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2.4B: Dehydration Synthesis - Biology

2.4B: Dehydration Synthesis - Biology


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In dehydration synthesis, monomers combine with each other via covalent bonds to form polymers.

Learning Objectives

  • Explain dehydration (or condensation) reactions

Key Points

  • During dehydration synthesis, either the hydrogen of one monomer combines with the hydroxyl group of another monomer releasing a molecule of water, or two hydrogens from one monomer combine with one oxygen from the other monomer releasing a molecule of water.
  • The monomers that are joined via dehydration synthesis reactions share electrons and form covalent bonds with each other.
  • As additional monomers join via multiple dehydration synthesis reactions, this chain of repeating monomers begins to form a polymer.
  • Complex carbohydrates, nucleic acids, and proteins are all examples of polymers that are formed by dehydration synthesis.
  • Monomers like glucose can join together in different ways and produce a variety of polymers. Monomers like mononucleotides and amino acids join together in different sequences to produce a variety of polymers.

Key Terms

  • covalent bond: A type of chemical bond where two atoms are connected to each other by the sharing of two or more electrons.
  • monomer: A relatively small molecule which can be covalently bonded to other monomers to form a polymer.

Dehydration Synthesis

Most macromolecules are made from single subunits, or building blocks, called monomers. The monomers combine with each other via covalent bonds to form larger molecules known as polymers. In doing so, monomers release water molecules as byproducts. This type of reaction is known as dehydration synthesis, which means “to put together while losing water. ” It is also considered to be a condensation reaction since two molecules are condensed into one larger molecule with the loss of a smaller molecule (the water.)

In a dehydration synthesis reaction between two un-ionized monomers, such as monosaccharide sugars, the hydrogen of one monomer combines with the hydroxyl group of another monomer, releasing a molecule of water in the process. The removal of a hydrogen from one monomer and the removal of a hydroxyl group from the other monomer allows the monomers to share electrons and form a covalent bond. Thus, the monomers that are joined together are being dehydrated to allow for synthesis of a larger molecule.

When the monomers are ionized, such as is the case with amino acids in an aqueous environment like cytoplasm, two hydrogens from the positively-charged end of one monomer are combined with an oxygen from the negatively-charged end of another monomer, again forming water, which is released as a side-product, and again joining the two monomers with a covalent bond.

As additional monomers join via multiple dehydration synthesis reactions, the chain of repeating monomers begins to form a polymer. Different types of monomers can combine in many configurations, giving rise to a diverse group of macromolecules. Three of the four major classes of biological macromolecules (complex carbohydrates, nucleic acids, and proteins), are composed of monomers that join together via dehydration synthesis reactions. Complex carbohydrates are formed from monosaccharides, nucleic acids are formed from mononucleotides, and proteins are formed from amino acids.

There is great diversity in the manner by which monomers can combine to form polymers. For example, glucose monomers are the constituents of starch, glycogen, and cellulose. These three are polysaccharides, classified as carbohydrates, that have formed as a result of multiple dehydration synthesis reactions between glucose monomers. However, the manner by which glucose monomers join together, specifically locations of the covalent bonds between connected monomers and the orientation (stereochemistry) of the covalent bonds, results in these three different polysaccharides with varying properties and functions. In nucleic acids and proteins, the location and stereochemistry of the covalent linkages connecting the monomers do not vary from molecule to molecule, but instead the multiple kinds of monomers (five different monomers in nucleic acids, A, G, C, T, and U mononucleotides; 21 different amino acids monomers in proteins) are combined in a huge variety of sequences. Each protein or nucleic acid with a different sequence is a different molecule with different properties.


Understanding clinical dehydration and its treatment

Dehydration in clinical practice, as opposed to a physiological definition, refers to the loss of body water, with or without salt, at a rate greater than the body can replace it. We argue that the clinical definition for dehydration, ie, loss of total body water, addresses the medical needs of the patient most effectively. There are 2 types of dehydration, namely water loss dehydration (hyperosmolar, due either to increased sodium or glucose) and salt and water loss dehydration (hyponatremia). The diagnosis requires an appraisal of the patient and laboratory testing, clinical assessment, and knowledge of the patient's history. Long-term care facilities are reluctant to have practitioners make a diagnosis, in part because dehydration is a sentinel event thought to reflect poor care. Facilities should have an interdisciplinary educational focus on the prevention of dehydration in view of the poor outcomes associated with its development. We also argue that dehydration is rarely due to neglect from formal or informal caregivers, but rather results from a combination of physiological and disease processes. With the availability of recombinant hyaluronidase, subcutaneous infusion of fluids (hypodermoclysis) provides a better opportunity to treat mild to moderate dehydration in the nursing home and at home.


Types of Dehydration Synthesis

Dehydration synthesis can be classified based on a number of criteria. They can be grouped based on the nature of the reactants. Some reactants are molecules that have two functional groups, which can react with one another. For instance, amino acids contain an amine group and a carboxylic acid functional group attached to the same carbon atom.

The amine group of one amino acid can react with the acid group of another to form an amide bond and release one molecule of water. The newly formed amino acid dimer again contains one free amine group and one free carboxylic acid group allowing the reaction to proceed with more amino acids.

Such bi-functional monomers, therefore, give rise to linear products with the monomers attached to each other end-to-end. Alternatively, the reactants could have multiple functional groups, which can create branched products, such as the formation of glycogen from glucose molecules.

Secondly, dehydration reactions can be classified on the nature of the catalyst. In the example on the formation of symmetrical ethers, the catalyst is a hydrogen ion. But for many reactions, especially within a living organism, the pH, salt concentrations and temperature cannot be altered. In these conditions, the presence of some other catalyst is important for driving a reversible reaction in one direction. Biological catalysts are called enzymes and often derive their name from the nature of the reaction they catalyze. For instance, enzymes that catalyze the formation of DNA from deoxyribonucleotides through condensation reactions are called DNA polymerases. Proteins are modified with carbohydrate moieties through glycosylases. Occasionally enzymes that catalyze a dehydration reaction are also named based on the nature of the enzyme itself. Ribosomes catalyze the formation of the amide bond (also known as the peptide bond) between two amino acids. Since the catalytic region within the ribosome is made predominantly of RNA rather than protein, it is also known as an RNA enzyme or ribozyme.

Alternatively, dehydration reactions can be classified based on the product they produce. In biological systems, most dehydration reactions create polymers. Therefore, these reactions can be grouped based on whether they create complex carbohydrates from simpler monosaccharides, form fatty acids from acetyl coA or synthesize proteins from amino acids.

Finally, dehydration reactions are also involved in the modification of biological molecules such as nucleosides, proteins and carbohydrates. These modifications include phosphorylation and glycosylation and are important for regulating the properties and functions of biopolymers. This is particularly crucial in many signaling cascades where protein kinases (enzymes that catalyze the phosphorylation of proteins) are involved.


Dehydration Synthesis and Hydrolysis Practice

2. Match the correct prefix or suffix or definition to its meaning/word below.

DEHYDRATE HYDRO- SYNTHESIS -LYSIS MONOMER POLYMER

  1. To split or break apart release -lysis
  2. To make something Synthesis
  3. Many monomers hooked together make a Polymer
  4. Means to lose or remove water to take water away Dehydrate
  5. Means water (as in gaining water) Hydro-
  6. Building block or single unit of a polymer is monomer

3. Examine each example. Indicate if each of the following is an example of dehydration synthesis or hydrolysis.

Reaction #2: Dehydration hydrolysis

Reaction #3: Dehydration hydrolysis

Our body digests a piece of streak into individual amino acids.

Our body links glucose molecules together to form glycogen, an energy storage molecule in your body.

4. Explain in your own words: How can you tell if a chemical equation represents:

  1. Dehydration synthesis? Water is taken away creates bonds
  1. Hydrolysis? Water is added and breaks bonds.

5. Analyze the following diagrams to answer the questions that follow.

Below is the chemical reaction to form a peptide, or very small protein.

  1. Is this an example of a synthesis or hydrolysis reaction?
  2. What are the reactants of this reaction? 2 amino acids
  3. What are the products of this reaction? Dipeptide
  4. If we were to create a protein chain with 9 amino acids, how many water molecules would be produced in the process? 4.5?
  5. During a dehydration synthesis reaction, if 42 water molecules were made how many amino acids were joined together to make the protein? 21

Below is the chemical reaction to form a triglyceride.

  1. Is this an example of a synthesis or hydrolysis reaction? Synthesis
  2. What are the reactants of this reaction? Glycerol and 3 fatty acid chains
  3. What are the products of this reaction? Triglyceride, or neutral fat
  4. If we were to break down 10 triglyceride molecules, how many water molecules would we need to complete the reaction? 30
  5. How many water molecules are lost when you create 32 triglyceride molecules?

6. Cellulose is an integral molecule in cell wall formation, the sturdy outer structure that surrounds every plant cell. It is the most abundant organic polymer on our planet! Below is a diagram that shows how cellulose is produced in a plant cell.


Biology 171


Elements in various combinations comprise all matter, including living things. Some of the most abundant elements in living organisms include carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus. These form the nucleic acids, proteins, carbohydrates, and lipids that are the fundamental components of living matter. Biologists must understand these important building blocks and the unique structures of the atoms that comprise molecules, allowing for cells, tissues, organ systems, and entire organisms to form.

All biological processes follow the laws of physics and chemistry, so in order to understand how biological systems work, it is important to understand the underlying physics and chemistry. For example, the flow of blood within the circulatory system follows the laws of physics that regulate the modes of fluid flow. The breakdown of the large, complex molecules of food into smaller molecules—and the conversion of these to release energy to be stored in adenosine triphosphate (ATP)—is a series of chemical reactions that follow chemical laws. The properties of water and the formation of hydrogen bonds are key to understanding living processes. Recognizing the properties of acids and bases is important, for example, to our understanding of the digestive process. Therefore, the fundamentals of physics and chemistry are important for gaining insight into biological processes.

Learning Objectives

By the end of this section, you will be able to do the following:

  • Understand macromolecule synthesis
  • Explain dehydration (or condensation) and hydrolysis reactions

As you’ve learned, biological macromolecules are large molecules, necessary for life, that are built from smaller organic molecules. There are four major biological macromolecule classes (carbohydrates, lipids, proteins, and nucleic acids). Each is an important cell component and performs a wide array of functions. Combined, these molecules make up the majority of a cell’s dry mass (recall that water makes up the majority of its complete mass). Biological macromolecules are organic, meaning they contain carbon. In addition, they may contain hydrogen, oxygen, nitrogen, and additional minor elements.

Dehydration Synthesis

Most macromolecules are made from single subunits, or building blocks, called monomers . The monomers combine with each other using covalent bonds to form larger molecules known as polymers . In doing so, monomers release water molecules as byproducts. This type of reaction is dehydration synthesis , which means “to put together while losing water.”


In a dehydration synthesis reaction ((Figure)), the hydrogen of one monomer combines with the hydroxyl group of another monomer, releasing a water molecule. At the same time, the monomers share electrons and form covalent bonds. As additional monomers join, this chain of repeating monomers forms a polymer. Different monomer types can combine in many configurations, giving rise to a diverse group of macromolecules. Even one kind of monomer can combine in a variety of ways to form several different polymers. For example, glucose monomers are the constituents of starch, glycogen, and cellulose.

Hydrolysis

Polymers break down into monomers during hydrolysis. A chemical reaction occurs when inserting a water molecule across the bond. Breaking a covalent bond with this water molecule in the compound achieves this ((Figure)). During these reactions, the polymer breaks into two components: one part gains a hydrogen atom (H+) and the other gains a hydroxyl molecule (OH–) from a split water molecule.


Dehydration and hydrolysis reactions are catalyzed, or “sped up,” by specific enzymes dehydration reactions involve the formation of new bonds, requiring energy, while hydrolysis reactions break bonds and release energy. These reactions are similar for most macromolecules, but each monomer and polymer reaction is specific for its class. For example, catalytic enzymes in the digestive system hydrolyze or break down the food we ingest into smaller molecules. This allows cells in our body to easily absorb nutrients in the intestine. A specific enzyme breaks down each macromolecule. For instance, amylase, sucrase, lactase, or maltase break down carbohydrates. Enzymes called proteases, such as pepsin and peptidase, and hydrochloric acid break down proteins. Lipases break down lipids. These broken down macromolecules provide energy for cellular activities.

View visual representations of Hydrolysis and Dehydration Synthesis (Flash interactivity).

Section Summary

Proteins, carbohydrates, nucleic acids, and lipids are the four major classes of biological macromolecules—large molecules necessary for life that are built from smaller organic molecules. Macromolecules are comprised of single units scientists call monomers that are joined by covalent bonds to form larger polymers. The polymer is more than the sum of its parts: it acquires new characteristics, and leads to an osmotic pressure that is much lower than that formed by its ingredients. This is an important advantage in maintaining cellular osmotic conditions. A monomer joins with another monomer with water molecule release, leading to a covalent bond forming. Scientists call these dehydration or condensation reactions. When polymers break down into smaller units (monomers), they use a water molecule for each bond broken by these reactions. Such reactions are hydrolysis reactions. Dehydration and hydrolysis reactions are similar for all macromolecules, but each monomer and polymer reaction is specific to its class. Dehydration reactions typically require an investment of energy for new bond formation, while hydrolysis reactions typically release energy by breaking bonds.

Free Response

Why are biological macromolecules considered organic?

Biological macromolecules are organic because they contain carbon.

What role do electrons play in dehydration synthesis and hydrolysis?

In a dehydration synthesis reaction, the hydrogen of one monomer combines with the hydroxyl group of another monomer, releasing a molecule of water. This creates an opening in the outer shells of atoms in the monomers, which can share electrons and form covalent bonds.

Amino acids have the generic structure seen below, where R represents different carbon-based side chains.

Describe how the structure of amino acids allows them to be linked into long peptide chains to form proteins.

Amino acids can be linked into long chains through condensation reactions. One of the hydrogen atoms bonded to the nitrogen atom of an amino acid reacts with the –OH group attached to the terminal carbon on another amino acid. Since both ends of the molecule can participate in condensation reactions, peptide bonds can be made in both directions to create a long amino acid chain.

Glossary


Dehydration Synthesis - Research Article from World of Biology

Dehydration synthesis is a chemical reaction in which one or more molecules of water are removed from the reactants to form a new product. These reactions can occur when one of the reactants has a hydroxyl group (OH) that can be cleaved, thus forming the negatively charged hydroxide ion (OH - ). The other reactant must have a hydrogen atom which can be cleaved to yield a hydrogen, or hydronium, ion (H + ). In solution, these ions are free to combine and form a water molecule. The respective reactants are then able to form a chemical bond that creates a new compound.

Because of the ready availability of their hydroxyl groups, many alcohols readily participate in dehydration reactions. When an alcohol loses its hydroxyl group it can combine with other electron loving species to form a new reaction product. If the alcohol loses the hydroxyl group in addition to one of its own hydrogen atoms, a new carbon-carbon bond can be formed within the molecule. When this occurs the alcohol is transformed into an unsaturated compound or into an ether. The nature of the reaction depends on the class of the alcohol. For example, ethyl alcohol, or ethanol, undergoes dehydration to form ethylene or diethyl ether. Certain polymers are also formed by this type of reaction. Dehydration synthesis is essentially the opposite of hydrolysis which is the process of breaking down compounds by dissolving in water.


Three AP2/ERF family members modulate flavonoid synthesis by regulating type IV chalcone isomerase in citrus

Flavanones and flavones are excellent source of bioactive compounds but the molecular basis of their highly efficient production remains elusive. Chalcone isomerase (CHI) family proteins play essential roles in flavonoid biosynthesis but little are known about the transcription factors controlling their gene expression. Here, we identified a type IV CHI (designated as CitCHIL1) from citrus which enhances the accumulation of citrus flavanones and flavones (CFLs). CitCHIL1 participates in a CFL biosynthetic metabolon and assists the cyclization of naringenin chalcone to (2S)-naringenin, which leads to the efficient influx of substrates to chalcone synthase (CHS) and improves the catalytic efficiency of CHS. Overexpressing CitCHIL1 in Citrus and Arabidopsis significantly increased flavonoid content and RNA interference-induced silencing of CitCHIL1 in citrus led to a 43% reduction in CFL content. Three AP2/ERF transcription factors were identified as positive regulators of the CitCHIL1 expression. Of these, two dehydration-responsive element binding (DREB) proteins, CitERF32 and CitERF33, activated the transcription by directly binding to the CGCCGC motif in the promoter, while CitRAV1 (RAV: related to ABI3/VP1) formed a transcription complex with CitERF33 that strongly enhanced the activation efficiency and flavonoid accumulation. These results not only illustrate the specific function that CitCHIL1 executes in CFL biosynthesis but also reveal a new DREB-RAV transcriptional complex regulating flavonoid production.

Keywords: DREB RAV citrus flavanone flavone type IV chalcone isomerase.

© 2020 The Authors. Plant Biotechnology Journal published by Society for Experimental Biology and The Association of Applied Biologists and John Wiley & Sons Ltd.


Breakdown and Synthesis of Sucrose, Starch and Cellulose

Sucrose is broken down or hydrolysed to yield glucose and fructose in the presence of the enzyme invertase or sucrase. The reaction is irreversible.

Synthesis of Sucrose:

Synthesis of sucrose in plants may take place by 3 different ways:

(1) From Glucose-1-Phosphate and Fructose in the presence of the enzyme sucrose phosphorylase e.g., in bacteria.

(2) From UDPG (Uridine Di-Phosphate Glucose) and Fructose in the presence of the en­zyme sucrose synthetase e.g., in higher plants.

(3) From UDPG and Fructose-6-phosphate in the presence of the enzyme sucrose phos­phate synthetase e.g., in higher plants.

Sucrose-phosphate thus produced is hydrolysed in the presence of the enzyme phosphatase to yield sucrose.

Breakdown of Starch:

Breakdown or the hydrolysis of starch to yield its constituent a-D-Glucose units may take place in two ways:

(1) By the enzyme diastase:

In fact diastase is not a single enzyme but a complex of many enzymes which are as follows:

α-amylase and β-amylase attack 1 : 4 linkages of amylose and amylopectin (which constitute the starch) while R-Enzyme attacks 1 : 6 linkages of amylopectin, so that starch is hydrolysed to yield disaccharide units i.e., maltose. Finally, the enzyme maltase converts maltose into glucose molecules.

(2) By the enzyme starch phosphorylase.

Glucose-1-Phosphate may be converted into glucose by the enzyme phosphatase.

Synthesis of Starch:

Synthesis of starch involves the simultaneous synthesis of amylose (with α-(1: 4) glyco­sidic linkages) and amylopectin (with α-(1: 6) glycosidic linkages), the two important constitu­ents of starch.

(A) Synthesis of Amylose (Or α-(1: 4) Glycosidic Linkages):

Synthesis of amylose may take place by any of the following ways:-

(1) According to Hanes (1940) amylose can be synthesised in the presence of the en­zyme starch phosphorylase from glucose-1-phosphate and an acceptor molecule consisting of about 3 to 20 glucose units joined together by α-(1: 4) glycosidic linkages.

(2) Formation of α-(1 : 4) glycosidic linkages may also take place in the presence of the enzyme UDPG-transglycosylase (amylose synthetase) by the transfer of glucose from UDPG (Uridine Di Phosphate Glucose) to an acceptor molecule consisting of 2 to 4 or more glucose units joined together by α-(1 : 4) glycosidic linkages or even a starch molecule.

The structure of UDPG is given below:

UDPG (Uridine Diphosphate Glucose)

(3) According to Akazawa et al (1964) glucose molecule obtained as a result of the hydrolysis of sucrose in the presence of enzyme sucrase is transferred to UDP (Uridine Di Phos­phate) molecule to form UDPG. Form UDPG the glucose molecule is transferred to starch (Fig. 13.2)

(4) Formation of α-(1: 4) glycosidic linkages leading to the synthesis of amylose may also take place in the presence of D-Enzyme by the transfer of two or more glucose units from maltodextrins (consisting of more than two glucose units) to a variety of acceptors such as maltotroise, maltotetrose molecules.

(B) Synthesis of Amylopectin (Or α-(1: 6) Glycosidic Linkages):

It takes place in the presence of Q-Enzyme by the transfer of small chains of glucose units joined together by α-(1: 4) glycosidic linkages to an acceptor molecule consisting of at least four α (1:4) linked glucose units. The α-(1: 6) glycosidic bond is established between C-1 of the terminal glucose unit of donor molecule and C-6 of one of the glucose units of the acceptor molecule (Fig. 13.3).

Breakdown of Cellulose:

Cellulose is a straight chain polymeric carbohydrate molecule (a glucan), composed of a large number of D-glucopyranose units joined together by β(1 → 4) glycosidic linkages. In nature, cellulose is broken down by enzymatic hydrolysis through the enzymes called celluloses. These enzymes which are often grouped under generic name cellulase, randomly attack β(1 → 4) glycosidic linkages of the cellulose chain first forming cellodextrins and then disaccharides called as cellobiose. Cellobiose is then hydrolyzed to glucose by the enzyme cellobiose.

Cellulose degrading enzymes are not found in plants or humans. These are found only in certain organisms such as ruminants, termites, some bacteria and certain protozoa.

(Division Ruminantia of even-toed ungulates such as a deer, antelope, sheep, goat or cow).

Synthesis of Cellulose:

Long un-branched chains of cellulose (consisting of β(1→4) linked glucose residues) are synthesized in plants by the enzymes called cellulose synthases. The enzyme cellulose synthase is a multi-submit complex that is situated on plasma membrane and transfers a glucose residue from a sugar nucleotide donor called uridine diphosphate glucose (UDPG) to an acceptor molecule forming β (1 → 4) glucosyl acceptor.

UDPG + Acceptor → UDP + β (1→4) glucosyl-acceptor

It is believed that sterol-glycosides (i.e., sterols joined to a chain of one or more glucose units) such as β-sitosterol glucoside (Fig. 13.4), probably act as initial acceptors that start the elongation of cellulose chain. The process continues, and after the cellulose chain has attained desired length, the sterol is cut off from the glucan (Cellulose Chain) by the enzyme endoglucanase present in the plasma membrane. The separated cellulose chains are then extruded on the outer side of the plasma membrane (Fig. 13.5).

There are evidences to suggest that glucose in UDPG comes from sucrose, by the action of the reversible enzyme sucrose synthetase (Fig. 13.5). Alternatively, UDP-glucose may be directly obtained from cytoplasm.


Symptoms and progression

The symptoms of dehydration depend in part on the cause and in part on whether there is associated salt deprivation as well. When loss of water is disproportionately greater than loss of electrolytes (salt), the osmotic pressure of the extracellular fluids becomes higher than in the cells. Since water passes from a region of lower to a region of higher osmotic pressure, water flows out of the cells into the extracellular fluid, tending to lower its osmotic pressure and increase its volume toward normal. As a result of the flow of water out of the cells, they become dehydrated. This results in the thirst that always accompanies “pure” water depletion.

In those diseases in which there is loss of salt in excess of water loss, the decreased concentration of sodium in the extracellular fluid and in the blood serum results in decreased osmotic pressure, and water therefore enters the cells to equalize the osmotic pressure. Thus there is extracellular dehydration and intercellular hydration—and no thirst.

Water deprivation produces distinctive symptoms in humans. Weight loss, amounting to two to three pounds per day, occurs. Thirst is the most prominent symptom, with the dryness of mouth, decreased production of saliva, and impaired swallowing that accompany it. It is probable that thirst is the result of this subsequent intracellular dehydration and increased intracellular osmotic pressure. Experimentally, thirst can be produced when the cells have lost about 1 percent of their intracellular water.

As dehydration progresses, the tissues tend to shrink, the skin becomes dry and wrinkled, and the eyes become sunken and the eyeballs soft. Fever develops, possibly from mild to marked, as dehydration progresses. Dehydration itself probably affects the temperature regulatory centres in the brain. As dehydration and salt loss progress, however, the plasma volume and heart output decrease, with a consequent decrease in blood supply to the skin. Sweating decreases and may stop completely, and the main avenue for heat loss is closed. The body temperature may then rise precipitously.

There are marked changes in the volume of the extracellular and intracellular fluids, but the blood plasma volume changes the last and the least. The plasma volume is maintained more or less constant at the expense of the tissue fluids. If, however, the plasma volume does fall, the output of the heart also falls, and the pulse rate climbs, all of which indicates a dangerous physical state.

The renal ( kidney) changes that occur in humans during prolonged water depletion similarly tend to maintain a normal balance. If water deprivation continues and the plasma volume falls, however, the output of urine will be drastically reduced. As long as urine output of more than 30 millilitres (1 ounce) per hour is maintained, the kidney can excrete nitrogenous and nonnitrogenous solids with maximum efficiency. Once the urine flow is decreased below this level, the kidney is unable to function efficiently, the substances are retained in the body, and their concentration in the blood rises.

The final result of prolonged dehydration is now apparent. The normal distribution of salt and water in the body is destroyed, the plasma volume decreases, and the blood viscosity increases. As a result of these changes renal function is impaired, the urinary output falls, and waste products accumulate. Far more life-threatening, however, is decreased loss of moisture from the skin, with the subsequent rise in temperature, and the fall in cardiac output with the attendant irreversible shock.

Once renal failure occurs, about 8 percent of the total body water has been lost (4 litres [about 4.25 quarts]). When 5 to 10 litres (about 5.25 to 10.5 quarts) of body water have been lost, a person is acutely and severely ill, with contracted plasma volume, increased concentration and viscosity of the blood, renal failure and excessive urea in the blood, and falling blood pressure. In a previously healthy adult, death follows the loss of 12 to 15 litres (about 12.5 to 15.8 quarts) of body water. In the very young, the very old, or the debilitated, death occurs at a lower level of dehydration.


2.4B: Dehydration Synthesis - Biology

The discovery of the double-helical nature of DNA by Watson & Crick explained how genetic information could be duplicated and passed on to succeeding generations. The strands of the double helix can separate and serve as templates for the synthesis of daughter strands. In conservative replication the two daughter strands would go to one daughter cell and the two parental strands would go to the other daughter cell. In semiconservative replication one parental and one daughter strand would go to each of the daughter cells.

Through experimentation it was determined that DNA replicates via a semiconservative mechanism. There are three possible mechanisms that can explain DNA's semiconservative replication.

(a) DNA synthesis starts at a specific place on a chromosome called an origin. In the first mechanism one daughter strand is initiated at an origin on one parental strand and the second is initiated at another origin on the opposite parental strand. Thus only one strand grows from each origin. Some viruses use this type of mechanism.

(b) In the second mechanism replication of both strands is initiated at one origin. The site at which the two strands are replicated is called the replication fork. Since the fork moves in one direction from the origin this type of replication is called unidirectional. Some types of bacteria use this type of mechanism.

(c) In the third mechanism two replication forks are initiated at the origin and as synthesis proceeds the two forks migrate away from one another. This type of replication is called bi-directional. Most organisms, including mammals, use bi-directional replication.

Requirements for DNA Synthesis

There are four basic components required to initiate and propagate DNA synthesis. They are: substrates, template, primer and enzymes.

Four deoxyribonucleotide triphosphates (dNTP's) are required for DNA synthesis (note the only difference between deoxyribonucleotides and ribonucleotides is the absence of an OH group at position 2' on the ribose ring). These are dATP, dGTP, dTTP and dCTP. The high energy phosphate bond between the a and b phosphates is cleaved and the deoxynucleotide monophosphate is incorporated into the new DNA strand.

Ribonucleoside triphosphates (NTP's) are also required to initiate and sustain DNA synthesis. NTP's are used in the synthesis of RNA primers and ATP is used as an energy source for some of the enzymes needed to initiate and sustain DNA synthesis at the replication fork.

The nucleotide that is to be incorporated into the growing DNA chain is selected by base pairing with the template strand of the DNA. The template is the DNA strand that is copied into a complementary strand of DNA.

The enzyme that synthesizes DNA, DNA polymerase, can only add nucleotides to an already existing strand or primer of DNA or RNA that is base paired with the template.

An enzyme, DNA polymerase, is required for the covalent joining of the incoming nucleotide to the primer. To actually initiate and sustain DNA replication requires many other proteins and enzymes which assemble into a large complex called a replisome. It is thought that the DNA is spooled through the replisome and replicated as it passes through.

The major catalytic step of DNA synthesis is shown below. Notice that DNA synthesis always occurs in a 5' to 3' direction and that the incoming nucleotide first base pairs with the template and is then linked to the nucleotide on the primer.

DNA Synthesis is Semidiscontinuous

Since all known DNA polymerases can synthesize only in a 5' to 3' direction a problem arises in trying to replicate the two strands of DNA at the fork.

Notice that the top strand must be discontinuously replicated in short stretches thus the replication of both parental strands is a semidiscontinuous process. The strand that is continuously synthesized is called the leading strand while the strand that is discontinuously synthesized is called the lagging strand.

Leading Strand Synthesis

DNA synthesis requires a primer usually made of RNA. A primase synthesizes the ribonucleotide primer ranging from 4 to 12 nucleotides in length. DNA polymerase then incorporates a dNMP onto the 3' end of the primer initiating leading strand synthesis. Only one primer is required for the initiation and propagation of leading strand synthesis.

Lagging Strand Synthesis

Lagging strand synthesis is much more complex and involves five steps.

1. As the leading strand is synthesized along the lower parental strand the top parental strand becomes exposed. The strand is then recognized by a primase which synthesizes a short RNA primer.

2. DNA polymerase then incorporates a dNMP onto the 3" end of the primer and initiates lagging strand synthesis. The polymerase extends the primer for about 1,000 nucleotides until it comes in contact with the 5' end of the preceding primer. These short segments of RNA/DNA are known as Okazaki fragments.

3. When the DNA polymerase encounters the preceding primer it dissociates. The RNA is then removed by a specialized DNA polymerase or by an enzyme called RNaseH. Ribonucleotides are then excised one at a time in a 5' to 3' direction. The RNaseH leaves a phosphate group at the 5' end of the adjoining DNA segment thus leaving a gap.

4. The gap is filled by a DNA polymerase which uses an Okazaki fragment as a primer.

5. The 3' hydroxyl group on the 3' nucleotide terminus is then covalently joined, using DNA ligase, to the free 5' phosphate of the previously made lagging segment.

Structure of DNA

There are many types of DNA polymerases which can excise, fill gaps, proofread, repair and replicate.

Other Factors Required for DNA Synthesis

Origins: Origins are unique DNA sequences that are recognized by a protein that builds the replisome. Origins have been found in bacterial, plasmid, viral, yeast and mitochondrial DNA and have recently been discovered in mammalian DNA. Specific origins are used for initiating DNA replication in humans. Most origins have a site that is recognized and bound by an origin-binding protein. When the origin-binding protein binds to the origin the A + T rich sequence becomes partially denatured allowing other replication factors known as cis-acting factors to bind and initiate DNA replication.

Origin-binding Protein: binds and partially denatures the origin DNA while binding to another enzyme called helicase.

Helicases: unwind double stranded DNA.

Single-stranded DNA Binding Protein (SSB): enhances the activity of the helicase and prevents the unwound DNA from renaturing.

Primase: synthesize the RNA primers required for initiating leading and lagging strand synthesis.

DNA Polymerase: recognizes the RNA primers and extends them in the 5' to 3' direction.

Processivity Factors: help load the polymerase onto the primer-template while anchoring the polymerase to the DNA.

Topoisomerase: removes the positive supercoils that form as the fork is unwound by the helicase.

RNaseH: removes RNA portions from Okazaki fragments.

Ligase: seals the nicks after filling in the gaps left by DNA polymerase.

Coordination of Leading and Lagging Strand Synthesis

Leading and lagging strand synthesis is thought to be coordinated at a replication fork. The two polymerases are held together by another set of proteins, tg , which are near the fork that is being unwound and simultaneously primed by helicase-primase. Both polymerases are bound by a processivity factor, b . Upon completing an Okazaki fragment the lagging strand polymerase release the b factor and dissociates from the DNA. The tg complex then loads the new b factor/primer complex onto the lagging strand polymerase which initiates a new round.

Leading strand synthesis can proceed all the way to the end of a chromosome however lagging strand synthesis can not. Consequently the 3' tips of each daughter chromosome would not be replicated.

Telomerase ( also AKA telomere terminal transferase) extends the 3' ends of a chromosome by adding numerous repeats of a six base pair sequence until the 3' end of the lagging strand is long enough to be primed and extended by DNA polymerase.

Telomerase recognizes the tips of chromosomes also know as telomeres. The DNA sequences of telomeres have been determined in several organisms and consist of numerous repeats of a 6 to 8 base long sequence, [TTGGGG]n.

Telomeres have been found to progressively shorten in certain types of cells. These cells appear to lack Telomerase activity. When telomeric length shortens to a critical point the cell dies. Cells derived from rapidly proliferating tissues, such as tumors, have telomeres that are unusually long. This indicates that Telomerase activity may be necessary for the proliferation of tumor cells. Telomerase activity is found in ovarian cancer cells but not in normal ovarian tissue. Thus it may be possible to develop anti-tumor drugs that function to inhibit telomerase activity.

Chemical Inhibitors of DNA Replication

Some types of drugs function by inhibiting DNA replication.

Substrate Analogs: analogs of dNTP's which function as chain terminators can be incorporated into DNA. These analogs are usually either missing the 3' hydroxyl group or have a chemical group, other than hydroxyl, in the 3' position.

Cytosine Arabinoside: is an anticancer drug used to treat leukemia.

Azidothymidine (AZT): was used as an anti-HIV drug that, while effective in tissue culture experiments, proved to be ineffective for treating HIV in humans.

Acyclovir: is an effective anti-herpes virus drug.

Intercalating Agents: are compounds with fused aromatic ring systems that can wedge (intercalate) between the stacked base pairs of DNA. This disrupts the structure of the DNA so that the replicative enzymes have difficulty in synthesizing DNA past the "intercalated" sites. Anthracycline glycosides and Actinomycin D are intercalators used to treat a variety of cancers.

DNA Damaging Agents: a variety of compounds such as Cisplatin, cause chemical damage to DNA and are used in the treatment of cancers.

Topoisomerase Inhibitors: Nalidixic acid and Fluoroquinolones are antibiotics used to inhibit bacterial topoisomerases.


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