Chapter 9 Cellular Reproduction Mitosis And The Cell Cycle

Cellular Growth Cells grow until they reach their size limit, then they either stop growing or divide. Cell size is limited. Most cells are less than 100 micrometers in diameter (smaller than the period at the end of the sentence in your book)

Cell Size Limitation Ratio of surface area to volume Area covered by the plasma membrane Volume – space taken by the inner contents of the cell (organelles, cytoplasm, nucleus) To calculate ratio: multiply length x width x height of cell The larger the cell, the less efficient it is Small cells have a higher ratio of surface area to volume and can sustain themselves more easily.

Why does a high ratio of surface area to volume benefit a cell? Easier to transport substances As cell grows, its volume increases much more rapidly than the surface area making it difficult for the cell to supply nutrients and to expel waste products. The smaller the cell, the more efficient it is Cells remain small to maximize the ability of diffusion and motor proteins to transport nutrients and waste products.

Cellular Communication The need for signaling proteins to move throughout the cell limits cell size If cell becomes too large, it becomes almost impossible for cellular communications i.e., the signals that trigger protein synthesis might not reach the ribosome fast enough for protein synthesis to occur to sustain the cell

The Cell Cycle Once a cell reaches its size limit – either it will stop growing or it will divide Most cells divide Prevents cell from getting too large Allows for cell to reproduce Allows for growth Allows injuries to heal Cell grows and divides – The Cell Cycle – One complete cycle produces two cells from each original cell – provides continuous production of new cells

The Cell Cycle Three main stages Interphase – cell growth, cell functions, copies DNA 3 stages Gap 1 – cell growth and normal functions Synthesis – DNA is replicated Gap 2 – cell prepares for Mitosis Mitosis – cell’s nucleus and nuclear material divide 4 stages Prophase Metaphase Anaphase Telophase Cytokinesis – cytoplasm divides, creating new cells

Synthesis stage of Interphase Involves: Chromosomes – structures that contain genetic material of cell – passed on from generation to generation Chromatin: relaxed form of DNA in cell’s nucleus

Prokaryotic Cell Division Reproduce by a method called binary fission. Eukaryotes use the method known as the Cell Cycle.

The Cell Cycle

Mitosis

Phases of Mitosis (PMAT) Interphase-prior to mitosis Mitosis: Prophase Metaphase Anaphase Telophase

Interphase Cell grows Carries out normal cell processes DNA replicates

Prophase Nuclear membrane disintegrates Nucleolus disappears Chromosomes condense Spindle apparatus begins to form between poles

Metaphase Chromosomes attach to spindle fibers and align along equator of cell

Anaphase Microtubules shorten, moving chromosomes to opposite poles

Telophase Chromosomes reach poles of cell Nuclear envelope re-forms Nucleolus reappears Chromosomes decondense

Cytokinesis In plant cells: Cell plate forms Dividing daughter cells In animal cells: *Cleavage furrow forms at equator of cell and pinches inward until cell divides in two

Cell Cycle Regulation The normal cell cycle is regulated by cyclin proteins. The timing and rate of cell division are important to the health of an organism. Rate varies depending on type of cell. Role of cyclins: cyclins bind to enzymes called cyclin-dependent kinases (CDKs) in the stages of the cell cyle

Cell Cycle Regulation Different cyclin/CDK combinations control different activities at different stages in the cell cycle. Figure 9.11 on page 253 (know when cyclins/CDK become active) In G1 stage of interphase , the combination of cyclin and CDK signals the start of the cell cycle. Different cyclin/CDK combinations signal other activities (DNA replication, protein synthesis, and nuclear division) The same cyclin/CDK combination also signals the end of the cell cycle.

Cell Cycle Regulation Quality control checkpoints A checkpoint near the end of the G1 stage monitors for DNA damage and can stop the cycle before entering the S stage of interphase Spindle checkpoints also in place – if a failure of the spindle fibers is detected, the cycle can be stopped before cytokinesis. Note checkpoints on figure 9.11

Abnormal Cell Cycle: Cancer Although there are checkpoints in cell cycle, the process sometimes fails. Cancer is the uncontrolled cell growth and division of cells – a failure in the regulation of the cell cycle. Cancer cells can kill an organism by crowding out normal cells, resulting in the loss of tissue function. Cancer cells spend less time in interphase than normal cells, can grow and divide unrestrained as long as they are supplied with essential nutrients.

Cancer Causes – mutations or changes in the segments of DNA that control the production of proteins Repair system of these changes fail Environmental factors can affect the occurrence as well – carcinogens: substances and agents known to cause cancer

Cancer Avoiding carcinogens can reduce the risk of cancer. FDA works to make sure that what you eat and drink are safe Known carcinogens – asbestos, tobacco, radiation Ways to protect yourself –sunscreen, don’t smoke or use tobacco, etc.

Cancer Cancer genetics – chance of cancer increases with age due to changes in DNA Inheriting risk increases chances of getting cancer

Apoptosis Not every cell is destined to survive Some cells go through a process called apoptosis – programmed cell death Shrink and shrivel in a controlled process All animal cells appear to have apoptosis Example – human hands and feet – prevents webbing Example – plants – loss of leaves Also occurs in cells that are damaged beyond repair- protecting organisms from developing cancerous growths

Stem Cells Most cells are designed for specialized function Stem cells are unspecialized cells that can develop into specialized cells when under the right conditions

Stem Cells Two basic types Embryonic stem cells Adult stem cells

Embryonic Stem Cells Once a sperm fertilizes the egg, the resulting mass of cells divides repeatedly until there are about 100 – 150 cells. Not specialized yet – embryonic stem cells If separated, each of these has the capability of developing into a wide variety of specialized cells Controversial because of ethical concerns

Adult Stem Cells Found in various tissues in the body Might be used to maintain and repair the same kind of tissue in which they are found A newborn has adult stem cells Might be able to develop into different kinds of cells, providing new treatments for many diseases and conditions Less controversial because the adult stem cells can be obtained with the consent of their donors

Treatment Using Adult Stem Cells In 1999, researchers at Harvard Medical School used nervous system stem cells to restore lost brain tissue in mice In 2000, a team of researchers at the University of Florida used pancreatic stem cells to restore pancreas function in a mouse with diabetes