Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 3 Cell Structures and Their Functions Dividing Cells

Other Cell Parts

Cytoplasm material between plasma membrane and nucleus –cytosol fluid part (site of chemical reactions), cytoskeleton, and cytoplasmic inclusions –cytoskeleton supports the cell and enables cell movements »Microtubules – provide support, aid in cell division, components of organelles »Actin filaments – support plasma membrane and define shape of the cell »Intermediate filaments – provide mechanical support to cell Cytoplasmic Inclusions are aggregates of chemicals either produced by the cell or taken in by the cell (lipids, glycogen, hemoglobin, melanin) –organelles

Fig Cytoskeleton

Cytoplasmic Organelles Specialized subcellular structures with specific functions Membranous –Mitochondria, peroxisomes, lysosomes, endoplasmic reticulum, and Golgi apparatus Nonmembranous –Centrioles and ribosomes

Nucleus nuclear envelope-two separate membranes with nuclear pores –Encloses jellylike nucleoplasm, contains essential solutes DNA and associated proteins are found inside the nucleus –hereditary material of cell and controls activities of cell –genetic library with blueprints for proteins Dictates kinds and amounts of proteins to be synthesized –Between cell divisions DNA is organized as chromatin –During cell division chromatin condenses to form chromosomes consisting of two chromatids connected by a centromere

Fig Nucleus

Fig Chromosome Structure

Nucleoli and Ribosomes Nucleoli: dark-staining spherical bodies within nucleus –Consist of RNA and proteins –Produces ribosomal ribonucleic acid (rRNA) –Site of ribosomal subunit assembly Ribosomes: sites of protein synthesis –Free ribosomes are not attached to any organelles synthesize proteins used inside the cell –Attached ribosomes are part of a network of membranes--rough endoplasmic reticulum (RER) produce proteins that are secreted from the cell

Fig Production of Ribosomes

Endoplasmic Reticulum (ER) Series of membranes forming sacs and tubules that extend from outer nuclear membrane into cytoplasm Two varieties: –Rough ER (RER) Studded with ribosomes Major site of protein synthesis –Smooth ER (SER) Does not have ribosomes attached Major site of lipid and carbohydrate synthesis –Catalyzes the following reactions in various organs of the body »Liver: lipid and cholesterol metabolism, breakdown of glycogen and along with the kidneys, detoxifiy drugs »Testes: synthesis of steroid-based hormones »Intestinal cells: absorption, synthesis, and transport of fats »Skeletal and cardiac muscle: storage and release of calcium

Endoplasmic Reticulum (ER) Fig. 3.17

Golgi Apparatus Series of closely packed membranous sacs that collect, package, and distribute proteins and lipids produced by the ER –Secretory vesicles: small, membrane-bound sacs that transport material from the golgi apparatus to the exterior of the cell Fig. 3.18

Function of the Golgi Apparatus 1.Some proteins are produced at ribosomes on the surface of the RER and are transferred into the cisterna as they are produced 2.The proteins are surrounded by a vesicle that forms from the membrane of the ER 3.This transport vesicle moves from the ER to the Golgi apparatus, fuses with its membrane, and releases the proteins into its cisterna 4.The Golgi apparatus concentrates and in some cases, modifies the proteins into glycoproteins or lipoproteins 5.The proteins are packaged into vesicles that form from the membrane of the Golgi apparatus 6.Some vesicles, such as lysosomes, contain enzymes that are used within the cell 7.Secretory vesicles carry proteins to the plasma membrane, where the proteins are secreted from the cell by exocytosis 8.Some vesicles contain proteins that become part of the plasma membrane Fig. 3.19

Lysosomes Spherical membranous bags containing digestive enzymes –Digest ingested bacteria, viruses, and toxins –Degrade nonfunctional organelles –Breakdown glycogen and release thyroid hormone –Breakdown non-useful tissue –Breakdown bone to release Ca2 + –Secretory lysosomes are found in white blood cells, immune cells, and melanocytes

Action of Lysosomes 1.A vesicle forms around material outside the cell 2.The vesicle is pinched off from the plasma membrane and becomes a separate vesicle inside the cell 3.A lysosome is pinched off the Golgi apparatus 4.The lysosome fuses with the vesicle Fig The enzymes from the lysosome mix with the material in vesicle, and the enzymes digest the material

Peroxisomes Membranous sacs containing oxidases and catalases –Breakdown fatty acids, amino acids, and hydrogen peroxide –Detoxify harmful or toxic substances –Neutralize dangerous free radicals Free radicals: highly reactive chemicals with unpaired electrons (i.e., O 2 – )

Mitochondria Fig major sites of ATP production via aerobic cellular respiration smooth outer membrane inner membrane infolded: cristae Contain own DNA, can produce some of their own proteins, and can replicate independently of the cell

Centrioles and Spindle Fibers Centrioles: cylindrical organelles located in centrosome –Pinwheel array of 9 triplets of microtubules –Centrosome: a specialized zone of the cytoplasm site of microtubule formation –Microtubule “spindle fibers” extend out in all directions from centrosome involved in separation of chromosomes during cell division –Form basis of cilia and flagella Fig. 3.22

Cilia, Flagella, and Microvilli Cilia –move substances over the surface of cells Flagella –much longer than cilia and propel sperm cells Microvilli –increase surface area of cell and aid in absorption and secretion

Cellular Processes Protein Synthesis –Transcription –Translation Cell Division (Mitosis)

Protein Synthesis DNA serves as master blueprint for protein synthesis DNA controls enzyme production and cell activity is regulated by enzymes (Proteins) Genes are segments of DNA carrying instructions for a polypeptide chain Triplets of nucleotide bases form the genetic library Each triplet specifies coding for an amino acid

Protein Synthesis Two step process –Transcription cell makes a copy of gene necessary to make a particular protein: messenger RNA (mRNA) mRNA travels from nucleus to ribosomes where info is translated into a protein –Translation requires both mRNA and transfer RNA (tRNA) tRNA brings amino acids necessary to synthesize the protein to ribosome

Overview of Protein Synthesis 1.DNA contains info necessary to produce proteins 2.Transcription of 1 DNA strand results in mRNA (complementary copy of info in DNA strand needed to make a protein) 3.mRNA leaves nucleus and goes to a ribosome 4.Amino acids, building blocks of proteins, carried to ribosome by tRNAs 5.In process of translation, info contained in mRNA is used to determine #, kinds, and arrangement of amino acids in polypeptide chain Fig. 3.23

Transcription Synthesis of mRNA, tRNA, and rRNA based on nucleotide sequence in DNA –Messenger RNA (mRNA) – carries genetic info from DNA in nucleus to ribosomes in cytoplasm –Transfer RNAs (tRNAs) – bound to amino acids base pair with codons of mRNA at ribosome to begin process of protein synthesis –Ribosomal RNA (rRNA) – structural component of ribosomes

Transcription 1.strands of DNA molecule separate from each other. 1 DNA strand serves as a template for mRNA synthesis 2.Nucleotides that will form mRNA pair with DNA nucleotides according to base-pair rules. Sequence of n’tides in template DNA strand (purple) determines sequence of n’tides in mRNA (grey). RNA polymerase (enzyme, not shown) joins n’tides of mRNA together 3.As n’tides are added, an mRNA molecule is formed Fig. 3.24

Transcription: RNA Polymerase enzyme that oversees synthesis of RNA Unwinds DNA template Adds complementary ribonucleoside triphosphates on DNA template Joins these RNA nucleotides together Encodes a termination signal to stop transcription

@ End of Transcription Posttranscriptional processing –modifies mRNA before it leaves nucleus –removes introns (non-coding) and –splices exons (coding) together with enzymes (spliceosomes) –Functional mRNA consists only of exons Alternative splicing –produces different combination of exons, allowing one gene to produce more than one type of protein

Translation Synthesis of proteins –Codon: set of 3 mRNA n’tides, codes for 1 amino acid during translation –Anticodon: set of 3 tRNA n’tides, complementary to codons of mRNA mRNA leaves nucleus through nuclear pores, to ribosomes tRNA carries amino acids, interacts at ribosome with mRNA. anticodons of tRNA bind to codons of mRNA, amino acids are joined to form a protein

Steps of Translation 1.Ribosome binds to mRNA-- has 2 binding sites for tRNA. **1 st codon is ALWAYS “AUG” (start codon), which codes for methionine. Codon and anticodon bind. The 2 nd tRNA binding site is open. 2.2 nd tRNA binds to 2 nd site on ribosome. 3.An enzyme within the ribosome catalyzes a synthesis reaction to form a peptide bond between the amino acids. **Amino acids are now associated with only 1 of the tRNAs. Fig. 3.25

Translation 4. Ribosome shifts position by 3 nucleotides. tRNA w/o amino acid is released from ribosome. tRNA w/amino acids takes its position. tRNA binding site is left open by the shift: more amino acids can be added by repeating steps 2 through Eventually a stop codon in the mRNA, such as UAA, ends the process of translation. mRNA and polypeptide chain are released from ribosome. 6. Multiple ribosomes attach to a single mRNA to form a polyribosome. As the ribosomes move down the mRNA, proteins attached to ribosomes lengthen and eventually detach from mRNA. Fig. 3.25