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Lecture 14

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Biological Sciences
Mark Fitzpatrick

th BIOA01H3 – Lecture 14 Week of October 1 : DNA, the stuff of heredity Chapter 8 8.1 The Cycle of Cell Growth and Division: An Overview  New progeny cells are needed for expanding population size (single-celled organisms), multicellular tissue growth (new leaves), asexual reproduction, and replacement of cells lose to wear (shedding skin and gut lining, and tear (wound repair, virus infection) 8.2 The Cell Cycle in Prokaryotic Organisms  Newly formed prokaryotic cell, must double in size, replicate its circular chromosome, and then move each of the resulting two daughter chromosomes into its own progeny cell during cell division  binary fission, can be thought of in three periods  Following birth, cells may grow for some time before initiating DNA synthesis (stage 1)  Once chromosomes are replicated and separate to opposite ends of cell (stage 2), membrane pinches together between them and two daughter cells are formed (stage 3) 8.2a Replication Occupies Most of the Cell Cycle in Rapidly Dividing Prokaryotic Cells  Chromosomes compacted in a central region called the nucleoid throughout cell cycle in bacteria and Achaea  When nutrients are abundant, prokaryotic cells have no need for stage 1 since they can grow quickly enough to divide their cytoplasm as soon as DNA replication is complete and chromosomes are separated 8.2b Replicated Chromosomes Are Distributed Actively to the Halves of the Prokaryotic Cell  In 1860s, Francois Jacob of The Pasteur Institute in Paris, France, proposed a model for the segregation of bacterial chromosomes to daughter cells, in which two chromosomes attach to plasma membrane near middle of the cell and separate as new plasma membrane added between two sites during cell elongation  Essence of model is that chromosome separation is passive  Current research shows bacterial chromosomes rapidly separate in active way that is linked to DNA replication event and is independent of cell elongation  Replication of bacterial chromosome commences at ori, which is in the middle of the cell where enzymes for DNA replication are located  Once ori has been duplicated, two new origins migrate toward two opposite ends (poles) of cell as replication continues for the rest of the chromosome  Mechanism that propels ends of the cell still unknown  Cytoplasmic division associate with an inward constriction of a cytokinetic ring of cytoskeletal proteins  New plasma membrane and cell wall material is assembled to divide cell into two equal parts 8.2c Mitosis Has Evolved from an Early Form of Binary Fission  Prokaryotic mechanisms works effectively b/c most prokaryotic organisms have only a single chromosome & if daughter cell receives at least one copy, its genetic info is complete  Bacterial and Achaea do not have nuclear membrane around their nucleoid  Therefore, eukaryotic cellular and chromosomal architecture demands diff. mechanism for distributing chromosomes to daughter cells  which is mitosis  One of the central innovation of mitosis is the ability to hold two newly created molecules of chromatids together following DNA synthesis  Enables cells to keep track of long replicated chromosomes and to orient them relative to cytoskeleton at proper time to ensure precise distribution to daughter cells  Variations in mitotic apparatus in modern-day organisms illustrate possible intermediates in evolutionary pathway to mitosis from some ancestral type of binary fission ie. In yeasts and diatoms, a spindle of microtubules made of polymerized tubulin protein forms & chromosomes segregate to daughter nuclei w/o disassembly and reassembly of nuclear envelope 1 8.3 Mitosis and Eukaryotic Cell Cycle  Mitosis serves very well to divide replicated DNA equally amongst daughter cells precisely, result of three interrelated systems  One component is elaborate master program of molecular checks & balances that ensures orderly & timely progression through cell cycle  Within overall regulation of cell cycle, process of DNA synthesis replicates each DNA chromosome into two copies w/ almost perfect fidelity  Final system is a structural and mechanical web of interwoven “cables” and “motors” of cytoskeleton that separates DNA copies precisely into daughter cells 8.3a Chromosomes Are the Genetic Units Divided by Mitosis  Most eukaryotes have two copies of each type of chromosome in nuclei, chromosome complement said to be diploid or 2n  Other eukaryotes, mostly microorganisms, have only one copy of each type of chromosome  haploid or n  Many plant species have three, four, etc. more copies of each chromosome and are called ploidy  Replication of DNA of each individual chromosome creates two new, identical, molecules called sister chromatids, which are held together by a centromere until being separated during mitosis  Equal distribution of daughter chromosomes into two new cells called chromosome segregation 8.3b Interphase Extends from the End of One Mitosis to the Beginning of the Next Mitosis Interphase - Cell grows & replicates its DNA prepping for mitosis (M phase) & cytokinesis - Begins as a daughter cell from prev. division cycle enters in initial period of cytoplasmic growth G1 Phase - During this growth, called G1 phase, cell makes various RNAs, proteins, and other types of cellular molecules but not nuclear DNA (G in G1 phase stands for gap, referring to absence of DNA synthesis) - If cell is going to divide, DNA replication begins, initiating S phase of cell cycle (S refers to synthesis) S Phase - Cell duplicates chromosomal proteins as well as DNA and continues synthesis of other cellular molecules - Moves into G2 phase G2 Phase - G in this phase refers to a second gap, during which there is no synthesis of DNA - Cell continues to synthesize RNAs and proteins, incl. those required for mitosis, and cell continues to grow - At end of G2 phase, which marks end of interphase, mitosis begins  During interphase, chromosomes are relatively loose but organized in nucleus  Usually G1 is phase that varies length between organisms  Whether cells divide rapidly or slowly depends primarily on length of G1  Once DNA replication begins, most mammalian cells take about 10-12hrs for S phase, 4-6hrs for G2 phase, and 1-4hrs for complete mitosis  G1 phase also stage where many cells stop dividing  State of division arrest often designated as G0 phase ie. In human, cells of nervous system enter G0 phase, once fully mature 8.3c After Interphase, Mitosis Proceeds in Five Stages  Mitosis occurs in five stages; prophase (pro = before), pro-metaphase (meta = between), metaphase, anaphase (ana = back), and telophase (telo = end) Prophase  Greatly extended chromosomes that were replicated during interphase begin to condense into compact, rod- like structures  Each diploid human cell, only about 40-50m in diameter, contains 2m of DNA distributed amongst 23 pairs of chromosomes 2  Condensation during prophase packs long DNA molecules into units small enough to be divided successfully during mitosis  As they condense, chromosomes appear as thin threads  Word mitosis (mitos = thread) derived from this threadlike appearance  While condensation progressing, nucleolus becomes smaller & eventually disappears in most species  Disappearance reflects shutdown of all types of RNA synthesis, incl. ribosomal RNA  In cytoplasm, mitotic spindle begins to form between two centrosomes as they start migrating toward opposite ends of cell to form spindle poles  Spindle develops as bundles of microtubules that radiate from spindle poles Prometaphase  At end of prophase, nuclear envelope breaks down, which is beginning of prometaphase  Bundles of spindle microtubules grow from centrosomes at opposing spindle poles toward center of cell  Some of developing spindle enters former nuclear are and attaches to chromosomes  A complex of several proteins, a kinetochore, has formed on each chromatid at the centromere  Kinetochore microtubules bind to kinetochore  These connections determine outcome of mitosis b/c they attach sister chromatid of each chromosome to microtubules leading to opposite spindle poles  Microtubules that do not attach to kinetochores overlap those from opposite spindle pole Metaphase  Spindle reaches its final form and spindle microtubules move chromosome into alignment at spindle midpoint also called metaphase plate  Chromosomes complete condensation at this stage and assume characteristic shape determined by location of centromere and length & thickness of chromatid arms  Complete collection of metaphase chromosomes, arranged according to size and shape, forms karyotype of a given species Anaphase  Sister chromatids separate and move to opposite spindle poles  First signs of chromosome movement can be seen at centromeres as kinetochores are first sections to move toward opposite poles  Movement continues until separated chromatids, now called daughter chromosomes, reached two poles  Chromosome segregation complete Telophase  Spindle disassembles and chromosomes at each spindle pole decondense & return to extended state typical of interphase  As decondensation proceeds, nucleolus reappears, RNA transcription resumes, and new nuclear envelope forms around chromosomes at each pole, producing two daughter nuclei 8.3d Cytokinesis Completes Cell Division by Dividing the Cytoplasm between Daughter Cells Cytokinesis is the division of the cytoplasm, usually follows the nuclear division stage of mitosis and produces two daughter cells, each containing one of the daughter nuc
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