Introduction to Veterinary Science Chapter 1 Basic Cell Biology PDF

Chapter 1 Basic Cell Biology 3 VetBooks.ir A Day in the Life There Just Never Seems to...

Chapter 1 Basic Cell Biology 3 VetBooks.ir A Day in the Life There Just Never Seems to be a Typical Day... I headed to the office with the thought of doing only cow work on this particular day. However, those plans were short lived. Shortly after I arrived at work, two nervous owners walked through the door with their Labrador re- triever. Poor Jake had just been run over by the own- er’s car! Amazingly, Jake was doing very well, although he was a bit excited. Apart from a couple of cuts on his jaw, he was ready to go home and play. Then, at my first farm call of the day, the farmer wanted me to look at his dog, Millie. Millie had a grape- © 2017 Cengage Learning® fruit-size lump under her jaw. The lump felt like it was full of fluid. I asked him to bring Millie to the office so I could work on her there. I finished my farm calls and headed back to the small animal clinic. Once there, I anesthetized Millie and made an in- cision into the skin. Pus flowed from the lump (Figure FIGURE 1–1 Draining an abscess on the side of the face of an 1–1). I flushed the large pocket left behind and started anesthetized cat. Millie on a course of antibiotics, drugs that fight bacte- rial infections. Although I do not know why it started, I surgery. The surgery went well, and I was able to re- do know Millie was fighting an infection with her body’s move the entire lump. cells. In private practice, cells affect me every day. Today Next I had the opportunity to remove a tumor from I saw Millie’s cells attacking the bacteria in her neck. Penny, a 12-year-old cocker spaniel. Last week I gave Penny, on the other hand, had cancer-causing r cells divid- Penny a physical examination and administered blood ing uncontrollably. To understand how mammals work and tests. Penny appeared healthy, and we elected to do how to treat them, I first had to learn how cells function. CELL MAKEUP of these molecules. They possess the basic structure of (CH2O)n (Figure 1–3). In this formula, n describes the Objective number of carbon atoms in the molecule. The genetic Explain the Molecular Makeup of Cells material in the cell has the five-carbon sugars ribose and deoxyribose. Glucose (blood sugar), a six-carbon Cells and their structures are composed of molecules. sugar, is used for energy in the cells. The amount of Biochemistry is the study of these molecules in living glucose in blood is routinely monitored. If there is too creatures. One goal of this chapter is to identify the dif- much or too little glucose in the blood, the animal will fering types of molecules and their properties. not function normally. In diabetes, the blood sugar Lipids or fats combine hydrogen, carbon, and oxy- increases to very high levels, but the animal does not gen in a form that is poorly dissolvable in water (this utilize it properly. Diabetes requires treatment to lower is why fat floats to the top of water). Fat consists of the blood sugar. a molecule of glycerol and three fatty acid molecules Polysaccharides are composed of many monosac- (Figure 1–2). Fats are stored in the cells of the body as charides. One example of a polysaccharide is starch, a source of high energy. such as glycogen, which is used to store energy within Phospholipids are similar but have only two fatty the cell. Glycogen is made when monosaccharides acid groups and a phosphate group (PO4). This is sig- are taken into the cell and then assembled into a long nificant because one end of the molecule is attracted to chain. Polysaccharides can be joined with protein mol- or soluble in water (hydrophilic) and the other end is ecules to form glycoproteins, which assist in building repelled by water (hydrophobic). These characteristics the cell structure. of phospholipids are important in the structure of the Proteins play a key role in the structure and func- cell membrane. tion of cells. Proteins make up 50% of the dry weight of Carbohydrates supply energy and provide struc- animals. Proteins are large molecules of many amino ture within the cell. Monosaccharides are the simplest acids. (Twenty-two different amino acids are used to  4 Unit 1 Comparative Anatomy and Physiology VetBooks.ir © 2017 Cengage Learning® © 2017 Cengage Learning® FIGURE 1–4 Chemical structure of selected amino acids. make proteins; Figure 1–4.) A single protein can include 200 to 300 of these amino acids. It was mentioned earlier that proteins could be joined to sugars. They may also FIGURE 1–2 Chemical structure of glycerol, a fatty acid, and a typical be joined with lipids and phosphate groups. Protein lipid. molecules are not only very large but also quite com- plex molecules. Chemical bonding between amino acids will fold the amino acid chains into a three-dimensional structure. This complex structure is essential for the function of certain protein molecules. Proteins have many functions in cells. Muscle is largely composed of protein that is specially arranged to allow cells to contract and move. Further, enzymes are protein molecules that speed the chemical reactions in the body (i.e., enzymes act as catalysts). Proteins also add strength to many of the structures in the body. Proteins are found within the cell membrane and are commonly found in the intercellular matrix of tissues. © 2017 Cengage Learning® Protein can bind with other molecules to aid in their transport in the bloodstream. In addition, proteins found in blood help to carry oxygen, stop bleeding, and fight off infection. These infection-fighting proteins are called antibodies. In practice, antibodies specific FIGURE 1–3 Chemical structure of selected sugars. to different diseases are measured in the blood, thus Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  Chapter 1 Basic Cell Biology 5 VetBooks.ir Base (Purine or Pyrimidine) H © 2017 Cengage Learning® OH FIGURE 1–5 Chemical structure of a nucleotide. allowing veterinarians to diagnose what specific organ- Notice that the bases are the same except for thymine ism is causing the sickness. and uracil. The order of base combination determines Nucleic acids provide plans for the differing con- what amino acids are used to make proteins. This infor- struction of proteins. Nucleic acids are fabricated with mation is stored in the cell’s genetic material. a series of nucleotides. The nucleotides are made up Both DNA and RNA have a backbone of sugar al- of a five-carbon sugar, a phosphate group, and a ni- ternating with phosphate. The nitrogenous bases are trogen-containing base (Figure 1–5). Ribonucleic acid attached to this backbone. In DNA, a double-stranded (RNA) claims ribose as its sugar, whereas deoxyribo- molecule is formed as the bases are loosely bonded nucleic acid (DNA) has deoxyribose as its sugar. There together. The molecule has a twisted structure, which are four different bases for RNA and DNA (Table 1–1). is described as a double helix (Figure 1–6). The bases join, specifically, thymine to adenine and cytosine to guanine. Later in the chapter, a process of tran- scription will be described, in which the sequence of Table 1–1 RNA and DNA Bases DNA nitrogenous bases is converted to a molecule DNA Bases RNA Bases of RNA. In this situation, adenine in the DNA mole- cule bonds to a uracil base of RNA. The sequence of 1. Adenine 1. Adenine nitrogenous bases is used to define the amino acids © 2017 Cengage Learning® 2. Cytosine 2. Cytosine used in protein synthesis. A group of three nitroge- nous bases is the code for a specific amino acid. The 3. Guanine 3. Guanine order of the nitrogenous bases makes up the genetic code of the animal. Each gene provides the code for 4. Thymine 4. Uracil one peptide chain. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  6 Unit 1 Comparative Anatomy and Physiology Prote Protein rotein in VetBooks.ir Phos Ph Phospholipid osph pho olipid © 2017 Cengage Learning® R epelllle epe Repelled ed by Water Water Attracted ttracted to W Attracted Water ater Phosphate Sugar FIGURE 1–7 Illustration of cell membrane. The cell membrane has a dou- ble layer of phospholipid. In addition, protein molecules are present on Hydrogen Bonds and within the phospholipid layers. the inside of the cell contained. The cell membrane is so fine that it cannot be seen with a normal light micro- scope. The cell membrane is about half protein and half lipid (phospholipid type). One end of phospholipids is attracted to water, whereas the other end is repelled by water. The cell membrane, which is surrounded by water on both sides, has two layers of lipid in its wall (Figure 1–7). The ends of the lipid that are attracted to water face outward. Protein is also included in the mem- brane, both between the lipid molecules and on the sur- face. The position of the protein molecules is not firmly established; rather, the molecules are mobile within the membrane. Cholesterol, another molecule in the cell membrane, provides stabilization of the membrane. Cell membranes are semipermeable, meaning they allow certain substances but not others to pass. Some © 2017 Cengage Learning® molecules, such as water, are able to pass through easily. The specialized proteins in the cell membrane influence Cytosine Guanine which molecules are able to pass readily. In addition, the intrinsic membrane proteins can act as receptors. These Thymine Adenine receptors can process a signal from the extracellular fluid to influence the cell’s interior (e.g., a hormone can FIGURE 1–6 DNA structure: The structure is described as a double helix. Phosphate and sugar groups make up the two strands. The strands are trigger a reaction within the cell). Other molecules, such joined by hydrogen bonds between two nitrogenous bases. as proteins, starches, and some ions, are unable to pass. Many of the organelles within the cell are also sur- rounded by a membrane. The basic structure remains CELL STRUCTURE the same for all the membranes. The specifics of the makeup differ, depending on function. Objective Cell contents are divided into the nucleus and the Identify the Basic Structures of the Cell and Their cytoplasm. Cytoplasm generally describes the organelles Corresponding Functions and fluid in the cell. A nucleus comes as a standard part of most cells (with a few exceptions such as the red blood Many cell types exist. These cells not only look dif- cell; Figure 1–8). The nucleus contains the genetic material ferent but function differently as well. Nevertheless, (i.e., DNA) of the cell, which controls cellular activities many features are common among cells. Specialized by coding for protein synthesis. The DNA in the nucleus structures within the cells are called organelles. These is called chromatin. As the cell divides, the chromatin organelles are present in most but not all cells. Red clumps into chromosomes. Identical DNA is passed to blood cells, for example, lack a nucleus. all daughter cells. All the cells in the body have the same The cell membrane (or plasma membrane) is com- chromatin. However, cells take on different roles by using mon to all cells. It serves as the boundary that keeps certain areas of the chromatin more than others.  Chapter 1 Basic Cell Biology 7 Ribosomes Centriole Lysosome VetBooks.ir © 2017 Cengage Learning® Smooth Endoplasmic Reticulum FIGURE 1–8 Illustration of cell structure. A membrane made of two lipid bilayers surrounds Smooth endoplasmic reticulum (SER) has no the nucleus. This membrane is often joined to other ribosomes attached. This form is not as common. Some organelles, such as the endoplasmic reticulum and liver cells contain a large amount of SER. The SER in ribosomes. Such a close association helps the nucleus these cells produces glycogen and lipids, and removes control cell function. toxins. In cells not dividing, a nucleolus is often seen in the The Golgi apparatus is formed with large amounts nucleus. The nucleolus produces RNA that forms the of folded membrane that looks similar to SER. The ribosomes, which in turn produce protein. Cells with Golgi apparatus produces polysaccharides and special large nucleoli actively produce protein. protein sacs called lysosomes. Protein produced in Ribosomes are small granular-like structures that the RER is moved to the Golgi apparatus. The Golgi can be found in the cytoplasm. They contain roughly apparatus then changes the protein and collects it in 60% RNA and 40% other protein. Ribosomes manufac- the lysosomes. These sacs are pinched from the Golgi ture the protein used in the cell. Growing cells require apparatus and then moved to the surface of the cell and large amounts of protein and, therefore, have a greater released. number of ribosomes. The specific proteins produced The proteins contained in the lysosomes are by a cell are governed by the nucleus. enzymes (remember, enzymes are molecules that help The endoplasmic reticulum (ER) is a collection speed chemical reactions in the body). Lysosomes con- of folded membrane. This membrane attaches to the tain enzymes that help to break down other mole- membrane of the nucleus. The ribosomes often line cules. Varying enzymes match differing molecules. this membrane, giving it a bumpy appearance and The membrane surrounding lysosome prevents the therefore its name, rough endoplasmic reticulum enzymes from attacking other parts of the cell. (RER). Protein produced by the ribosomes is then Lysosomes are used to digest food taken in by the deposited into the RER. These proteins can be further cell and to destroy cell structures no longer needed. In changed in the RER. This protein may be used by the Millie, the dog with the abscess, her white blood cells cell or moved to the surface of the cell for secretion. were using lysosomes to destroy bacteria. Cells that The protein is moved through the membrane in a pro- die in the body are eliminated when enzymes within cess called exocytosis, which will be discussed later lysosomes are released into the cytoplasm. This process in the chapter. of autolysis makes room for replacement cells. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  8 Unit 1 Comparative Anatomy and Physiology Mitochondria are small rod-shaped organelles Table 1–2 Components of the Extracellular Fluid VetBooks.ir found in varying numbers in cells. The more active the cell, the more mitochondria are present. Mitochondria Water have a double membrane, similar to the cell membrane. 2. Dissolved gases: oxygen, carbon dioxide The outer membrane is smooth and forms the shape of the mitochondria. The inner membrane is highly folded. 3. Inorganic ions These shelflike infolded ridges are called cristae. Macrominerals: sodium, potassium, chloride, phosphate, The role of mitochondria is well defined. The mito- calcium, bicarbonate chondria convert food substances into energy that can Trace minerals: copper, zinc, manganese, cobalt, be used by the cell. Mitochondria contain the enzymes selenium, fluoride, iron necessary for this process. Because of this role, mito- 4. Organic compounds (carbon-containing compounds): chondria are called the powerhouses of the cell. The proteins, amino acids, lipids, carbohydrates, vitamins © 2017 Cengage Learning® mitochondria are found within cells at their areas of highest activity. 5. Hormones: compounds produced by glands to influence metabolism of cells CELL FUNCTION 6. Waste products Objective eliminated by the ECF. Without elimination, the waste Review the Basic Function of the Cell products actually become toxic to the cell. Many of the products in ECF must be maintained The cell constantly reacts to its environment. Metabolism at constant normal concentrations. Cells will be unable describes all the reactions going on in cells. Metabo- to function properly if there is too much or too little of lism can be categorized into two main types. Anabolism certain products. Glucose provides an excellent exam- describes reactions in which smaller molecules are ple. Small puppies can become low in blood sugar if combined into larger ones. The joining of amino acids they have too many parasites robbing them of nutri- to form proteins serves as an example. Catabolism, the ents. When the sugar in ECF becomes too low, the cells opposite, occurs when large molecules are broken down do not have adequate energy. The puppy can become into smaller ones. The breaking down of glycogen to weak or, in severe cases, develop a seizure. Homeo- release energy is an example of catabolism. stasis is the maintenance of ECF. Homeostasis allows A liquid called extracellular fluid (ECF) surrounds maintenance of normal concentrations of molecules in living cells. The ECF supplies cells with all the prod- spite of a wide variety of external conditions. ucts necessary for their functions. ECF is derived from Cells must be able to obtain products from the blood. The outermost skin cells are not covered in liq- ECF. It is not enough that the chemicals just exist in uid; however, they are no longer living. the ECF; there must be means for their exchange with Other cells exposed to the surface, such as those of the cell. Table 1–3 summarizes the mechanisms by the eye, need moisture. In the eye, tears produced by which materials are exchanged across the cell mem- glands act as the source of moisture and nutrients. The brane. The first mechanism is a process called diffusion eyelids help to sweep the tears across the surface of the (Figure 1–9), in which molecules move from higher to eye. Certain breeds of dogs, such as the pug, have eyes lower concentrations. Because molecules are always that bulge from the eye socket. The bulging can be so moving, there is a greater chance that they will move severe that the eyelids cannot keep the surface of the toward areas of lower concentration. This movement eye moist with tears. This results in a disease condition continues until the concentrations are equalized. on the surface of the eye. Artificial tears are often used The cell membrane does not allow totally free to keep the surface moist. diffusion. Diffusion is influenced by the size of the Table 1–2 summarizes the makeup of ECF. Water molecule, its charge, and its ability to dissolve in lipid. is the major component of ECF. Oxygen passes to the In general, the smaller the molecule, the more easily cells through the ECF. Conversely, carbon dioxide passes from the cells through it. There are many inor- Table 1–3 Mechanisms of Cellular Exchange ganic ions in the ECF. Some ions, macrominerals, are present in large amounts. Trace minerals are present in 1. Diffusion much smaller amounts. Both macrominerals and trace minerals are essential for cellular function. Many of 2. Osmosis © 2017 Cengage Learning® the trace minerals are needed for enzymes to function. 3. Active transport Organic compounds, including the lipids, proteins, and carbohydrates, are also delivered by the ECF. 4. Endocytosis Metabolism produces waste products, which must be 5. Exocytosis removed from the cells. These waste products are Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  Chapter 1 Basic Cell Biology 9 Molecule Hypotonic Solution VetBooks.ir Hypertonic Solution Water © 2017 Cengage Learning® Selectively Permeable Membrane FIGURE 1–10 Osmosis: The semipermeable membrane prevents the passage of large molecules. In this situation, water moves across the membrane to equalize the concentration. In certain situations, a cell may require a higher concentration of a molecule than is found in the ECF. For example, red blood cells have higher levels of potassium than the surrounding fluid. Diffusion con- stantly attempts to equalize the concentrations (e.g., potassium continually diffuses from the cell). In this case, the potassium is pumped back into the cell, and the higher concentration is maintained. This process is referred to as active transport (Figure 1–11). Active transport requires the cell to burn energy and use en- zymes to aid the process. Many different cell types per- form the function. Another example occurs in intestinal cells, which transport glucose into the bloodstream, © 2017 Cengage Learning® where it is present at higher levels. Large molecules, such as proteins, must be moved through the membrane in a process called endocytosis Molecule at Equilibrium Receptor Site FIGURE 1–9 Diffusion: Random movement of molecules allows Outside equalization of concentrations across a membrane. the diffusion occurs. Some large molecules such as proteins are unable to diffuse through the membrane and must be transported in other ways. As previously learned, the property of allowing Cell Membrane only certain molecules to diffuse through the mem- brane is called semipermeability. This characteristic sets the stage for a special type of diffusion, called osmosis. A solvent (in the following case, water) moves across © 2017 Cengage Learning® the membrane to equalize the concentration; however, the molecules dissolved in the water (called solutes) cannot pass through the membrane (Figure 1–10). This Energy process can be observed in red blood cells when they Inside are placed in a concentrated solution. The water from Transport Channel the cell moves outward into the solution. Microscopi- FIGURE 1–11 Active transport: Energy is used to actively pump molecules cally, the red blood cells can be seen to shrink. into a region of higher concentration. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  10 Unit 1 Comparative Anatomy and Physiology Cytoplasm Extracellular enzyme begins at a specific series of bases (thymine, VetBooks.ir Fluid adenine, cytosine) called a promoter. The RNA poly- merase moves along the length of the DNA molecule, creating a complementary strand of RNA. The RNA bases are added in the specific order that bonds to the © 2017 Cengage Learning® bases of the DNA. The corresponding bases were dis- cussed earlier in the chapter. This process continues un- til the polymerase reaches a terminator series of bases (adenine, thymine, thymine). The mRNA is released and the DNA helix reconnects. FIGURE 1–12 Endocytosis: A large particle is engulfed by the cell membrane and brought into the cytoplasm within a vacuole. DNA mRNA Double Helix Strand Cytoplasm Extracellular Fluid © 2017 Cengage Learning® FIGURE 1–13 Exocytosis: A membrane-bound sac joins with the cell C membrane to release the particle. (Figure 1–12). During endocytosis, the cell membrane wraps around the particle, pinches off, and moves into the cytoplasm as a vacuole. Lysosomes then join with the vacuole, providing the enzymes necessary to break down the particle. The smaller fragments produced are then released into the cell. RNA In cells producing protein, the opposite process oc- Polymerase curs. In exocytosis, a membrane-bound sac containing the protein joins with the cell membrane and releases it into the ECF (Figure 1–13). These sacs are produced within the Golgi apparatus. In intestinal cells, fat drop- lets can be taken into the cell through endocytosis. The vacuole is transported across the cell and released into RNA Nucleotide the bloodstream by exocytosis. PROTEIN SYNTHESIS Objective G Describe the Process of Protein Synthesis As mentioned previously, every cell contains all the genetic material of the animal. The expression of cer- © 2017 Cengage Learning® tain genes produces specific proteins that allow cell Cytosine Uracil specialization. Protein synthesis begins within the nucleus on the basis of the DNA structure. During transcription, Adenine Thymine information within the DNA is transferred to a strand of messenger RNA (mRNA) that moves into the cytoplasm. Guanine An enzyme called RNA polymerase binds to FIGURE 1–14 Transcription of mRNA: RNA polymerase separates the DNA, causing a separation of the double-helix strands strands of DNA and creates a strand of mRNA coded by the nucleotides (Figure 1–14). This pulling apart exposes a gene. The of the DNA molecule. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  Chapter 1 Basic Cell Biology 11 Color codes for the bases VetBooks.ir A G C T U Nontranscribed Strand 5' 5' 3' Transcription DNA 3' 3' mRNA Transcribed Strand (Complementary Copy of Transcribed mRNA DNA Strand) 5' Codon 1 Codon 2 Codon 3 Codon 4 Codon 5 Codon 6 © 2017 Cengage Learning® Translation O H2N C OH FIGURE 1–15 Translation: The mRNA created in transcription is used to code the amino acid sequence in protein formation. Translation, which occurs in the ribosomes, is the as the epithelia lining the intestinal tract, divide fre- process in which the code of bases in the mRNA is con- quently to maintain the integrity of the layer. Other verted to a series of amino acids. Each series of three cells, such as skeletal muscle, do not divide in an adult. bases in the mRNA is a codon (Figure 1–15). The codon When these normal controls break down, the cells provides the signal for a specific amino acid. The mol- can begin to undergo frequent mitosis. Uncontrolled ecule of mRNA is bound by ribosomes. A molecule of mitosis results in cancer. New cells are produced more transfer RNA (tRNA) that contains the three complemen- quickly than needed, resulting in an accumulation or tary bases (anticodon) attaches to the mRNA. Each mole- mass of cells in a region. This mass of rapidly dividing cule of tRNA carries the amino acid specific to the codon. cells is called a tumor. Enzymes on the ribosome allow release of the amino In a nondividing cell, the genetic material is called acid from the tRNA. A peptide bond is created between chromatin. In this form, the chromatin is loosely ar- adjacent amino acids. This process is repeated along the ranged in the nucleus. The individual chromosomes length of the mRNA molecule, creating a polypeptide. cannot be seen with a light microscope. These cells are The proteins created may be used within the cytoplasm described as being in the interphase. In this stage, the or processed further within the endoplasmic reticulum. cell is in the process of doubling its DNA. The steps of division are broken down into four phases (Table 1–4). The phases are identified to help understand the MITOSIS AND CANCER Objective Table 1–4 Stages of Mitosis Discuss Mitosis and its Clinical Significance in Diseases Such as Cancer 1. Interphase Cells must reproduce. In mitosis, the cells divide, pro- 2. Prophase © 2017 Cengage Learning® ducing two identical cells. Mitosis is necessary for the 3. Metaphase growth and maintenance of the animal. Cell division is controlled by a number of factors present within the 4. Anaphase cell and the extracellular fluid. The rate of cell division 5. Telophase is adapted to the needs of the animal. Some cells, such Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  12 Unit 1 Comparative Anatomy and Physiology VetBooks.ir © 2017 Cengage Learning® Interphase Early Prophase Late Prophase Metaphase Anaphase Telophase Interphase FIGURE 1–16 Mitosis. Interphase: Cell in its normal state, as the chromosomes begin to replicate. Prophase: The chromatin thickens and becomes visible, taking on an X shape. The nucleoli and nuclear membrane begin to disappear. Metaphase: The spindle forms between two centrioles. The chromosomes align on the spindle. Anaphase: Chromosomes split at the centromeres, with each half moving to opposite ends. Telophase: The nucleus reforms and a groove divides the two new cells. process. However, actual cell division is a continuous the cells. These factors can either stimulate cell divi- process, as seen in Figure 1–16. sion or inhibit it. Other factors found in the ECF help Prophase begins as the chromatin thickens into vis- to control cell death in a process called apoptosis. A ible chromosomes. This is the first time that the individ- classic example of this balance occurs in the cells that ual chromosomes can be seen with a light microscope. line the gastrointestinal tract. Cells at the base of the Along with this process, the nucleoli and nuclear mem- lining divide frequently at a rate that balances with the brane begin to disappear. At this point, the chromosomes cells undergoing apoptosis and death at the surface of show the doubling that occurred during interphase. The the lining. The programmed cell death is designed to chromosomes have an X shape. The two identical halves, occur whenever there is significant cell damage such or chromatids, are joined at a small point called the cen- as mutations. tromere. Two small organelles, the centrioles, separate Different cells divide at varying rates. Cells found and move to opposite ends of the cell. in certain areas such as the bone marrow and linings of In metaphase, a spindle is formed between the the gastrointestinal tract have stem cells that actively two centrioles. This is a collection of microtubules that divide on a regular basis. Other cells found in organs stretch between the two centrioles. The chromosomes such as the liver, kidney, and pancreas do not routinely move to the center of the cell and align themselves on divide. However, following injury or disease, these the spindle. cells can become activated into frequent mitosis to al- As anaphase begins, the chromosomes split at the low repair of the organ. A few specialized cells such as centromere. At this point, the chromosomes are still nerve and muscle cells have very limited or no ability on the spindle. Each chromatid begins to move out- to divide. ward. The centromere portion moves first, giving the chromosome a V shape. The chromosomes move to opposite ends of the cell. MAMMALIAN REPRODUCTION Telophase is basically the reverse of prophase. The chromosomes become loosely organized into chro- Objective matin. The nuclear membrane and nucleoli return. A Detail Meiosis in Mammalian Reproduction groove then forms down the center of the cell. This groove deepens until two identical cells are produced Mammals rely on sexual reproduction for species sur- in a process called cytokinesis. vival. In sexual reproduction, a sperm cell and egg cell Mitosis is essential in maintaining the population join to form the new embryo. In this process, half of the size of cells in the body. The number of cells is estab- genetic material is provided by each of the cells. Mei- lished on the basis of the frequency of mitosis, the dif- osis is the division in which the resulting cells contain ferentiation of cells, and cell death. An increase in cell only half of the genetic material. number can occur if the rate of cell division increases or There are two cell divisions during meiosis, with the rate of death decreases. In certain instances, a com- only one doubling of the chromatin. The final result is bination of these two changes has a cumulative effect. the formation of four cells, each with half the number The rate of cell division is controlled by soluble of chromosomes. Just as in mitosis, meiosis divides into factors found in the extracellular fluid surrounding phases (Figure 1–17). Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  Chapter 1 Basic Cell Biology 13 The second division of meiosis is similar to mitosis. VetBooks.ir The net effect of meiosis is the formation of four cells, each with half of the original number of chromosomes. Prophase I: Mammals should have an even number of chromosomes. The chromosomes come in pairs, and each member of a pair is called a homologue. Before prophase I, the homologue of each pair replicates and is formed by two strands (or chromatids). The chro- Early Prophase I Middle Prophase I Late Prophase I matids are joined by a centromere. Prophase I is very (Leptotene) (Zygotene) (Diplotene) complex. The basic process allows the homologues to pair up near the center of the cell. In this arrangement, the homologues are joined at several points. At these points, an exchange of DNA fragments occurs. Metaphase I: This step is very similar to mitosis. The nuclear membrane and nucleoli begin to disap- pear. The paired chromosomes move into alignment on the spindle. The important distinction in meiosis is that the homologues align themselves where they will Metaphase I Anaphase I be divided into opposite cells. Anaphase I and Telophase I: In anaphase, the chro- mosomes begin to move to opposite ends of the cell. In this step, the centromere does not split. Rather, the pairs of chromosomes are divided. The length of interphase between the two divisions Telophase I is variable, and may even be zero (i.e., this phase may not occur at all). The two cells produced enter into the second division. The second division of meiosis is basi- cally the same as mitosis. In this division, the chromo- somes align on the spindle, separate at the centromere, Prophase II and send one strand to each new cell. The stages are named just as they are in mitosis. The final result of meiosis is the formation of four cells, each with half the number of chromosomes of the original cell. Meiosis allows genetic material to be provided from each parent. The exchange of genetic material be- Metaphase II tween homologues in prophase I produces variability in each cell. Offspring acquire traits from each parent. With the variation, no two sperm or egg cells will pro- vide the same genetic material. Twinning can result in two animals having the same genetic makeup. Identical twins occur when an embryo splits. Each half then develops into a new em- bryo. The resulting offspring begin life with identical Anaphase II chromosomes. Even identical twins, however, do not appear completely identical. There is variation in the way the genes are expressed. © 2017 Cengage Learning® CLINICAL PRACTICE Telophase II Objective FIGURE 1–17 Meiosis: A two-division process. Prophase I: Homologous Connect Cellular Parts and Function to Clinical Veterinary chromosomes align in the center of the cell. The homologues exchange Practice segments of genetic material. Metaphase I: Similar to the step in mitosis, except that the homologues will separate into opposite cells. Anaphase I: In clinical practice, the appearance of cells is often The chromosomes move to opposite ends of the cell. Telophase I: The cell is evaluated. A biopsy takes tissues or cells from an divided into two daughter cells, each with half of the chromosome numbers animal for microscopic review. This procedure allows of the original cell. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  14 Unit 1 Comparative Anatomy and Physiology diagnosis. In tumors, the cells divide without normal Photograph courtesy Dr. Arthur Hattel, Pennsylvania State University. VetBooks.ir control. This leads to a mass in the tissue or in an organ. Tumors are divided into two major groups, benign and malignant. Benign tumors are localized to one area, have a well-defined margin, and do not spread to other parts of the body. Malignant tumors are more likely to invade surrounding tissues and spread to other parts of the body. For example, cells from a tumor may break away and move into the bloodstream or lymph vessels and then settle into a new location. The spread of a tumor from its primary location is called metastasis. The word tumor, or neoplasm, can be used to describe either a benign or malignant mass. The term cancer is typically used in FIGURE 1–18 Photomicrograph (high power) of cancer (squamous cell reference to a malignant tumor. With biopsy, cells are carcinoma) in the skin of a horse. There is variation in the shape and size evaluated to determine the type of tumor that is pres- of the nucleus and cells. Many mitotic figures can also be seen. ent. Penny’s tumor was submitted for biopsy. Fortu- nately, the pathologists (who interpret and diagnose changes in cells and tissues) found the tumor to be bloodstream. The walls of the smallest blood vessels benign and did not find any tumor cells at the margins (capillaries) act as a semipermeable membrane. They of the sample. This was great news for Penny, and we do not allow the protein within the blood to freely were optimistic that her tumor would not cause any move to the extracellular fluid. Water, on the other more problems. hand, is able to move freely in either direction. Tumors develop because the cells are growing Osmosis occurs across the walls of the capillar- rapidly and dividing without normal control. Changes ies. With normal blood protein levels, a balance is that occur in DNA allow a tumor cell to divide, inde- reached between the extracellular fluid and the liq- pendent of the inhibitory and stimulating control of uid portion of blood. As protein levels decline, there normal growth factors. The tumor begins from a single is less osmotic pressure to keep water within the defective cell. Further division can allow even further blood vessels. As protein continues to decline, water mutations that influence some other aspect of the cell accumulates within the tissues. In sheep with severe cycle. For example, tumor cells lose susceptibility to parasitism, the most common site for this fluid accu- factors that control apoptosis. Normal cells require at- mulation to occur is below the jaw. tachment to other cells and surrounding matrix. Cancer Sadie, a nine-year-old Papillon, presented with a cells develop the ability to replicate without this at- distended abdomen. Previous workup by a specialist tachment. This feature is critical to allow for metastasis had discovered a kidney problem that allowed protein with cells spread from the original tumor being able to to leak into the urine. Table 1–5 shows that Sadie’s replicate at a new location. total protein had dropped below normal. This was Many features of cancer can be predicted with primarily due to the decrease in albumin. Sadie’s liver this information. In cancerous cells, there is often a was unable to make albumin quickly enough to replace large nucleus with many nucleoli. The chromatin is what was lost. As a result, Sadie developed ascites, often clumped and visible. There are many more cells an accumulation of free fluid within the abdomen involved in the process of cell division than in normal (Figure 1–19). On presentation, Sadie weighed four tissue. Furthermore, many of the dividing cells have pounds, and more than one half pound of fluid was an abnormal spindle. Only with an understanding of the normal cell cycle can we interpret these abnormal findings (Figure 1–18). Bottle jaw is a term used to describe an accumula- Table 1–5 Blood Results for Sadie, a 9-Year-Old Spayed tion of fluid within the tissues under the jaw. Edema Female Papillon describes excess fluid within a tissue. In bottle jaw, it is described as pitting edema, because finger pres- Sadie Reference Range © 2017 Cengage Learning® sure into the tissue creates a “pit” that only slowly Total protein 3.4 g/dl 5.2–8.2 resolves. Bottle jaw occurs in severely parasitized sheep. Parasites within the intestinal tract consume Albumin 1.0 g/dl 2.2–3.9 such a large amount of protein that the animal is Globulin 2.4 g/dl 2.5–4.5 unable to maintain normal protein levels within the Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  Chapter 1 Basic Cell Biology 15 VetBooks.ir © 2017 Cengage Learning® FIGURE 1–19 Photograph of Sadie showing her abdomen distended with ascites. drawn from the abdomen. This lowered the pressure In peritoneal dialysis, an electrolyte solution is on her internal organs and provided immediate relief. infused into the abdomen through a catheter. Urea The low protein level meant the ascites would rapidly is able to diffuse across the membrane that lines the recur. To slow this progression, Sadie was given a solu- abdomen. Because there is no urea within the infused tion of hetastarch into her bloodstream. Hetastarch is electrolyte solution, the urea diffuses from the higher a very large molecule that increases osmotic pressure concentration within the bloodstream into the solution. within the bloodstream, helping to slow the recurrence The solution is then removed from the body, effectively of ascites. The hetastarch slowly leaves the blood and reducing the level of urea in the bloodstream. Dialysis thus provides only a temporary benefit. The goal in this must be repeated to remove the urea that is subse- case is to improve the osmotic pressure until a more quently produced. Dialysis is used to maintain the permanent cure can be found. animal in the hope that the kidneys will recover from Diffusion and osmosis are used therapeutically in the toxic effect. the process of dialysis. Dialysis is a treatment option in Understanding cellular function is essential for kidney failure. As will be discussed later in the text, the sound veterinary practice. Although there is great kidneys serve to maintain fluid and electrolyte balance similarity among all mammals, there are species dif- within the body. In addition, the kidneys function to ferences. This chapter has discussed enzymes and their remove many forms of soluble waste products. Ethyl- importance in cellular function. Cells contain enzymes ene glycol found in certain antifreeze products is ex- that break down medication (that is why medicines tremely toxic to the kidneys. Unfortunately, pets often must be given more than once). Furthermore, differ- find the antifreeze palatable (i.e., tasty) and will ingest ences between animals can lead to reactions to med- enough to severely damage the kidneys. ications. Cats possess much less of certain enzymes When the kidneys do not function adequately, that break down many medications. Acetaminophen toxins increase in the bloodstream, and many electro- (Tylenol), a common over-the-counter pain reliever, lyte levels become imbalanced. The complete process has been proved quite safe for use in humans. Because of dialysis is quite complex and requires extensive of the differences in the enzymes between species, this monitoring to be done correctly. Urea will be used as product is very dangerous for cats. As little as half a an example to describe the function of diffusion in di- tablet designed for adult humans can make a cat sick. alysis. Urea is a breakdown product in the metabolism Cellular detail seems far removed from a complete an- of protein. The kidneys normally excrete the urea into imal. However, the details of cells guide treatment of the urine. With kidney failure the urea level increases animals in clinical practice. in the bloodstream and acts as a toxin. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  16 Unit 1 Comparative Anatomy and Physiology SUMMARY VetBooks.ir Successful students of veterinary science begin their turn can help veterinarians assist producers in making study by mastering the understanding of cells. A thor- financial decisions concerning their livestock. Similarly, ough knowledge of cellular makeup, including struc- another cellular activity, mitosis, can result in cancer if tures and functions, will help veterinary students build uncontrolled. Here again, veterinary practitioners rely a strong foundation of information, based on which a on their cellular understanding to assist in diagnosis more comprehensive investigation of body processes and treatment of that dreaded disease. The examina- can start. Moreover, knowing how cellular activities tion of cells sets the groundwork for the study of vet- such as meiosis take place gives a more complete erinary science. understanding of the reproductive system, which in REVIEW QUESTIONS 1. Define any 10 of the following terms: 3. True or False: Larger molecules diffuse more read- ily than smaller molecules. anesthetize antibiotics 4. True or False: Smooth endoplasmic reticulum cancer (SER) contains ribosomes. lipid 5. All cellular reactions are collectively called hydrophilic. hydrophobic glucose 6. Extracellular fluid (EFC) surrounds all living cells diabetes and is derived from. glycogen 7. Too much blood sugar indicates which disease? enzymes antibodies 8. How many different types of amino acids are used exocytosis to make proteins? metabolism 9. Give another name for the cell membrane. anabolism 10. Where is chromatin found within a cell? catabolism homeostasis 11. Do mammals have an even or odd number of diffusion chromosomes? osmosis 12. Do enzymes function identically in all species? active transport endocytosis 13. How might cancer cells differ from a normally benign dividing cell? malignant 14. List five mechanisms of cellular exchange. pathologists 15. List the four stages of mitosis. 2. True or False: Fats easily dissolve in water. ACTIVITIES Materials needed for completion of activities: 1. Does fat really float as explained in the cellular several beakers molecular component section? Assemble several assorted fats, such as cooking and motor oil beakers of water. Add drops of assorted fats, water such as cooking and motor oil. Observe the food coloring results. heat source 2. Pretend the classroom is a cell, with the walls being eggs the cell membrane. Your instructor will assign you dissecting pins a cellular part. Review your function within the cell Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203  Chapter 1 Basic Cell Biology 17 and your relationship to other cell parts (other stu- 6. Research the latest treatments available for ca- VetBooks.ir dents). Present your information to the group. nine cancers. Report the findings to classmates. Advancing technologies continually give new 3. Add one drop of food coloring to a beaker of wa- hope to owners whose pets are diagnosed with ter. Observe for evidence of diffusion. Does heat cancer. influence diffusion? Investigate by trying the ac- tivity with water samples of varying temperature. 7. Students can self-test for pitting edema of the foot or ankle by pressing a thumb against the skin 4. Using a small dissecting pin, pick away the shell and watching to see how quickly the depression at the air sac end of an egg. Leave the inner shell resolves. If any concerns regarding pitting arise, membrane intact. Place the egg under water in a medical attention should be sought. beaker. Watch osmosis cause the inner shell mem- brane to rupture. 5. Several interactive quizzes can be found online to assist students in learning the processes of mitosis and meiosis. Investigate these options as a means of further study. Copyright 2017 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203