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BIOL 1020 12

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Auburn University
BIOL 1020
Anne- Marie Singh

BIOL1020 – CHAPTER 12 LECTURE NOTES Chapter 12: The Cell Cycle I. Cell division in prokaryotes a. typically, a prokaryotic cell divides by binary fission, splitting into two nearly equal halves b. the main circular DNAmolecule of the cell is replicated i. replication begins at a replication origin and proceeds in both directions ii. two complete, identical circles are present by the end; each is attached to the plasma membrane c. new plasma membrane and cell wall materials are laid down between the two DNAcircles, eventually separating the daughter cells d. prokaryotic cells can have a generation time (general term for the period from the start of one cell division to the start of the next cell division) as short as 20 minutes II. Eukaryotic DNAmolecules are organized in chromosomes a. each chromosome is made of chromatin, a long DNAmolecule with associated proteins i. chromatin is packaged into dense chromosomes during cell division 1. protects the DNA 2. helps assure proper distribution of DNAduring cell division 3. the dense bodies can be stained and show up well under light microscopy ii. the chromosomes are unpacked (“decondensed”) when cells are not dividing b. each chromosome contains hundreds to thousands of genes i. genes are the functional units of heredity ii. typically, a gene contains the instructions to make a protein or RNAmolecule iii.the complete DNAsequence for an organism is the genome; it contains the complete set of instructions for that organism iv. humans apparently have ~40,000 genes in the now- sequenced human genome c. each species has a characteristic number of chromosomes i. the number varies between species ii.chromosome number does not reflect the complexity of the organism iii.the assortment of chromosomes for an individual is the karyotype iv. humans have 46 chromosomes d. chromosomes carry the genetic information of a cell from one cell generation to the next, and from one organism to its offspring III. The eukaryotic cell cycle a. the cell cycle describes the status of cells in relationship to growth and division i. when cells reach a certain size, growth either stops or the cell must divide ii.most, but not all, eukaryotic cells are capable of dividing iii.cell division is generally a highly regulated process iv. the generation time for eukaryotic cells varies widely, but is usually 8-20 hours b. cell cycle has two main phases – interphase and cell division (mitosis + cytokinesis) c. interphase is divided into three parts, defined with respect to DNAreplication i. the DNAis completely replicated (genetic information duplicated) during the synthesis phase or S phase ii.the period before the S phase is a “gap” phase, G1 phase 1. most cellular growth occurs in this phase 2. this phase is usually the most variable with respect to time, and is typically longest 3. cells that do not divide become arrested in this phase, then called G0 iii.the period between the S phase and cell division is the G2 phase d. cell division has two main parts – mitosis and cytokinesis1. mitosis is the process that distributes a complete copy of the duplicated genetic information to each daughter cell i. cytokinesis is the process of dividing the cytoplasm into two separate cells ii.some cells can have mitosis without cytokinesis (most common in fungi and slime molds) the current model of cell cycle regulation involves a highly conserved, genetically-controlled program that can be influenced by external signals 1. there are three major checkpoints, found in G1, G2 and mitosis 2. key regulatory components for checkpoints are cyclins and cyclin- dependent protein kinases 3. hormones such as cytokinins in plants and various protein growth factors in animals can stimulate progression through checkpoints in the right cells under the right conditions 4. other factors can serve as suppressors of cell division 5. cancer cells generally grow without needing stimulation by external growth factors and fail to respond to normal suppressors of cell division iii. IV. mitosis is generally be divided into 4 stages: prophase, metaphase, anaphase, and telophase (PMAT) a. be aware that mitosis is a continuous process, the stages are defined only for our convenience b. prophase – chromatin condenses to form
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