growing. They increase in size and make new proteins and organelles• S – Chromosomes are replicated• G2 – Organelles and molecules required for cell division are produced• M – cell divides
Prophase • The chromatin condenses into chromosomes • The centrioles separate • A spindle begins to form • The nuclear membrane breaks down
Metaphase • Chromosomes line up down the middle of the cell • Each chromosome is connected to a spindle fiber at its centromere
Anaphase • Centromeres split, allowing sister chromatids to separate • The chromosomes move to opposite ends of the cell
Knowing When to Stop•Suppose you had a paper cut on yourfinger. Although the cut may have bled andstung a little, after a few days, it will havedisappeared, and your finger would be asgood as new.1. How do you think the body repairs an injury, such as a cut on a finger?2. How long do you think this repair process continues?3. What do you think causes the cells to stop the repair process?
Controls on Cell Division• When cells come into contact with other cells, they respond by not growing• When an injury such as a cut occurs, cells at the edges of the injury are stimulated to divide rapidly, the cell division stops when the healing process is complete
The sample is injected into a second cell in G2 of interphase.A sample of cytoplasm isremoved from a cell in As a result, themitosis. second cell enters mitosis.
Cyclins• Protein that regulates the cell cycle• There are 2 types of regulatory proteins; those that occur inside the cell and those that occur outside the cell.
Internal Regulators• Allow the cell cycle to proceed only when certain processes have happened inside the cell• Ex.) several regulatory proteins make sure that a cell does not go through mitosis until all chromosomes have been replicated
External Regulators• Direct cells to speed up or slow down the cell cycle• Ex.) growth factors• Molecules found on the surface of neighboring cells often have the opposite effect causing cells to slow down or stop their cell cycles• These signals prevent excessive cell growth
Cancer• A disorder in which some of the body’s own cells lose the ability to control growth• When cells divide uncontrollably, masses of cells called tumors can damage the surrounding tissues• Cancer cells may break loose from the tumors and spread throughout the body*(DNA interactive DVD)*
Q: What causes the loss of growth control?A: smoking tobacco, radiation and chemical exposure, viral infectionsAll cancers have one thing in common: The control over the cell cycle has been broken• Some cancer cells will no longer respond to external growth regulators• Others fail to produce the internal regulators
p53• An astonishing number of cancer cells have a defect in a gene called p53• p53 normally halts the cell cycle until all the chromosomes have been properly replicated• if we could fix p53 we could cure many forms of cancer
Is a cure in sight?• Cancer is a serious disease• As difficult as a cure would be, at least scientists know where to start• Cancer is a disease of the cell cycle , and conquering cancer will require a much deeper understanding of the processes that control cell division