1 Cytoskeleton Microfilaments Thinnest elements are composed of actin (a protein)  Most prevalent at periphery of cell Two general functions 1. Providing mechanical support Basic strength and shape of cell Anchor integral proteins Support microvilli 2. Helping generate movements Muscle contraction Cell division Cell locomotion

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3 Cytoskeleton Intermediate filaments Thicker than microfilaments but thinner than microtubules Several different proteins Exceptionally strong Found in parts of cells subject to mechanical stress Stabilize position of organelles such as nucleus Help attach cells to one another

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5 Cytoskeleton Microtubules Largest of cytoskeletal components Long, unbranched hollow tubes Composed mainly of tubulin (a protein) Assembly begins in centrosome growing outward into cell Help determine cell shape Involved in the movement of vesicles, chromosomes, cilia, and flagella

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7 Centrosome Located near the nucleus Consists of  Pair of centrioles Cylindrical structures with nine clusters of three microtubules arranged in circular pattern  Pericentriolar material Hundreds of ring-shaped complexes composed of tubulin Organizing centers for growth of mitotic spindle in cell division Replicate during cell division

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10 Cilia and Flagella Cilia  Numerous, short, hair like projections extending from surface of cell  Function to move fluid along cell surface Flagella  Similar in structure to cilia, but much longer  Function to move entire cell  In humans, only found in sperm

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16 Ribosomes Sites of production of protein synthesis Contain high content of RNA Consists of two subunits Made in nucleus  Large subunit  Small subunit Some attach to endoplasmic reticulum Free ribosomes produce proteins used in cytosol

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18 Endoplasmic Reticulum Network of membranes Rough ER  Continuous with nuclear envelope  Studded with ribosomes  Some proteins enter spaces in ER for processing and sorting. Carbohydrates added to glycoproteins. Smooth ER  Lacks ribosomes  Contains unique enzymes that synthesize fatty acids and steroids  Specialized function depending on location

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20 Golgi Complex Most proteins synthesized on rough ER transported to other regions of cell All pass through the Golgi complex 3-20 cisternae  Small flattened membranous sacs  Two surfaces Entry face on rough ER surface Exit face on plasma membrane surface, functions in secretion.

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23 Lysosomes Membrane enclosed vesicles Form from Golgi complex Contain as many as 60 enzymes  Break down molecules  Help recycle worn out cell parts Autophagy digestion of other organelles Work best in acidic pH (~5)  Membrane pumps import hydrogen ions keeping pH low

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26 Mitochondria Site of most ATP production Numbers vary from hundreds to thousands per cell Double membraned organelle Contains its own DNA  Can self replicate Inner membrane characteristics  Inward folds are called cristae  Inner space called matrix  Cellular respiration reactions occur here

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32 Concept 3.5 Nucleus

33 Nucleus Usually most prominent feature of cell A few cells have multiple nuclei but most have only one Have a double membrane  Outer membrane contiguous with rough ER  Openings in envelope called nuclear pores Control movement of substances

34 Nucleus Contains DNA  DNA has genes  Genes arranged along chromosomes  Humans have 46 chromosomes (23 pair)  Entire genetic information is called genome  Chromosomes are DNA coiled around specialized proteins  Uncoiled DNA with its associated proteins (histones) and RNA is called chromatin

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41 Concept 3.6 Transcription and Translation

42 Gene Expression DNA is the blue print for RNA RNA is the blue print for protein Proteins determine the physical and chemical characteristics of cells Three RNA nucleotides (codon) code for a particular tRNA (anticodon) which carries a particular amino acid Therefore, the sequence of DNA nucleotides determines sequence of RNA nucleotides which in turn determines the sequence of amino acids

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44 Transcription DNA is the blue print for RNA DNA stays in the nucleus, RNA is transcribed from DNA and is mobile mRNA strand is “read” by ribosomes and signal for particular tRNAs carrying specific amino acids RNA polymerase is the enzyme catalyzes transcription Promoter sequence before coding region

45 Transcription Base pairing Sequence of RNA determined by base pairing: DNARNA AdenineUracil ThymineAdenine GuanineCytosine CytosineGuanine Coding region of gene stops at terminator site which signals enzyme to release

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47 Translation Nucleotide sequence is read by ribosome Occurs in cytosol Codon on mRNA signals for tRNA carrying particular amino acid Small subunit has binding site for mRNA Large subunit has two binding sites for tRNA P site and A site. See figure 3.26

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56 Concept 3.7 Cell Division

57 Cell Division Cytoplasmic division called cytokinesis Somatic cell division  Produces two new identical cells  Nuclear division called mitosis Reproductive cell division  Produces gametes with half as many chromosomes as primary cell  Nuclear division two-step division called meiosis

58 Somatic Cell Division Cell cycle  Sequence of events that cell undergoes until it divides  Cells with pairs of chromosomes are diploid symbolized 2n  One member of each pair is inherited from biological father and the other member is inherited from biological mother Pair are called homologues Each one called homologous chromosome

59 Somatic Cell Division Cell cycle  Interphase G1  Cell is metabolically active replicates most everything BUT its DNA (8-10 hours) S phase  Cell duplicates its DNA (8 hours) G2  Growth continues  Cell prepares for division  Replication of centrosomes completed  Cell now ready for mitotic phase

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62 Mitotic Phase of Cell Cycle Nuclear division followed by cytokinesis  Prophase  Metaphase  Anaphase  Telophase See figure 3.30a, b, c, d, e

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64 Cytokinesis Process begins during anaphase Formation of cleavage furrow When complete two new identical cells results See figure 3.32

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67 End Chapter 3