Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 3 Cell Structure and Genetic Function 3-1

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cell  Is the basic unit of structure and function in body  Is a highly organized molecular factory  Has 3 main components: plasma membrane, cytoplasm and organelles 3-4

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Plasma Membrane  Surrounds and gives cell form; is selectively permeable  Formed by a double layer of phospholipids  Which restricts passage of polar compounds  Proteins customize membranes  Provide structural support  Serve as transporters, enzymes, receptors and identity markers 3-5

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cytoplasm and Cytoskeleton  Cytoplasm is the jellylike matrix within a cell  Consists of fluidlike cytosol plus organelles 3-15

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cytoplasm and Cytoskeleton continued  Cytoskeleton is a latticework of microfilaments and microtubules filling cytoplasm  Gives cell its shape and structure  Forms tracks upon which things are transported around cell 3-16

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Organelles  Are cytoplasmic structures that perform specialized functions for cells 3-17

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lysosomes  Are vesicle-like organelles containing digestive enzymes and matter being digested (Fig 3.9)  Involved in recycling cell components  Involved in programmed cell death  Are vesicle-like organelles containing oxidative enzymes  Involved in detoxification in liver 3-18 Peroxisomes

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Mitochondria  Are energy-producing organelles  Believed to have originated from symbiotic bacteria 3-20

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Ribosomes  Are protein factories  Where cell's proteins are synthesized  Composed of 2 rRNA subunits 3-21

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Endoplasmic Reticulum (ER)  A system of membranes specialized for synthesis or degradation of molecules  Rough ER contains ribosomes for protein synthesis  Smooth ER contains enzymes for steroid synthesis and inactivation 3-22

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Golgi Complex  Is a stack of flattened sacs  Vesicles enter from ER, contents are modified, and leave other side  Lysosomes and secretory vesicles are formed in Golgi  Secretory proteins leave ER in vesicles and go to Golgi  In the Golgi complex carbohydrates are added to make glycoproteins  Vesicles leave Golgi for lysosomes or exocytosis 3-23

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Nucleus  Contains cell's DNA  Enclosed by a double membrane nuclear envelope  Outer membrane is continuous with ER 3-24

Gene Expression 3-26

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gene Expression  Genes are lengths of DNA that code for synthesis of RNA  mRNA carries info for how to make a protein  Is transported out of nucleus to ribosomes where proteins are made 3-27

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gene Expression continued  Takes place in 2 stages:  Transcription occurs when DNA sequence in a gene is turned into a mRNA sequence  Translation occurs when mRNA sequence is used to make a protein 3-28

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Genome and Proteome  Genome refers to all genes in an individual or in a species  Proteome refers to all proteins produced by a genome 3-30

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. RNA Synthesis continued  One gene codes for one polypeptide chain  Each gene is several thousand nucleotide pairs long  For transcription, RNA polymerase binds to a “start” sequence on DNA and unzips strands  Nearby are promoter regions, which regulate levels of transcription  Transcription factors must bind to promoter to initiate transcription 3-35

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. RNA Synthesis continued  Only one strand of DNA contains the gene and is transcribed  Its bases pair with complementary RNA bases to make mRNA  G pairs with C  A pairs with U  RNA polymerase detaches when hits a "stop" sequence 3-36

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. RNA Synthesis continued  Transcription produces four types of RNA:  pre-mRNA - altered in nucleus to form mRNA  mRNA - contains the code for synthesis of a protein  tRNA (transfer RNA) - decodes the info contained in mRNA  rRNA - forms part of ribosomes 3-37

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Protein Synthesis  Occurs one amino acid at a time according to sequence of base triplets in mRNA  In cytoplasm, mRNA attaches to ribosomes forming a polysome where translation occurs 3-41

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Protein Synthesis continued  Ribosomes read 3 mRNA bases (= a triplet) at a time  Each triplet is a codon, which specifies an amino acid  Ribosomes translate codons into an amino acid sequence that becomes a polypeptide chain 3-42

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Protein Synthesis continued 3-43

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Protein Synthesis continued  In a ribosome, anticodons on tRNA bind to mRNA codons  Amino acids on adjacent tRNAs are brought together and linked enzymatically by peptide bonds  This forms a polypeptide; at a stop codon it detaches from ribosome 3-45

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Protein Degradation  The activity of many enzymes and regulatory proteins is controlled by rapidly degrading them  By proteases in lysosomes  And by cytoplasmic proteasomes  Proteins are directed to proteasomes by ubiquitin tags 3-48

Cell Cycle 3-53

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cell Cycle continued  Interphase is subdivided into:  G 1 - cell performs normal physiological roles  S - DNA is replicated in preparation for division  G 2 - chromatin condenses prior to division 3-55

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Oncogenes  Are genes whose mutations are associated with cancer  Tumor suppressor genes inhibit cancer development  Tumor suppressor gene p53 encodes a transcription factor  It halts cell division when DNA is damaged  Then either promotes repair of the DNA; or apoptosis (cell death)  Mutations in p53 are found in 50% of all cancers 3-57

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cell Death  Occurs in 2 ways:  Necrosis occurs when pathological changes kill a cell  Apoptosis occurs as a normal physiological response  Also called programmed cell death  Involves activation of cytoplasmic caspases, which lead to cell death 3-58

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Telomeres  Are non-coding regions of DNA at ends of chromosomes  Each time a cell divides, a length of telomere is lost  Because DNA polymerase can’t copy the very end of DNA strand  When telomere is used up, cell becomes senescent  Believed to represent a molecular clock for aging  That ticks down with each division  Germinal and cancer cells can divide indefinitely and do not age  Have the enzyme telomerase which replaces telomere nucleotides not duplicated by DNA polymerase during divisions 3-67