Chapter 3 The Cellular Level of Organization PDF

Chapter 3 The Cellular Level of Organization Copyright 2009 John Wiley & Sons, Inc. Fig. 3.1 Generalized Body Cell Copyright 2009 John Wiley & Sons, Inc. A Generalized Cell 1. Plasma membrane - forms the cell’s outer boundary - separates the cell’s interna...

Chapter 3 The Cellular Level of Organization Copyright 2009 John Wiley & Sons, Inc. Fig. 3.1 Generalized Body Cell Copyright 2009 John Wiley & Sons, Inc. A Generalized Cell 1. Plasma membrane - forms the cell’s outer boundary - separates the cell’s internal environment from the outside environment - is a selective barrier - plays a role in cellular communication Copyright 2009 John Wiley & Sons, Inc. A Generalized Cell 2. Cytoplasm - all the cellular contents between the plasma membrane and the nucleus - cytosol - the fluid portion, mostly water - organelles - subcellular structures having characteristic shapes and specific functions Copyright 2009 John Wiley & Sons, Inc. A Generalized Cell 3. Nucleus - large organelle that contains DNA - contains chromosomes, each of which consists of a single molecule of DNA and associated proteins - a chromosome contains thousands of hereditary units called genes Copyright 2009 John Wiley & Sons, Inc. Cytoplasm - 2 components 1. Cytosol - intracellular fluid, surrounds the organelles - the site of many chemical reactions - energy is usually released by these reactions - reactions provide the building blocks for cell maintenance, structure, function and growth 2. Organelles Specialized structures within the cell The cytoskeleton - network of protein filaments throughout the cytosol -provides structural support for the cell -three types according to increasing size: microfilaments, intermediate filaments, and microtubules Copyright 2009 John Wiley & Sons, Inc. The Cytoskeleton Copyright 2009 John Wiley & Sons, Inc. Organelles ◼ Centrosome - located near the nucleus, consists of two centrioles and pericentriolar material (Fig. 2.7) ◼ Cilia - short, hair-like projections from the cell surface, move fluids along a cell surface ◼ Flagella - longer than cilia, move an entire cell; only example is the sperm cell’s tail (Fig. 2.8) Copyright 2009 John Wiley & Sons, Inc. The Centrosome Copyright 2009 John Wiley & Sons, Inc. Cilia and Flagella Copyright 2009 John Wiley & Sons, Inc.  Organelles Ribosomes - sites of protein synthesis ◼ Endoplasmic reticulum ◼ - network of membranes in the shape of flattened sacs or tubules - Rough ER - connected to the nuclear envelope, a series of flattened sacs, surface is studded with ribosomes, produces various proteins -Smooth ER - a network of membrane tubules, does not have ribosomes, synthesizes fatty acids and steroids, detoxifies certain drugs Copyright 2009 John Wiley & Sons, Inc. Ribosomes Copyright 2009 John Wiley & Sons, Inc. Endoplasmic Reticulum Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc.  Synthesized Ribosome protein 1 Transport vesicle Entry face cisterna 2 Medial cisterna 3 Exit face cisterna 9 4 Transport vesicle (to lysosome) 8 Rough ER 6 Transfer vesicle Proteins in vesicle 4 Membrane membrane merge 7 vesicle 5 with plasma membrane Transfer vesicle Proteins exported Secretory from cell by exocytosis vesicle Plasma membrane Organelles ◼ Golgi complex - consists of 3-20 flattened, membranous sacs called cisternae - modify, sort, and package proteins for transport to different destinations - proteins are transported by various vesicles Lysosomes - vesicles that form from the Golgi complex, contain powerful digestive enzymes Copyright 2009 John Wiley & Sons, Inc. Golgi Complex Copyright 2009 John Wiley & Sons, Inc. Lysosomes Copyright 2009 John Wiley & Sons, Inc. Organelles ◼ Peroxisomes - smaller than lysosomes, detoxify several toxic substances such as alcohol, abundant in the liver ◼ Proteasomes - continuously destroy unneeded, damaged, or faulty proteins, found in the cytosol and the nucleus Copyright 2009 John Wiley & Sons, Inc. Organelles Mitochondria - the “powerhouses” of the cell ❑ Generate ATP ❑ More prevalent in physiologically active cells: muscles, liver and kidneys ❑ Inner and outer mitochondrial membranes ❑ Cristae - the series of folds of the inner membrane ❑ Matrix - the large central fluid-filled cavity ❑ Self-replicate during times of increased cellular demand or before cell division Copyright 2009 John Wiley & Sons, Inc. Mitochondria Copyright 2009 John Wiley & Sons, Inc. Organelles - Nucleus ◼ Spherical or oval shaped structure ◼ Usually most prominent feature of a cell ◼ Nuclear envelope - a double membrane that separates the nucleus from the cytoplasm ◼ Nuclear pores - numerous openings in the nuclear envelope, control movement of substances between nucleus and cytoplasm ◼ Nucleolus - spherical body that produces ribosomes Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. Organelles - Nucleus ◼ Genes - are the cell’s hereditary units, control activities and structure of the cell ◼ Chromosomes - long molecules of DNA combined with protein molecules Copyright 2009 John Wiley & Sons, Inc. Packing of DNA into a Chromosome of a Dividing Cell Copyright 2009 John Wiley & Sons, Inc. Can you name the parts of the cell? Cytoplasm Nucleus Cell Membrane Mitochondrion Cell Wall Permanent Vacuole Chloroplast Mini Quiz ◼ What is the primary function of the nucleus? Answer: The nucleus is the control center of the cell. It houses the cell's DNA, which contains the genetic instructions for building and maintaining the cell. Copyright 2009 John Wiley & Sons, Inc. Mini Quiz ◼ What is the role of the endoplasmic reticulum in protein synthesis and modification? ◼ Answer: The endoplasmic reticulum (ER) is a network of membrane-bound sacs and tubules. ❑ The rough ER is studded with ribosomes and is involved in protein synthesis. ❑ The smooth ER modifies proteins and lipids, and is also involved in other cellular functions like detoxification and calcium storage. Copyright 2009 John Wiley & Sons, Inc. Mini Quiz ◼ How do lysosomes help maintain cellular health? ◼ Answer: Lysosomes are membrane-bound organelles that contain digestive enzymes. They break down cellular waste products, debris, and foreign substances. This helps prevent the accumulation of harmful materials and maintains cellular health. Copyright 2009 John Wiley & Sons, Inc. Structure of the Plasma Membrane Copyright 2009 John Wiley & Sons, Inc. Plasma Membrane ◼ Flexible yet sturdy barrier ◼ The fluid mosaic model - the arrangement of molecules within the membrane resembles a sea of lipids containing many types of proteins ◼ The lipids act as a barrier to certain substances ◼ The proteins act as “gatekeepers” to certain molecules and ions Copyright 2009 John Wiley & Sons, Inc. Structure of a Membrane Main constituents: Lipids: 40% Phospholipid bilayer: Made up of two layers of lipids opposed to each other like a sandwich. Proteins: 55% For transport and cell communication Carbohydrates: 5% Serve as receptors. Participate in immune reaction. Copyright 2009 John Wiley & Sons, Inc. Structure of a Membrane ◼ Integral proteins - extend into or through the lipid bilayer ◼ Transmembrane proteins - most integral proteins, span the entire lipid bilayer ◼ Peripheral proteins - attached to the inner or outer surface of the membrane, do not extend through it Copyright 2009 John Wiley & Sons, Inc. Functions of Membrane Proteins ◼ Some integral proteins are ion channels ◼ Transporters - selectively move substances through the membrane ◼ Receptors - for cellular recognition; a ligand is a molecule that binds with a receptor ◼ Enzymes - catalyze chemical reactions ◼ Others act as cell-identity markers Copyright 2009 John Wiley & Sons, Inc. Structure of a Membrane ◼ Glycoproteins - membrane proteins with a carbohydrate group attached that protrudes into the extracellular fluid ◼ Glycocalyx - the “sugary coating” surrounding the membrane made up of the carbohydrate portions of the glycolipids and glycoproteins Copyright 2009 John Wiley & Sons, Inc. Figure 3.3 Copyright 2009 John Wiley & Sons, Inc. Membrane Permeability ◼ The cell is either permeable or impermeable to certain substances ◼ The lipid bilayer is permeable to oxygen, carbon dioxide, water and steroids, but impermeable to glucose ◼ Transmembrane proteins act as channels and transporters to assist the entrance of certain substances, for example, glucose and ions Copyright 2009 John Wiley & Sons, Inc. Passive vs. Active Processes ◼ Passive processes - substances move across cell membranes without the input of any energy; use the kinetic energy of individual molecules or ions ◼ Active processes - a cell uses energy, primarily from the breakdown of ATP, to move a substance across the membrane, i.e., against a concentration gradient Copyright 2009 John Wiley & Sons, Inc.  Active Transport Solutes are transported across plasma membranes with the use of energy, from an area of lower concentration to an area of higher Concentration Sodium-potassium pump Extracellular fluid Na+ Na+/K+ ATPase 3 Na+ expelled 2K+ gradient 3 Na+ P Cytosol K+ ATP + gradient 1 2 ADP 3 P 4 2K imported Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. Diffusion ◼ the movement of molecules from a region of higher concentration to a region of lower concentration down the concentration gradient. Copyright 2009 John Wiley & Sons, Inc. Factors affecting the rate of Diffusion ◼ Steepness of concentration gradient ◼ Temperature ◼ Mass of diffusing substance ◼ Surface area ◼ Diffusion distance Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. Simple Diffusion, Channel-mediated Facilitated Diffusion, and Carrier-mediated Facilitated Diffusion Copyright 2009 John Wiley & Sons, Inc. Channel-mediated Facilitated Diffusion of Potassium ions through a Gated K + Channel Copyright 2009 John Wiley & Sons, Inc.  Extracellular fluid Plasma membrane Cytosol Glucose Glucose transporter 1 Glucose gradient 2 3 Glucose Osmosis ◼ Net movement of water through a selectively permeable membrane from an area of high concentration of water (lower concentration of solutes) to one of lower concentration of water Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. Osmosis ◼ Water can pass through plasma membrane in 2 ways: 1. through lipid bilayer by simple diffusion 2. through aquaporins, integral membrane proteins Copyright 2009 John Wiley & Sons, Inc. Tonicity and its effect on RBCS Copyright 2009 John Wiley & Sons, Inc.  1… 2… Copyright 2009 John Wiley & Sons, Inc. 1… 2… 3… Copyright 2009 John Wiley & Sons, Inc. Transport in Vesicles ◼ Vesicle - a small spherical sac formed by budding off from a membrane ◼ Endocytosis - materials move into a cell in a vesicle formed from the plasma membrane three types: receptor-mediated endocytosis phagocytosis bulk-phase endocytosis (pinocytosis) ◼ Exocytosis - vesicles fuse with the plasma membrane, releasing their contents into the extracellular fluid ◼ Transcytosis - a combination of endocytosis and exocytosis Copyright 2009 John Wiley & Sons, Inc. Types of Endocytosis: Phagocytosis Copyright 2009 John Wiley & Sons, Inc. Phagocytosis Copyright 2009 John Wiley & Sons, Inc. Types of Endocytosis: Pinocytosis Copyright 2009 John Wiley & Sons, Inc. Receptor-Mediated Endocytosis Copyright 2009 John Wiley & Sons, Inc.  1 Binding Receptor-LDL LDL particle Plasma complex Receptor membrane Invaginated plasma Clathrin-coated membrane pit 2 Vesicle formation Clathrin-coated vesicle 3 Uncoating Transport Uncoated vesicle vesicle 4 Fusion with endosome 5 Recycling of receptors to plasma membrane Endosome 6 Degradation Transport in lysosome vesicle Digestive enzymes Lysosome Exocytosis Copyright 2009 John Wiley & Sons, Inc. Transcytosis Copyright 2009 John Wiley & Sons, Inc. Mini Quiz 1… 2… Copyright 2009 John Wiley & Sons, Inc. Mini Quiz ◼ What is the fundamental difference between endocytosis and exocytosis? ◼ Endocytosis is the process by which cells take in substances from the outside by engulfing them in a vesicle. ◼ Exocytosis is the process by which cells release substances enclosed in vesicles to the outside environment. ◼ In essence, endocytosis is a cellular "ingestion" process, while exocytosis is a cellular "excretion" process. Copyright 2009 John Wiley & Sons, Inc. Mini Quiz ◼ Can you describe the three main types of endocytosis and their specific functions? ◼ Phagocytosis: This is a type of endocytosis where cells engulf large particles, such as bacteria or cellular debris. It's essential for immune function and tissue repair. ◼ Pinocytosis: This is a type of endocytosis where cells take in small amounts of fluid and dissolved substances. It's crucial for nutrient uptake and maintaining cell volume. ◼ Receptor-mediated endocytosis: This is a highly specific type of endocytosis where cells take in substances that bind to specific receptors on the cell surface. It's important for nutrient uptake, hormone signaling, and viral entry. Copyright 2009 John Wiley & Sons, Inc. Mini Quiz ◼ What is transcytosis, and how does it differ from endocytosis and exocytosis? ◼ Transcytosis is a process where substances are taken in by endocytosis on one side of a cell, transported through the cell, and then released by exocytosis on the other side. ◼ Unlike endocytosis and exocytosis, which involve the movement of substances into or out of the cell, transcytosis involves the movement of substances through the cell. Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. For next week: Read pages 87-100 Protein Synthesis DNA Replication, Translation, Transcription Cell Division: Mitosis and Meiosis Copyright 2009 John Wiley & Sons, Inc. Protein Synthesis ◼ Protein synthesis(translation) is the production of a polymer of a chain of amino acids which produces a functioning protein. ◼ Ribosomes are the structures that synthesize the protein chain. ◼ involves reading the information from mRNA (messenger RNA) to put together a chain of amino acids. Copyright 2009 John Wiley & Sons, Inc. What is the difference between genes, chromosomes and DNA? Copyright 2009 John Wiley & Sons, Inc. RNA vs. DNA DNA RNA ◼ Double stranded ◼ Single stranded ◼ Deoxyribose sugar ◼ Ribose sugar ◼ Bases: C,G A,T ◼ Bases: C,G,A,U Both contain a sugar, phosphate, and base. What is the difference between DNA and RNA? Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. The bases 5 The most common organic bases are Adenine (A) Thymine (T) Cytosine (C) Guanine (G) 2-stranded DNA 9 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 PO4 Bonding 1 10 The bases always pair up in the same way Adenine forms a bond with Thymine Adenine Thymine and Cytosine bonds with Guanine Cytosine Guanine CENTRAL DOGMA transcription DNA translation RNA Proteins ◼ 3 different RNA molecules involved in protein synthesis: mRNA (messenger RNA) rRNA (ribosomal RNA tRNA (transfer RNA) PROTEIN SYTNESIS ◼ 1 step: Transcription occurs in the nucleus ◼ 2 step: Translation occurs in the cytoplasm  1st step Transcription: DNA → RNA ◼ Transcription occurs in the nucleus First: DNA unwinds in a section Next: mRNA (messenger RNA) is formed by base pairing with the parent strand of DNA. This begins transcription. Then: mRNA carries the message about what type of protein to make from the DNA in the nucleus to the ribosome  The Genetic Code ◼ Each 3 consecutive bases on the mRNA is a code word, CODON, that specifies an amino acid. ◼ The genetic code consists of 64 codons, ◼ but only 61 code amino acids. ◼ Three codons act as signal to stop the process ◼ One codon, AUG, codes for methionine, and is also the Start signal for translation. Transcription Copyright 2009 John Wiley & Sons, Inc.  2 nd step Translation: RNA → Protein ◼ Translation: synthesizing a code from amino acids, according to the sequences of the nucleotides in mRNA. ◼ Occurs at the ribosome, in cytoplasm of cell ◼ Ribosomal RNA, rRNA, is needed for protein synthesis – helps mRNA bind to the ribosome ◼ tRNA, brings specific amino acids to the ribosome to be assembled as proteins.  Translation ◼ Ribosomes are the sites of______________ Amino Acids to Proteins Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc.  Transfer RNA (tRNA) ◼ Transport molecule that carries specific amino acid to a ribosome ◼ Each tRNA recognizes the correct codon on the mRNA molecule Translation  Steps in Translation 1. mRNA leaves the nucleus and migrates to ribosome 2. mRNA binds to small ribosomal subunit 3. tRNA brings an amino acid to the ribosome, where anticodon on the tRNA binds to the codon of the mRNA 4. The amino acid bonds to its adjoining amino acid to form a growing polypeptide molecule 5. The tRNA without the amino acid is released from the ribosome 6. Other tRNA’s bring amino acids to the ribosome to complete the protein molecule  Large Amino acid P site subunit Initiator tRNA tRNA UAC A site GG A U G C Small P site subunit Anticodon A site UAC 2 Large and small ribosomal GG A U G subunits join to form a functional ribosome and initiator tRNA mRNA Amino acid fits into P site. Codons (methionine) Initiator tRNA 3 Anticodon of incoming tRNA pairs Anticodon with next mRNA codon at A site. mRNA UAC GG A U G Small mRNA subunit binding site Start codon UAC U 1 Initiator tRNA attaches to a GG A U G U A G start codon. New peptide bond 4 Amino acid on tRNA at P site forms a peptide bond with amino acid at A site. C U G G A U G U GC mRNA Stop codon movement 6 Protein synthesis stops when 5 tRNA at P site leaves ribosome, the ribosome reaches stop ribosome shifts by one codon; codon on mRNA. tRNA previously at A site is now at the P site. Key: Growing Complete protein = Adenine mRNA protein tRNA = Guanine = Cytosine = Uracil Summary of movement of ribosome along mRNA Protein Synthesis Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc.  What are the two main stages of protein synthesis, and where do they occur? Answer: The two main stages of protein synthesis are transcription and translation. Transcription occurs in the nucleus, where the DNA sequence of a gene is copied into messenger RNA (mRNA). Translation occurs in the cytoplasm, where the mRNA sequence is used to build a protein at the ribosome. Copyright 2009 John Wiley & Sons, Inc.  what are the types of RNAs involved in protein synthesis mRNA (Messenger RNA): carries the genetic code from DNA in the nucleus to the ribosome in the cytoplasm. It is a transcription of the gene and contains the instructions for building a specific protein. tRNA (Transfer RNA): transfers specific amino acids to the ribosome. Each tRNA molecule has an anticodon that pairs with a complementary codon on the mRNA, ensuring that the correct amino acid is added to the growing polypeptide chain. rRNA (Ribosomal RNA): the cellular machinery where protein synthesis occurs. It helps to catalyze the formation of peptide bonds between amino acids and ensures proper alignment of mRNA and tRNA during translation. Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. The Cell Cycle Copyright 2009 John Wiley & Sons, Inc. Cell Cycle ◼ The cell cycle is a sequence of events in which a body cell duplicates its contents and divides in two ◼ Human somatic cells contain 23 pairs of chromosomes (total = 46) ◼ The two chromosomes that make up each pair are called homologous chromosomes (homologs) ◼ Somatic cells contain two sets of chromosomes and are called diploid cells Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. Cell Cycle ◼ Interphase - the cell is not dividing - the cell replicates its DNA - consists of three phases, G1, S, and G2, replication of DNA occurs in the S phase M phase - consists of a nuclear division (mitosis) and a cytoplasmic division (cytokinesis) to form two identical cells in Mitosis and four haploid cells in meiosis Copyright 2009 John Wiley & Sons, Inc. The Cell Cycle Copyright 2009 John Wiley & Sons, Inc. DNA Replication Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. Nuclear Division: Mitosis ◼ Prophase - the chromatin fibers change into chromosomes ◼ Metaphase - microtubules align the centromeres of the chromatid pairs at the metaphase plate ◼ Anaphase - the chromatid pairs split at the centromere and move to opposite poles of the cell; the chromatids are now called chromosomes ◼ Telophase - two identical nuclei are formed around the identical sets of chromosomes now in their chromatin form Copyright 2009 John Wiley & Sons, Inc. Cytoplasmic Division: Cytokinesis ◼ Division of a cell’s cytoplasm to form two identical cells ◼ Usually begins in late anaphase ◼ The plasma membrane constricts at its middle forming a cleavage furrow ◼ The cell eventually splits into two daughter cells ◼ Interphase begins when cytokinesis is complete Copyright 2009 John Wiley & Sons, Inc.  1 Centrosome: Centrioles Pericentriolar material Nucleolus Nuclear envelope Chromatin Plasma membrane 6 LM all at 700x Cytosol (a) INTERPHASE 2 Kinetochore Centromere Mitotic spindle Chromosome (microtubules) (two chromatids (f) IDENTICAL CELLS IN INTERPHASE Fragments of joined at 5 centromere nuclear envelope Early (b) PROPHASE Late Metaphase plate 3 Cleavage furrow (c) METAPHASE 4 (e) TELOPHASE Cleavage furrow Chromosome Early Late (d) ANAPHASE What are the main stages of the cell cycle, and what happens in each stage? G1 Phase (Gap 1): The cell grows and performs normal functions. It also prepares for DNA replication. S Phase (Synthesis): DNA replication occurs, resulting in the duplication of chromosomes. G2 Phase (Gap 2): The cell continues to grow and prepares for mitosis by producing necessary proteins and organelles. M Phase (Mitosis): The cell divides into two daughter cells. Mitosis includes prophase, metaphase, anaphase, and telophase, followed by cytokinesis, which splits the cytoplasm. Copyright 2009 John Wiley & Sons, Inc. What happens during each stage of mitosis? Prophase: Chromosomes condense and become visible, the nuclear envelope breaks down, and the mitotic spindle begins to form. Metaphase: Chromosomes align along the metaphase plate (center of the cell), and spindle fibers attach to the centromeres of the chromosomes. Anaphase: Sister chromatids are pulled apart towards opposite poles of the cell. Telophase: Chromatids reach the poles, the nuclear envelope re-forms around each set of chromosomes, and the chromosomes begin to de-condense. Copyright 2009 John Wiley & Sons, Inc. What is the difference between mitosis and meiosis in terms of their outcomes? Mitosis results in two genetically identical daughter cells, each with the same number of chromosomes as the parent cell (diploid). It is used for growth, repair, and asexual reproduction. Meiosis, on the other hand, results in four genetically diverse daughter cells, each with half the number of chromosomes as the parent cell (haploid). Meiosis is used for sexual reproduction to produce gametes (sperm and eggs). Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. Reproductive Cell Division ◼ During sexual reproduction each new organism is the result of the union of two gametes (fertilization), one from each parent ◼ Meiosis - reproductive cell division that occurs in the gonads (ovaries and testes) that produces gametes with half the number of chromosomes ◼ Haploid cells - gametes contain a single set of 23 chromosomes ◼ Fertilization restores the diploid number of chromosomes (46) Copyright 2009 John Wiley & Sons, Inc. Reproductive Cell Division ◼ Meiosis occurs in two successive stages: meiosis I and meiosis II ◼ Each of these two stages has 4 phases: prophase, metaphase, anaphase, and telophase ◼ Summary - Meiosis I begins with a diploid cell and ends with two cells having the haploid number of chromosomes; in Meiosis II, each of the two haploid cells divides, the net result is four haploid gametes that are genetically different from the original diploid starting cell Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. ◼ Leptotene stage- condensation of chromatin fibers into long, threadlike structures, similar to what occurs at the beginning of mitosis. ◼ Zygotene stage- individual chromosomes become distinguishable and homologous chromosomes become closely paired with each other via synapsis. Copyright 2009 John Wiley & Sons, Inc. ◼ Pachytene stage- crossing over, or the exchange of DNA segments between the homologous chromosomes. ◼ This accounts for genetic recombination. ◼ Each chromosome undergoes compacting process where the size is reduced to less than ¼ of its previous length. Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. ◼ Diplotene stage- homologous chromosome begin to separate from each other particularly near the centromere. They however, remain attached by connections known as chiasmata. Chiasmata – regions where exchange of DNA materials has occured. ◼ Diakinesis- chromosomes recondense to their maximally compacted state. Nucleoli disappear, the spindle forms and the nuclear envelope breaks down. Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. Copyright 2009 John Wiley & Sons, Inc. Cellular Diversity ◼ The average adult has nearly 100 trillion cells ◼ There are about 200 different types of cells ◼ Cells come in a variety of shapes and sizes ◼ Cellular diversity permits organization of cells into more complex tissues and organs Copyright 2009 John Wiley & Sons, Inc. End of Chapter 3 Copyright 2009 John Wiley & Sons, Inc. All rights reserved. Reproduction or translation of this work beyond that permitted in section 117 of the 1976 United States Copyright Act without express permission of the copyright owner is unlawful. Request for further information should be addressed to the Permission Department, John Wiley & Sons, Inc. The purchaser may make back-up copies for his/her own use only and not for distribution or resale. The Publishers assumes no responsibility for errors, omissions, or damages caused by the use of theses programs or from the use of the information herein. Copyright 2009 John Wiley & Sons, Inc. Lab homework Bring your lab gown Books and References about next week’s topic Bring 1 piece each group: Celery, Pears, Cucumber, Stem of Soybean Copyright 2009 John Wiley & Sons, Inc.