THE CELL CYCLE HOW DO CELLS DIVIDE?

Introduction – Answer the following questions: 1.Why do cells divide? (Try to come up with multiple explanations) 2.What is the difference between asexual and sexual reproduction? 3.What conditions and molecules are necessary for cell division? 4.If you would be a scientist who studies cell division, what kinds of organisms and tools would you need?

I. WHY DO CELLS DIVIDE? Most living things grow by increasing the number of cells not by increasing the size of cells. Larger cells have two demands on them:  “Information crisis” – DNA is not able to fulfill the demands for information in larger cells (not able to give enough information for protein synthesis)  Exchange of materials become inefficient – lack of nutrients and oxygen, too much waste and CO 2

Cell division is also necessary for healing the organisms’ injuries. It provides a way to pass on genetic information to the next generation and with that to upkeep the species. Cell division is a vital component of the reproduction of organisms as well as the growth of organisms. Sexual reproduction is vital for adapting to new environments and avoiding parasites. (watch only parts 1 and 4)

II. CHROMOSOME STRUCTURE Cells need to supply their entire DNA to the two new cells – DNA replicates than forms chromosomes. Chromosome is a very long DNA molecule and associated proteins, that carry portions of the hereditary information of an organism. Animation on chromosome packaging: basic.htmlhttp:// basic.html

DNA needs to be tightly packaged before cell division, so it can be evenly divided between the two new cells. First DNA is wrapped around some small round proteins called histones, that wrapped again and again by other non- histone proteins like a wrapping paper wraps a present, until we get the X-shaped chromosome of eukaryotic cells. Prokaryotic cells don’t have histones only naked, circular shaped chromosomes.

III. Cell Division in Prokaryotes Simple asexual reproduction takes place after DNA replication. Binary fission only splits the cell in half.

The cell cycle is a series of events that cells go through as they grow and divide. During the cell cycle the cell grows, prepares for division and divides to form two (or four) daughter cells each of which begins a new cell cycle. IV. THE CELL CYCLE

V. MITOSIS AND CYTOKINESIS The last stage of the cell cycle when the nucleus of a cell divides to produce two new daughter cells (with cytokinesis) each with the same amount and type of chromosomes as the parent cells. Mitosis is divided into four phases:  Prophase  Metaphase  Anaphase  Telophase

Prophase – The chromatin condenses into chromosomes. The centrioles separate, a spindle begins to form. The nuclear envelope breaks down. Metaphase – The chromosomes line up across the center of the cell. Each chromosome is connected to a spindle fiber at its centromere. Anaphase – The sister chromatids separate into individual chromosomes and are moved apart.

Telophase – The chromosomes gather at opposite ends of the cell and lose their distinct shapes. Two new nuclear envelopes will form.

The cytoplasm pinches in half. Each daughter cell has an identical set of chromosomes -- cytokinesis.

In most animal cells cytokinesis takes place when the cell membrane pinches in until the cytoplasm is pinched into two equal halves. In most plant cells a cell plate forms midway between the divided nuclei. This cell plate gradually becomes a new cell membrane than the cell wall develops.

VI. CONTROL OF CELL DIVISION Cell division is a complex process that needs to be regulated. These regulators determine when and how the cell should divide. External regulators:  proteins produced by other cells, speed up or slow down the cell cycle.  If the cell touches other cells, the cell cycle slows down.  If enough space and nutrients are available growth factors make cells divide or speed up their cell cycle.

Internal regulators:  Cyclins – proteins that regulate the timing of the cell cycle in eukaryotic cells when their concentration increase and they bind with other proteins called kinases, the cell moves to an other stage of the cell cycle.  Other regulator proteins – they make sure the cell is ready to move to the next stage (3 major checkpoints in the cell cycle).  The age of the cell.    To review everything:

VI. CANCER Cancer cells lack normal checkpoints and continue to grow without inhibition do not respond to normal signals within the cell are not inhibited by other cells – form tumors will divide indefinitely. (Cancer cells isolated from a woman in the 1950s continue to grow today.)

Tumor Progression 1. Tumor growth 2. Blood vessels feed tumor 3. Tumor cells enter blood and lymph vessels 4. Secondary tumors form in other parts of the body Movie clips on cancer, its nature and experiments to treat it (Parts 2 and 6)

Treatment of Cancer Surgical removal of tumor – Most effective when tumor is in a defined area Chemotherapy – Medicines that disrupt the process of mitosis in rapidly growing cells Radiation Therapy - High energy gamma radiation is aimed at the growing tumour. This damages the DNA in rapidly dividing cells and helps to destroy the tumor.

VII. CHROMOSOME NUMBER Every species has a determined number of chromosomes. If a cell has two sets of these chromosomes the cell is diploid. If a cell has only one set of its chromosomes, this cell is haploid. Chromosomes that carry the same kinds of genes are called homologous chromosomes.

VIII. MEIOSIS Meiosis is a division of the nucleus in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. Meiosis results in four haploid daughter cells that are genetically somewhat different from the parent cells.

Prophase I – Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. The tetrads overlap and exchange some of their genetic material – crossing-over. Metaphase I – Spindle fibers attach to the chromosomes. Anaphase I – The fibers pull the homologous chromosomes toward opposite ends of the cell.

Telophase I and cytokinesis – Nuclear membranes form the cell separates into two cells. Prophase II – Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell. Metaphase II – The chromosomes line up in a similar way to the metaphase stage of mitosis.

Anaphase II – The sister chromatids separate and move toward opposite ends of the cell. Telophase II and cytokinesis – Meiosis II results in four haploid (N) daughter cells.

Crossing over in Prophase I results in great diversity because new genetic variations can result from it.

IX. WHY DO CELLS NEED TWO TYPES OF CELL DIVISIONS? Mitosis and meiosis serve different purpose in living organisms. Mitosis is used for asexual reproduction in single celled organisms or to growth in multicellular organisms. Human body cells reproduce by mitosis and damaged tissues are also repaired with the help of mitosis. Meiosis is used for sexual reproduction in organisms. Human’s egg and sperm cells for by meiosis.

X. GAMETE FORMATION In females:

In males:

X. KARYOTYPES The chromosomes from a cell that is in prophase or metaphase of mitosis, can be separated and lined up. This method is used to detect various chromosomal disorders in a dividing cell and frequently used to diagnose various chromosomal abnormalities in fetuses during pregnancy.

XI. MUTATIONS A mutation is a sudden change in the number or structure of chromosomes or in a small section of the chromosome. We are going to further organize chromosomal mutations on a concept map.