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Chapter 2

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McMaster University
Bhagwati Gupta

CHAPTER 2: Chromosomes and Cellular Reproduction 2.1 Prokaryotic and Eukaryotic Cells Differ in a Number of Genetic Characteristics - In prokaryotic cells, reproduction is simple because prokaryotic cells possess a single chromosome  Eubacteria: true bacteria  Archaea: ancient bacteria  Eubacteria and archaea are similar in cell structure, yet some genetic processes in archaea are more similar to those in eukaryotes, and the archaea are evolutionarily closer to eukaryotes than to eubacteria - In eukaryotic cells, multiple chromosomes must be copied and distributed to each new cell - Eukaryotic cells have nuclear envelopes which surround the genetic material and separates the DNA from the cellular content - In eukaryotes, DNA is closely associated with histones to form chromosomes  Histones limit the accessibility of enzymes and other proteins that copy and read the DNA, but they enable the DNA to fit into the nucleus  Archaea also have histone proteins - Genes of prokaryotic cells are on a single circular molecule of DNA  Important bacterial genes are frequently found on plasmids - Genes of eukaryotic cells are located on multiple linear DNA molecules - Viruses do not possess a cellular structure  Composed of an outer protein coat surrounding nucleic acid (DNA or RNA)  Reproduce only within host cells indicating that they evolved after cells  Viruses are closely related to their host 2.2 Cell Reproduction Requires the Copying of the Genetic Material, Separation of the Copies, and Cell Division - For any cell to reproduce successfully its genetic information must be copied, the copies of genetic information must be separated from each other, and the cell must divide - Prokaryotic cell reproduction:  Circular chromosome replicates and the cell divides by binary fission  Replication of the chromosome begins at the origin of replication  The origins of the two replicated chromosomes move away from each other toward opposite ends of the cell  Proteins bind near the replication origins and anchor the chromosomes to the plasma membrane  Cell wall forms between the two chromosomes producing two cells with identical chromosomes  Some bacterial cells divide every 20 minutes - Eukaryotic cell reproduction:  Chromosomes are separated from the cytoplasm by the nuclear envelope  The nucleus has a organized internal scaffolding, the nuclear matrix, which consist of a network of fibers that maintain spatial relations among the nuclear components and takes part in DNA replication, expression of genes and modification of gene products  Eukaryotic chromosomes:  Humans have 46 chromosomes per cell  Presence of two sets is a consequence of sexual reproduction  The two chromosomes of a homologous pair are usually alike in structure and size, and each carries genetic information for the same set of hereditary characteristics  Diploid: cells that carry two sets of genetic information  Haploid: cells with a single set of chromosomes  Chromosomes structure:  Linear, highly folded, and condensed  Before cell division, the chromosomes condense further into thick, observable structures  Centromere: attachment point of the spindle microtubules  Kinetochore: a multiprotein complex which assembles on the centromere to which the spindle microtubules attach o Metacentric o Submetacentric o Acrocentric o Telocentric  One of the two arms of a chromosome is designated by p, the other by q  Telomeres: the tips of a whole linear chromosome that protect and stabilize the chromosome ends, as well as limit cell division and play roles in aging and cancer  Origins of replication: sites where DNA synthesis begins  Sister chromatids: identical copies of a chromosome held together at the centromere - The cell cycle and mitosis:  Progression through the cell cycle is regulated at checkpoints  Consists of two major phases: interphase and M phase  Interphase:  Period of growth and development between cell divisions  DNA is being synthesized, RNA and proteins are being produced, and biochemical reactions are taking place  Includes checkpoints, which regulate the cell cycle by allowing or prohibiting cell division  In G1the cells grows, proteins for cell division are synthesized  G1/S checkpoint holds the cell in 1 until the cell has all the enzymes necessary for the replication of DNA  G0a nondividing phase, which is a stable state  S phase, in which each chromosome duplicates  In G several biochemical events take place 2  G2/M checkpoint is passed if the DNA is undamaged  Typical dividing mammalian cells spend about 10 hours in G ,1 9 hours in S, and 4 hours in 2  M phase:  Chromosomes separate and the cell undergoes division  Prophase: o Chromosomes become visible o Mitotic spindle forms and grows from the centrosomes that migrate to opposite sides of the cell  Prometaphase: o Disintegration of the nuclear membrane o Spindle microtubules enter the nuclear region o The ends of the microtubules anchor to the kinetochore  Metaphase: o Chromosomes become arranged in a single plane o Spindle-assembly checkpoint ensures that each chromosome is aligned on the metaphase plate and attached to spindle fibers from opposite poles o Passage through the spindle-assembly checkpoint depends on tension generated at the kinetochore as the two conjoined chromatids are pulled in opposite directions by the spindle fibers  Anaphase: o Sister chromatids separate and move toward opposite spindle poles o Chromosome movement is due to the disassembly of tubulin molecules at both the kinetochore end (+) and the spindle end (-) of the spindle fiver o Molecular motors disassemble tubulin molecules from the spindle and generate forces that pull the chromosome toward the spindle pole  Telophase: o Chromatids have separated o Nuclear membrane re-forms around each set of chromosomes producing two separate nuclei - Genetic consequences of the cell cycle:  From a single cell, the cell cycle produces two cells that contain the same genetic instructions  Each of the cells produced contains a full complement of chromosomes: there is not net reduction or increase in chromosome number - Counting chromosomes and DNA molecules  To determine the number of chromosomes, count the number of functional centromeres  To determine the number of DNA molecules, first determine if sister chromatids are present, if present, the DNA molecules are twice the number of chromosomes  The number of chromosomes increases only in anaph
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