K.Muma Bio 6 Appendix C DNA Replication & Mitosis Study Objectives: Appendix C: DNA replication and Mitosis 1. Describe the structure of DNA and where it is found. 2. Explain complimentary base pairing: A-T and C-G 3. Describe the steps involved in DNA replication. Include the enzymes involved and their functions. 4. Compare the replication of the leading strand vs. the lagging strand. 5. Explain why DNA replicates and when during the cell cycle it happens. 6. Describe the structure and function of a eukaryote chromosome. What is a chromosome? How many do you have? What are homologous chromosomes? 7. Explain why cells undergo mitosis. 8. Briefly describe the events that occur in each phase of interphase: G1, S, G2 9. Briefly describe the events that occur in each phase of mitosis: prophase, metaphase, anaphase, telophase. What is the final result of mitosis? Which types of cells undergo mitosis? 10. Define cytokinesis. 11. Compare and contrast the characteristics of normal cells vs. cancer cells 12. Describe factors that control cell division and the role they play in the development of cancer: proto-oncogenes, tumor suppressor genes, apoptosis (cell death), telomeres
Outline DNA Replication I. Review of DNA Structure a. Double stranded i. The chemical side groups of the nitrogen bases form hydrogen bonds, connecting the two strands. 1. Adenine forms two hydrogen bonds with thymine 2. Guanine forms three hydrogen bonds with cytosine ii. Sugar-Phosphate Backbones run antiparallel to each other 1. Each DNA strand has a 3 end with a free hydroxyl group and a 5 end with a free phosphate group (See figure 14-6 in textbook) 2. One strand runs 5 to 3 and the other runs 3 to 5 3. Sketch two DNA strands running antiparallel to each other and label the 5 and 3 ends. II. DNA Replication a. Purpose - before a cell divides it must replicate its DNA so each daughter cell has a full set of DNA b. Mechanism: Semiconservative the two parent strands serve as a template for the synthesis of the new complementary strands (See figure 14.7 and 14.8) c. Steps in replication: i. Replication Origin - begins at special sites called origins of replication 1. There may be hundreds or thousands of origin sites per chromosome. 2. Strands separate forming a replication bubble with replication forks at each end. 3. The replication bubbles elongate as the DNA is replicated and eventually fuses with other replication bubbles 4. Enzymes Involved a. Helicase unwinds and unzips the DNA helix at the replication forks b. Topoisomerases relieves the torque from the unwinding DNA
ii. Elongation of new strand 1. Primase constructs RNA primer complementary to the DNA templates 2. After formation of the primer, DNA polymerase III elongates the new strand by adding nucleotides to the 3 end (~50 per second) 3. Replication always occurs in the 5 to 3 direction a. Leading strand elongates toward the replication fork, continuous b. Lagging strand elongates away from the replication fork i. Results in Okazaki fragments: discontinuous short segments c. Sketch your original template strands and then add your leading and lagging strands. Label your 5 and 3 ends on your template strands so you can determine your leading and lagging strand: 4. DNA polymerase I replaces RNA primers with DNA nucleotides 5. Ligase enzyme that joins fragments into a single DNA strand III. Proofreading and Repair a. Replication is extremely accurate i. 1 error per billion nucleotides b. DNA polymerase proofreads each nucleotide against the template and fixes any mismatches c. Causes of mistakes i. replication errors ii. physical and chemical agents **Note: DNA replication only occurs when the cell is preparing to divide. When the cell is NOT dividing DNA is used as a blueprint for protein synthesis.
Outline Cell Division Mitosis I. Why do cells divide? a. To grow b. To repair damaged cells c. To replace lost cells d. To reproduce II. Chromosomes in Eukaryotes -long threads of DNA wrap around proteins called histones a. Chromatin -when a cell is not dividing the DNA is loose & unwound b. Chromosomes - when the cell divides the chromatin condenses into tightly c. Chromosome number humans have 46 chromosomes (23 pairs) i. n is the number of different types of chromosomes 1. in humans n =23 ii. Humans are diploid (2n) therefore each somatic cell has two sets of chromosomes 1. in humans 2n = 46 iii. Homologous chromosomes - are matching pairs of chromosomes 1. 22 pairs of homologous (matching) chromosomes, called autosomes 2. 1 pair of sex chromosomes, XY or XX d. Before a cell divides, it must duplicates all of its chromosomes so that each new cell gets a complete copy of DNA i. A duplicated chromosome consists of 2 sister chromatids, which are identical molecules of DNA III. Phases of Cell Division a. The cell cycle consists of two phases: i. Interphase cell carrying out daily functions or preparing to divide ii. Mitosis- division of the nucleus b. Phases of Interphase i. G1 phase cell spends 90-95% of its time in this phase growing and carrying out its everyday functions ii. S-phase - DNA is replicated iii. G2 phase - final check point : make sure everything is ready for mitosis
c. Phases of Mitosis i. Prophase 1. Chromatin condenses into chromosomes 2. Centrioles migrate to opposite poles 3. Nuclear membrane breaks down 4. Mitotic spindles form ii. Metaphase 1. Sister chromatids are aligned in the center of the mitotic spindle iii. Anaphase 1. Mitotic spindles shorten pulling chromosomes to opposite poles 2. Sister chromatids separate becoming daughter chromosomes 3. The cell begins to elongate iv. Telophase 1. Chromosomes uncoil into chromatin 2. Spindle breaks down 3. Nuclear membrane reforms d. Cytokinesis division of the cytoplasm i. Original cytoplasmic mass divides resulting in two identical daughter cells IV. Cell division out of control!!!! a. Cell division is usually under strict control in order to ensure appropriate proportions. When cell cycle regulation fails, cells start dividing uncontrollably and result in abnormal masses of dividing cells called tumors. b. Types of Tumors i. Benign does not invade adjacent tissues, encapsulated ii. Malignant invades adjacent tissues 1. Metastasize cells break away from primary tumor and travel to other areas of the body c. Normal cells vs. Cancerous cells i. Characteristics of Normal Cells 1. Stop dividing after a certain # of divisions 2. Have contact inhibition 3. Differentiated (have a job) 4. Undergo apoptosis if DNA is damaged
ii. Characteristics of Cancer cells 1. Immortal, divide indefinitely 2. No contact inhibition, pile up on each other 3. Non differentiated (no job) 4. Large abnormal nuclei or chromosome numbers d. Factors that control cell division i. External signals from neighboring cells 1. Proto-oncogenes produce proteins that turn on cell division a. Mutations in proto-oncogenes (now called oncogenes) may cause these genes to stay turned on and produce excess growth stimulating proteins resulting in uncontrolled cell division 2. Tumor suppressor genes code for proteins that receive stop signals from neighboring cells. If mutated they cannot suppress cell division ii. Apoptosis programmed cell death, back up system for cells with damaged DNA iii. Telomeres regions at the end of chromosomes that become shorter each time the cell divides 1. Keeps cells from becoming immortal 2. Cancer cells have enzyme telomerase which repairs telomeres *Optional reading to better understand how cells become cancerous: the article How Cancer Arises posted on Canvas.
Post-Lecture Practice 1. Draw and label the events occurring in the stages of cell division for a cell that is 2n = 4 and give a brief description for each event. Include the phases of interphase, mitosis (prophase, metaphase, anaphase, telophase), and cytokinesis. Other things you should include somewhere in your drawing is the labeling of chromatin, homologous chromosomes, sister chromatids, and daughter chromosomes.