Biology 1406 Chapter 12 Lecture Notes(2)
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BIOLOGY 1406
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Chapter 12When cells reach a certain size they must divide in order to function. Processes like diffusion areonly effective in a small space also the cell must maintain a certain surface to volume ratio to beefficient. Remember substances like oxygen and carbon dioxide move into and out of the cell through the plasma membrane at the cell surface. When a cell divides the hereditary material must be distributed to the daughter cells without any loss of the genetic information. The genomeis the cell’s total amount of DNA and its size varies from species to species. Prokaryotic cells reproduce by a simple process called binary fission. (fig. 12.12 page 243) Eukaryotic cells with larger genomes use the more complex processes of mitosis and meiosis.The Chromosome CycleIn eukaryotes the chromosomes are long strands of DNA and protein. In the eukaryotic chromosome a single DNA molecule is wound around complexes of histone proteins forming nucleosomes separated by strands of DNA like beads on a string(pages 330 and 331).The genes are units of heredity composed of DNA are located along the chromosome in a line one after another. The chromosomes undergo the same cycle in both mitosis and meiosis:Step 1Each chromosome duplicates or replicates itself. The DNA molecule in the chromosome replicates; two exact copies of the original DNA molecule are made. The duplicates of theoriginal chromosome are called sister chromatids held together by the centromere. The genes onone chromatid are identical to the ones on its sister because the original DNA molecule has.replicated. Step 2The chromatid fibers become coiled and compacted. Each chromosome is composed of twosister chromatids. The chromosomes are now visible as distinct threads in the nucleus whenusing the microscope. Step 3The centromere splits and the chromatids migrate in opposite directions as chromosomes.Summary:A chromosome duplicates and forms two identical sister chromatids. The chromatids will separate and move into the two new daughter cells as daughter chromosomes. (fig.12.5 page 236)
The Cell CycleEukaryotic cells when dividing have a cell cycle which includes: interphase, mitosis and cytokinesis. (fig. 12.6 page 237)Interphase is the phase of normal cell activities such as growth and metabolism. It is also when the chromosomes are duplicated in preparation for the division process. Interphase occupies 90%of the cell cycle.. Chromosomes are not distinctly visible in interphase.Subphases of InterphaseG1subphase – period of cell growthS subphase – period of growth and replication of DNA and duplication of chromosomes formingchromatidsG2– period of growth before mitosis In animal cells the centrioles are duplicated in interphase.MitosisMitosis is the division of the cell nucleus in which the chromosome number is not changed or remains the same. The phases of mitosis are: prophase, prometaphase, metaphase, anaphase and telophase. (fig. 12.7 pages 238 and 239)ProphaseIn prophase the chromosomes become distinctly visible due to the coiling and compaction of the chromatid fibers of the duplicated chromosomes. The coiling and compaction reduces the chanceof damage during the division process. The nucleolus begins to disappear. The spindle apparatus begins to form in prophase. The spindle is composed of microtubules derived from the cell’s cytoskeleton. The spindle is the structure involved in moving the chromosomes in mitosis. In animal cells the centrioles move to opposite ends of the cell withspindle fibers forming between them. PrometaphaseThe nuclear envelope breaks down. Microtubules of the spindle fibers connect with the chromosomes which begin to move.MetaphaseIn metaphase the duplicated chromosomes are lined up in the equatorial plane in the middle of the cell. The spindle apparatus is fully developed. In animal cells the poles of the spindle containcentrioles and aster fibers. Aster fibers are short microtubules which radiate from the centrioles.In plant cells there are no centrioles and asters in the poles of the spindle. Plant cell mitosis can be called anasteral mitosis.
AnaphaseSister chromatids separate and migrate toward the poles of the spindle as chromosomes.TelophaseChromosomes reach the end of their migration. A nuclear envelope begins to form around each group of chromosomes. Cytokinesis occurs.CytokinesisCytokinesis is the division of the cytoplasm to form two new daughter cells. The daughter cellsare identical to each other in chromosome number and genetic content and identical to theparent cell in chromosome number and genetic content.In plants cytokinesis is produced by the formation of a cell plate which is the beginning of a new cell wall. In animals cytokinesis is produced by the formation of a cleavage furrow in which aconstriction occurs in the cytoplasm pinching the two new daughter cells apart.Eukaryotes have multiple chromosomes. The function of chromosomes is to carry the genescomposed of DNA. Each species has a certain number of chromosomes. It is possiblefor two species to have the same number of chromosomes. Therefore, it is the genetic information carried on the chromosomes that is important.Somatic cells in animals are the cells of the body except those cells which produce gametes or sex cells. Somatic cells increase in number during growth, development and repair by the processes of mitosis and cytokinesis. In muscle cells mitosis occurs with no cytokinesis resultingin cells with multiple nuclei.The Mitotic Spindle(fig. 12.8 page 240)The mitotic spindle is a football shaped structure composed of bundles of microtubulesinvolved in moving the chromosomes in mitosis. These bundles are called spindle fibers.In animal cells centrioles in centrosomes compose the poles of the spindle. Centrosomes in animal cells organize the formation of the spindle. Kinetochore microtubules connect the centrosome with the kinetochore in the centromere region of a chromosome. One hypothesis states that chromosomes move on the kinetochore microtubules by the use of motor proteins. During anaphase motor proteins walk on the microtubules carrying the chromosomes toward the poles of the spindle. Behind the moving chromosome, tubulin molecules depolymerize shortening the spindle fiber.(fig. 12.9 page 241) Plants do not have centrioles but spindle fibers converge to polar regions. Nonkinetochore microtubules from opposite poles overlap in the middle of the cell. Nonkinetochore microtubules do not contact chromosomes.Nonkinetochore microtubules produce elongation of the cell as they slide past each other away from the middle of the cell allowing cytokinesis to occur.
Control of the Cell CycleExternal and internal cues control cell division by mitosis and cytokinesis. Late in G1a checkpoint is reached. If all external and internal cues are favorable the cell will duplicateits DNA and divide. When cells become specialized such as nerve cells they exit the cell cycleand will not divide. This nondividing state is called the G0phase. Most cells of the human bodyare in the G0phase. G0cells may go back into the cell cycle if there is an injury to repair.External CuesGrowth factors bind to receptor sites on the plasma membrane and stimulate cell division.The crowding of cells inhibits cell division. This is density-dependent inhibition. Most animalcells must be attached to a substrate in order to divide. This is called anchorage dependence.Internal CuesA cell will divide when a certain volume to genome ratio is reached. Thus the size of the cell is an internal cue. Cyclins are regulatory proteins that accumulate inside the cell during interphase. A cyclin willactivate a protein kinase enzyme which catalyzes the phosphorylation of a protein important inthe cell cycle. These enzymes are called cyclin dependent kinases or Cdks. MPF is a cyclin-Cdk complex that causes the cell to move from interphase into mitosis. When enough cyclin has been synthesized during interphase then the right amount of MPF(mitosis promoting factor) can be produced to cause the mitotic division of the nucleus. This is the second checkpoint(G2) in the control of the cell cycle. After mitosis is completed the cyclin breaks down and must be resynthesized causing the cell to remain in interphase until enough has been produced. (fig. 12.16 page 246)A third checkpoint is reached when a certain set of conditions is met for the cell to go frommetaphase into anaphase in mitosis.CancerCancer cells do not respond to the normal controls of cell division and divide excessively producing tumors. For example, cancer cells are not density-dependent or anchorage dependent.(fig. 12.19 page 248).Read the section on cancer. (pages 248 to 250). Be able to define: tumor, benign tumor,malignant tumor and metastasis.Cell division is very ATP expensive. The histone proteins in the chromatid fiber arephosphorylated by ATP which causes these proteins to change shape producing thecoiling and compaction of the chromatid fibers during prophase. Motor proteins arebelieved to be involved in moving the chromosomes and elongating the cell. As a result metabolism is higher in a tumor than in surrounding tissue and the skin above the tumor will bewarmer than other areas. As a result tumors can be detected by thermal imaging.