Chapter 3 - Cell Structures and Their Function.pdf

01/09/2023 Because learning changes everything.® 1...

01/09/2023 Because learning changes everything.® 1 Because learning changes everything.® Generalized Cell. This generalized cell shows the major organelles contained in cells. However, no single cell contains all of these organelle types. Furthermore, one kind of cell may contain many organelles of one type, whereas another kind of cell may contain very few. 2 1  01/09/2023 Because learning changes everything.® Functions of the Cell: Cell metabolism and energy use - chemical reactions that occur within cells ; energy released during metabolism is used for cell activities Synthesis of molecules – proteins, nucleic acids, lipids determined the structure and functional characteristics of cell Communication - through chemical and electrical signals Reproduction and inheritance - genetic information are transmitted to the next generation 3 Cell Structure Organelles: specialized structures in cells that perform specific functions Example: nucleus, mitochondria, ribosomes Cytoplasm: jelly-like substance that holds organelles Cell membrane: also termed the plasma membrane a structure that encloses the cytoplasm Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 4 2  01/09/2023 Cell Membrane Structure outermost component; encloses the cytoplasm and forms a boundary between material inside the cell and material outside extracellular substances - substances outside the cell intracellular substances – substances inside the cell supports the cell contents, acts as a selective barrier that determines what moves into and out of the cell, and plays a role in communication between cells made up of two major types of molecules: phospholipids and proteins also contains other molecules, such as cholesterol and carbohydrates. Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 5 Cell Membrane Structure Fluid-mosaic model of the cell membrane The membrane is composed of a bilayer of phospholipids and cholesterol with proteins “floating” in the membrane. The nonpolar hydrophobic region of each phospholipid molecule is directed toward the center of the membrane, and the polar hydrophilic region is directed toward the fluid environment either outside or inside the cell. Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 6 3  01/09/2023 Phospholipid Structure double layer of phospholipids forms a lipid barrier between the inside and outside of the cell polar head region - hydrophilic faces the extracellular and intracellular fluids of the cell; exposed to water around the membrane nonpolar, fatty acid region – hydrophobic, faces the interior of the membrane Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 7 Movement through the Cell Membrane cell membrane has selective permeability, which allows only certain substances to pass in and out of the cell. substances such as enzymes, glycogen, and K+ are found in higher concentrations inside the cell substances such as Na+, Ca2+ and Cl− are found in higher concentrations outside the cell movement through the cell membrane may be passive or active passive membrane transport does not require the cell to expend energy active membrane transport does require the cell to expend energy, usually in the form of ATP Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 8 4  01/09/2023 Movement through the Cell Membrane Passive membrane transport mechanisms - diffusion, osmosis, and facilitated diffusion Active membrane transport mechanisms - active transport, secondary active transport, endocytosis, and exocytosis Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 9 Diffusion solution - composed of solutes and solvent solutes - substances dissolved in a predominant liquid or gas, which is called the solvent solutes (ex., ions or molecules), tend to move from an area of higher concentration of a solute to an area of lower concentration of that same solute in solution; this movement from high concentration to a low concentration is called diffusion Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 10 5  01/09/2023 Diffusion Illustration of diffusion of a salt (green) in a beaker of water - diffusion is the gradual spread of salt throughout a beaker of still water involves movement of substances in a solution down a concentration gradient - difference in the concentration of a solute in a solvent between two points divided by the distance between the two points Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 11 Cell Membrane Passage some substances, like O2 and CO2, can pass directly through the cell membrane’s phospholipid bilayer some substances must pass through transmembrane protein channels, such as Na+ through its channels route of transport through the membrane depends on the size, shape, and charge of the substance. Diffusion Through the Cell Membrane. Non-lipid-soluble molecules (red) diffuse through membrane channels. Lipid- soluble molecules (orange) diffuse directly through the cell membrane. Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 12 6  01/09/2023 Leak and Gated Channels with two classes of cell membrane channels Leak channels constantly allow ions to pass through Gated channels limit the movement of ions Leak and Gated Membrane Channels across the membrane In this example, the K+ leak channel (purple) is always open, allowing K+ to by opening and closing diffuse across the cell membrane. The gated Na+ channel (pink) regulates the movement of Na+ across the membrane by opening and closing. Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 13 Osmosis diffusion of water (a solvent) across a selectively permeable membrane from a region of higher water concentration to one of lower water concentration exerts a pressure, termed osmotic pressure, which is the force required to prevent movement of water across cell membrane important to cells because large volume changes caused by water movement can disrupt normal cell functions Osmosis. Water moves from the beaker across the selectively permeable membrane into a tube containing a solution with a higher salt concentration Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 14 7  01/09/2023 Osmotic Pressure and the Cell Osmotic pressure depends on the difference of solution concentrations inside a cell relative to outside the cell A cell may be placed in solutions that are either hypotonic, isotonic, or hypertonic compared to the cell cytoplasm Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 15 Hypotonic solution has a lower concentration of solutes and a higher concentration of water relative to the cytoplasm of the cell solution has less tone, or osmotic pressure, than the cell water moves by osmosis into the cell, causing it to swell if the cell swells enough, it can rupture, a process called lysis Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 16 8  01/09/2023 Red Blood Cell Changes in Differing Solutions David M. Phillips/Science Source Figure 3.5 Access the text alternative for slide images. Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 17 Isotonic A cell immersed in an isotonic solution has the same solute concentrations inside and outside the cell The cell will neither shrink nor swell Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 18 9  01/09/2023 Hypertonic When a cell is immersed in a hypertonic solution, the solution has a higher concentration of solutes and a lower concentration of water relative to the cytoplasm of the cell Water moves by osmosis from the cell into the hypertonic solution Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 19 Carrier-Mediated Transport Mechanisms many nutrient molecules (amino acids and glucose), cannot enter the cell by diffusion; many polar molecules produced in cells cannot leave the cell by diffusion carrier molecules, which are proteins within the cell membrane, are involved in carrier-mediated transport mechanisms, which move large, water-soluble molecules or electrically charged ions across the cell membrane; - exhibit specificity; only specific molecules are transported by the carriers three kinds of carrier mediated transport: facilitated diffusion, active transport, and secondary active transport Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 20 10  01/09/2023 Facilitated Diffusion a carrier-mediated transport process that moves substances across the cell membrane from an area of higher concentration to an area of lower concentration of that substance because movement is with the concentration gradient, Facilitated Diffusion. metabolic energy in the form A carrier molecule transports glucose of ATP is not required across the cell membrane from an area of higher glucose concentration (outside the cell) to an area of lower glucose concentration (inside the cell). Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 21 Active Transport a carrier-mediated process, requiring ATP, that move substances across the cell membrane from regions of lower concentration to those of higher concentration against a concentration gradient accumulate necessary substances on one side of the cell membrane at concentrations many times greater than those on the other side Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 22 11  01/09/2023 Active Transport: Sodium-Potassium Pump. The sodium-potassium pump requires ATP to move a higher concentration of Na+ out of the cell and a higher concentration of K+ into the cell. Access the text alternative for slide images. Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 23 Secondary Active Transport 1. A Na+–K+ pump maintains a concentration of Na+ that is higher outside the cell than inside. 2. Na+ move back into the cell by a carrier molecule that also moves glucose. The concentration Secondary Active Transport gradient for Na+ provides the energy required to move glucose, by cotransport, against its The active transport of Na+ out of concentration gradient. the cell (step 1) maintains a Na+ concentration gradient, which provides the energy for moving glucose against its concentration gradient (step 2) Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 24 12  01/09/2023 Endocytosis uptake of material through the cell membrane by the formation of a vesicle the cell membrane invaginates (folds inward) to form a vesicle containing the material to be taken into the cell the vesicle then moves into the cytoplasm exhibits specificity requires energy in the form of ATP for the formation and movement of vesicles Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 25 Endocytosis Receptor-mediated endocytosis - occurs when a specific substance binds to the receptor molecule and is transported into the cell Phagocytosis - used for endocytosis when solid particles are ingested Receptor-Mediated Endocytosis. Pinocytosis has much smaller vesicles formed, and they contain Molecules bind to receptors on the cell membrane, and a vesicle liquid rather than solid particles forms to transport the molecules into the cell. Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 26 13  01/09/2023 Exocytosis involves the use of membrane- bound sacs called secretory vesicles that accumulate materials for release from the cell the vesicles move to the cell membrane and fuse, ultimately releasing the material by exocytosis. examples of exocytosis are the secretion of digestive enzymes Diagram of exocytosis allows the release of materials from cell; requires energy in the form of ATP for the formation and movement of vesicles Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 27 Organelles the interior of a cell is composed of the cytoplasm, which a jelly-like fluid that surrounds the organelles. specialized structures that perform certain functions includes the nucleus, ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, mitochondria, cytoskeleton, centrioles, cilia, flagella, and microvilli Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 28 14  01/09/2023 Cell Nucleus a large organelle usually located near the center of the cell bounded by a nuclear envelope, which consists of outer and inner membranes with a narrow space between them nuclear membrane contains nuclear pores, through which materials can pass into or out of the nucleus Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 29 Nucleus (a) The nuclear envelope consists of inner and outer membranes, which become fused at the nuclear pores. The nucleolus is a condensed region of the nucleus not bounded by a membrane and consisting mostly of RNA and protein. (b) Transmission electron micrograph of the nucleus. (c) Scanning electron micrograph showing the membranes of the nuclear envelope and the nuclear pores. (b,c) Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 30 15  01/09/2023 Cell Nucleus nuclei of human cells contain 23 pairs of chromosomes which consist of DNA and proteins during most of a cell’s life, the chromosomes are loosely coiled and collectively called Transmission electron micrograph of the nucleus chromatin when a cell prepares to divide, the chromosomes become tightly coiled and are visible when viewed with a Structure of a Chromosome. microscope Chromosomes consist of DNA and proteins. When loosely coiled, chromosomes are collectively referred to as chromatin. During cell division, chromosomes become tightly coiled and visible as individual structures. Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 31 Cell Nucleus within the nucleus are nucleoli, which are diffuse bodies with no surrounding membrane that are found within the nucleus there are usually one to several nucleoli within the nucleus the subunits of ribosomes, a type of cytoplasmic organelle, are produced within a nucleolus ribosomes may be attached to other organelles, such as the ribosomes are the organelles where endoplasmic reticulum. proteins are produced ribosomes that are not attached these ribosomal components exit to any other organelle are the nucleus through nuclear pores called free ribosomes Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 32 16  01/09/2023 Ribosome Production Production of Ribosomes. Ribosomal subunits are produced in the nucleus and then move into the cytoplasm, where they form ribosomes during protein synthesis Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 33 Endoplasmic Reticulum (ER) a series of membranes forming sacs and tubules that extends from the outer nuclear membrane into the cytoplasm rough ER - involved in protein synthesis and is rough due to attached ribosomes The endoplasmic reticulum is smooth ER has no attached continuous with the nuclear envelope and can exist as either ribosomes and is a site for lipid rough endoplasmic reticulum synthesis, cellular (with ribosomes) or smooth detoxification, and it stores endoplasmic reticulum (without ribosomes). calcium ions in skeletal muscle cells. Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 34 17  01/09/2023 Golgi Apparatus also called the Golgi complex, consists of closely packed stacks of curved, membrane-bound sacs collects, modifies, packages, and distributes proteins and lipids manufactured by the ER forms vesicles, some of which are secretory vesicles, lysosomes, and other vesicles Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 35 Lysosomes membrane-bound vesicles formed from the Golgi apparatus contain a variety of enzymes that function as intracellular digestive systems vesicles formed by endocytosis may fuse with lysosomes in order to breakdown materials in the endocytotic vesicles. one example is white blood cells phagocytizing bacteria; then enzymes within lysosomes destroy the phagocytized bacteria Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 36 18  01/09/2023 Peroxisomes small, membrane-bound vesicles containing enzymes that break down fatty acids, amino acids, and hydrogen peroxide (H2O2) H2O2 is a by-product of fatty acid and amino acid breakdown and can be toxic to a cell enzymes in peroxisomes break down hydrogen cells active in detoxification, such as liver and kidney cells, have many peroxisomes Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 37 Mitochondrion/Mitochondria small organelles responsible for producing considerable amounts of ATP by aerobic (with O2) metabolism with inner and outer membranes separated by a space outer membranes have a smooth contour; inner membranes have numerous folds, called cristae, which project into the interior of the mitochondria (a) Typical mitochondrion structure. (b) Transmission electron micrograph of mitochondria in longitudinal and cross sections. Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 38 19  01/09/2023 Mitochondria material within the inner membrane is the mitochondrial matrix and contains enzymes and mitochondrial DNA (mtDNA) major sites of adenosine triphosphate (ATP) production within cells carry out aerobic respiration, a series of chemical reactions that require O2 to break down food molecules to produce ATP ATP is the main energy source for most chemical reactions within the cell, and cells cells with a large energy requirement have more mitochondria than cells that require less energy Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 39 Cytoskeleton gives internal framework to the cell consists of protein structures that support the cell, hold organelles in place, and enable the cell to change shape protein structures are microtubules, microfilaments, and intermediate filaments Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 40 20  01/09/2023 Microtubules hollow structures formed from protein subunits perform a variety of roles, including helping to support the cytoplasm of cells, assisting in cell division, and forming essential components of certain organelles, such as cilia and flagella Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 41 Microfilaments small fibrils formed from protein subunits that structurally support the cytoplasm, determining cell shape some are involved with cell movement in muscle cells enable the cells to shorten, or contract Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 42 21  01/09/2023 Intermediate Filaments formed from protein subunits that are smaller in diameter than microtubules but larger in diameter than microfilaments provides mechanical support to the cell keratin - a specific type of intermediate filament, a protein associated with skin cells Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 43 Centrioles a specialized area of cytoplasm close to the nucleus where microtubule formation occurs contain two centrioles, which are normally oriented perpendicular to each other each centriole is a small, cylindrical organelle composed of microtubules involved in the process of mitosis Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 44 22  01/09/2023 Cilium / Cilia cilium – an eyelash project from the surface of certain cells cylindrical structures that extend from the cell and are composed of microtubules numerous on surface cells that line the respiratory tract responsible for the movement of materials over the top of cells, such as mucus Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 45 Flagella similar structure to that of cilia but are much longer, and they usually occur only one per cell sperm cells each have one flagellum, which propels the sperm cell Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 46 23  01/09/2023 Microvilli specialized extensions of the cell membrane that are supported by microfilaments do not actively move as cilia and flagella do numerous on cells that have them and they increase the surface area of those cells abundant on the surface of cells that line the intestine, kidney, and other areas in which absorption is an important function Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 47 Whole Cell Activity a cell’s characteristics are determined by the type of proteins it produces proteins produced are in turn determined by the genetic information in the nucleus in order to understand how a cell functions, we must consider the relationship between genes and proteins information in DNA provides the cell with a code for its cellular processes whole-cell activities determine the characteristics of a functioning cell and the growth and maintenance of the human body understanding how genetic information is used in the cell and distributed to daughter cells is important for understanding basic cellular activity Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 48 24  01/09/2023 DNA contains the information that directs protein synthesis; a process called gene expression a DNA molecule consists of nucleotides joined together to form two nucleotide strands the two strands are connected and resemble a ladder that is twisted around its long axis each nucleotide consists of a 5- carbon sugar, a phosphate group, and a nitrogenous base each nucleotide on one DNA strand has a specific bonding pattern to a gene is a sequence of nucleotides another nucleotide on the opposite that provides a chemical set of strand instructions for making a specific protein Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 49 DNA serve as structural components inside the cell, proteins secreted to the outside of the cell, and enzymes that regulate chemical reactions in the cell influences the structural and functional characteristics of the entire organism because it directs protein synthesis 1. The strands of the DNA molecule separate from each whether an individual has blue other. One DNA strand serves as a template for mRNA synthesis. eyes, brown hair, or other 2. Nucleotides that will form mRNA pair with DNA inherited traits is determined nucleotides according to the base-pair combinations shown in the key at the top of the figure. Thus, the ultimately by DNA sequence of nucleotides in the template DNA strand (purple) determines the sequence of nucleotides in mRNA (gray). An enzyme (not shown) joins the nucleotides of mRNA together. 3. As nucleotides are added, an mRNA molecule is formed Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 50 25  01/09/2023 Gene Expression Gene expression, which is protein synthesis, involves transcription and translation Transcription involves copying DNA into messenger RNA (mRNA); takes place in the nucleus of the cell Translation involves mRNA being used to produce a protein; occurs in the cell cytoplasm after mRNA has exited the nucleus through the nuclear pores; mRNA attaches to a ribosome Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 51 Overview of Gene Expression 1. DNA contains the information necessary to produce proteins. 2. Transcription of one DNA strand results in mRNA, which is a complementary copy of the information in the DNA strand needed to make a protein. 3. The mRNA leaves the nucleus and goes to a ribosome. 4. Amino acids, the building blocks of proteins, are carried to the ribosome by transfer RNAs (tRNAs). 5. In the process of translation, the information contained in mRNA is used to determine the number, kinds, and arrangement of amino acids in the polypeptide chain Access the text alternative for slide images. Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 52 26  01/09/2023 1.To start protein synthesis, a ribosome binds to mRNA. The ribosome also has two binding sites for tRNA, one of which is occupied by a tRNA with its amino acid. Note that the codon of mRNA and the anticodon of tRNA are aligned and joined. The other tRNA binding site is open. 2. By occupying the open tRNA binding site, the next tRNA is properly aligned with mRNA and with the other tRN. 3. An enzyme within the ribosome catalyzes a synthesis reaction to form a peptide bond between the amino acids. Note that the amino acids are now associated with only one of the tRNAs. 4. The ribosome shifts position by three nucleotides. The tRNA without the amino acid is released from the ribosome, and the tRNA with the amino acids takes its position. A tRNA binding site is left open by the shift. Additional amino acids can be added by repeating steps 2 through 4. Eventually, a stop codon in the mRNA ends the addition of amino acids to the protein (polypeptide), which is released from the ribosome. Translation of mRNA to Produce a Protein Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 53 The Cell Cycle during growth and development, cell division occurs to increase the number of cells or replace damaged or dying ones this cell division involves a cell cycle includes two major phases: a non-dividing phase, called interphase, and a cell dividing phase, termed mitosis Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 54 27  01/09/2023 1. Interphase is the time between cell divisions. DNA is found as thin threads of chromatin in the nucleus. DNA replication occurs during interphase. 2. In prophase, the chromatin condenses into chromosomes. Each chromosome consists of two chromatids joined at the centromere. 3, In metaphase, the chromosomes align in the center of the cell 4. In anaphase, the chromatids separate at the centromere and migrate to opposite poles 5. In telophase, the chromosomes disperse, the nuclear envelopes and the nucleoli form, and the cytoplasm continues to divide to form two daughter cells. 6. Mitosis is complete, and a new interphase begins. The chromosomes have unraveled to become chromatin. Cell division has produced two daughter cells, each with DNA that is identical to the DNA of the parent cell. The Cell Cycle Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 55 Differentiation a sperm cell and an oocyte unite to form a single cell, then a great number of mitotic divisions occur to give the trillions of cells of the body the process by which cells develop with specialized structures and functions is called differentiation during differentiation of a cell, some portions of DNA are active, but others are inactive active and inactive sections of DNA differ with each cell type results from the selective activation and inactivation of segments of DNA Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 56 28  01/09/2023 Apoptosis Apoptosis, termed programmed cell death, is a normal process by which cell numbers within various tissues are adjusted and controlled in the developing fetus, apoptosis removes extra tissue, such as cells between the developing fingers and toes in some adult tissues, apoptosis eliminates excess cells to maintain a constant number of cells within the tissue damaged or potentially dangerous cells, virus-infected cells, and potential cancer cells are also eliminated by apoptosis Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 57 Because learning changes everything. ® www.mheducation.com Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 29