Vitamins and Nucleic Acids PDF

BBCCT-101 Indira Gandhi National MOLECULES OF LIFE Open University School of Sciences Block 4 VITAMINS AND NUCLEIC ACIDS UNIT 12 Vitamins 197 UNIT 13 Nucleic Acids 225 UNIT 14 Dynamics of Nucleic Acids 243 Block 4...

BBCCT-101 Indira Gandhi National MOLECULES OF LIFE Open University School of Sciences Block 4 VITAMINS AND NUCLEIC ACIDS UNIT 12 Vitamins 197 UNIT 13 Nucleic Acids 225 UNIT 14 Dynamics of Nucleic Acids 243 Block 4 Vitamins and Nucleic Acids.......................................................................................................................................................................... BLOCK 4 : VITAMINS AND NUCLEIC ACIDS In block 3 you have studied the structure and functions of lipids and their specialised properties like the immediate energy source, energy storage, signaling molecule and as plant steroids etc. Also, you had an account of membrane lipids and their significance in maintaining the integrity of a cell. The present block consists of 3 units (unit 12 to unit 14). In unit 12 you will study about structure and functions of water soluble and fat soluble vitamins. The unit also describes significance of vitamins as antioxidants and coenzymes. Unit 13 deals with the different aspects of nucleic acids like their constituents, arrangement and properties. In first part of this unit you will study how DNA is acting as a genetic material. The second part of this unit describes about structure, various types and functions of DNA & RNA. In Unit 14 you will study how various chemical and physical factors influence the structure and functions of DNA. The unit also deals with other significant biological functions of nucleotides such as energy source, coenzymes and secondary messengers. Objectives After studying this block you will be able to: l list various types vitamins soluble in water and fat; l describe the structure and biological significance of vitamins; l describe the basic unit of nucleic acids and its structural hierarchy; l explain the difference between DNA and RNA; l illustrate how DNA is acting as genetic material; l explain the biological significance of nucleic acids; and l recognise and explain the effect of temperature, acid and alkali on DNA structure. 196  UNIT 12 VITAMINS Structure 12.1 Introduction 12.5 Hypervitaminosis Expected Learning Outcomes 12.6 Vitamins as coenzymes 12.2 Classification of Vitamins and Antioxidants 12.3 Structure, Active forms and 12.7 Summary Functions of Water-Soluble Vitamins 12.8 Terminal Questions B-complex Vitamins 12.9 Answers Vitamin C 12.4 Structure, Active forms and 12.10 Further Reading Functions of Fat-Soluble Vitamins Vitamin A Vitamin D Vitamin E Vitamin K 12.1 INTRODUCTION So far, you have learned about proteins, carbohydrates and lipids which are the major constituents of living cells. In unit 4, you studied about proteins. As you know all enzymes are proteins that function as catalysts in regulating the metabolic pathways and other life processes. Enzymes employ coenzymes or metal ions (cofactor) to assist them in catalysis. Coenzymes are non-protein organic molecules that are mostly precursors of vitamins. Therefore, this unit deals with vitamins. Vitamins are organic molecules that are essential for proper functioning and growth of the human body. As many of these molecules perform the role of coenzymes and antioxidants in the human body. Vitamins are considered as micronutrients that require in small amounts for maintaining health. Although, human body cannot synthesize vitamins and we must take them from the dietary sources. Generally, whole grains, green vegetables, fruits and animal products are rich sources of vitamins. However, an inadequate amount of vitamins, may result in vitamin-deficiency (hypovitaminosis) diseases such as night blindness, scurvy and rickets etc. In addition, abnormal accumulation of vitamins in the body leads to toxicity and this condition is known as hypervitaminosis. 197 Block 4 Vitamins and Nucleic Acids.......................................................................................................................................................................... Therefore, in this unit, you would learn about classification, structure, active forms and biochemical function of water soluble vitamins and fat soluble vitamins. This unit will also discuss the deficiency diseases, symptoms and recommended daily allowance (RDA) of vitamins. Hypervitaminosis and the role of vitamins as coenzymes and antioxidants will be discussed in this unit. Objectives ____________________________________ After studying this unit, you should be able to:  classify vitamins into water soluble vitamins and fat soluble vitamins;  describe structure, active forms and functions of the water soluble vitamin and fat-soluble vitamins;  explain deficiency diseases, symptoms, dietary requirements and dietary sources of vitamins;  illustrate hypervitaminosis; and  enlist role of vitamins as coenzymes and antioxidants. 12.2 CLASSIFICATION OF VITAMINS A Polish biochemist Casimir Funk coined the term ‘vitamin’ in 1912 who designated it as essential factor for life (Vita means “important” and amine means “possessing amine group”). However, all vitamins are not amines. The structure of all vitamins contain hetrocyclic ring of hydrogen, nitrogen, carbon, oxygen. Vitamins are classified into two broad categories on the basis of their solubility (Fig. 12.1). Vitamins generally are designated by alphabets A, B, C, D, E and K in which alphabet B is referred to the B-complex vitamins by B 1, B2, B3, B4, B6, B12. Vitamins Water soluble vitamins Fat soluble vitamins  Vitamin A  Vitamin D B-Complex vitamins Ascorbic acid (Vitamin C)  Vitamin E Thiamin (Vitamin B1)  Vitamin K Riboflavin (Vitamin B2) Niacin (Vitamin B3) Partoethenic acid (Vitamin B5) Pyridoxin (Vitamin B6) Biotin Folic acid Cobalamin (Vitamin B12) Fig. 12.1 : Classification of Vitamins 198 Unit 12 Vitamins.......................................................................................................................................................................... 12.3 STRUCTURE, ACTIVE FORMS AND FUNCTIONS OF WATER-SOLUBLE VITAMINS Let us first study about the water soluble vitamins. As the name implies, water-soluble vitamins are soluble in water and excreted out in urine, thus preventing the toxicity due to overdose. This group composed of B-complex vitamins and vitamin C. 12.3.1 B-complex Vitamins B-complex vitamins are so called because they are group of eight vitamins like thiamin (vitamin B1), riboflavin (vitamin B2), niacin (vitamin B3), pantothenic acid (vitamin B5), pyridoxine (B6), biotin, folic acid and cobalamin (vitamin B12). They serve as a coenzymes in many enzymatic reactions and help to release energy by metabolizing food stuff in the human body. 1. Thiamin (Vitamin B1) Thiamin is the first vitamin of B-complex group which was discovered in people affected with beriberi disease. It can be synthesized by bacteria, fungi and plants. It is a colorless compound which is found in cereals, groundnuts, pistachios, walnuts, mustard seeds, soybean and capsicum. The structure of thiamin consists of pyrimidine and a thiazole ring that are linked by methylene group (CH2,) as shown in the (Fig. 12.2). Thiamin named as thiol (sulfur group, SH-) containing vitamin. Thiamin is converted into biologically active form, thiamin pyrophosphoate in the human body. Fig. 12.2: Strucutre of Thiamin Active form: Thiamin pyrophosphate (TPP) is active form of thiamin. The OH- group of thiazole ring is phosphorylated (two phosphate groups) (Fig. 12.3). Fig. 12.3: Strucutre of Thiamin pyrophosphate Biochemical functions: TPP is involved in the dehydrogenation and decarboxylation reactions of carbohydrate and amino acid metabolis. 199 Block 4 Vitamins and Nucleic Acids.......................................................................................................................................................................... It participates in the oxidative decarboxylation of -ketoacids like pyruvate, - ketogltarate and ketoacids derived from branched chain amino acids, leucine, isoleucine and valine. It serves as a coenzyme in many enzymes like transketolase, pyruvate dehydrogenase complex, -ketoglutarate dehydrogenase and branched-chain amino acid dehydrogenase complex. Deficiency diseases and symptoms: Thiamin deficiency causes beriberi disease which occurs in three forms: Recommended 1) Dry beriberi: It is associated with neurological manifestations. Dietary Allowance (RDA): Is the average 2) Wet beriberi: It effects on mental confusion, muscle atrophy and edema. daily dietary nutrient intake level sufficient to 3) Infantile beriberi: Infant’s cardiovascular system is generally affected. meet the nutrient requirement of nearly The common symptoms are muscular weekness, loss of sensation, vomiting, all (97 to 98 percent) confusion. It mosly affects neurological and cardiovascular system. Thiamine healthy individuals in a particular life stage and containing balanced diet or supplements require for the treatment of beriberi gender group (ICMR, disease. 2009) Recommended Dietary Allowance (RDA): 1.4 mg/day for male and 1.1 mg/ day for female. 2) Riboflavin (Vitamin B2) Riboflavin is the second vitamin in the the B-complex group known as vitamin B2. Yeast, milk, liver, eggs and leafy vegetables are good sources of this vitamin. It consists of ribose (a pentose sugar) and a flavin molecule (an isoalloxazine ring) (Fig. 12.4). It was isolated by Kuhn, Gyorgy and Wagner in 1933. Riboflavin has intense yellow color and is widely used as a food additive. Ribose Flavin (Isoalloxazine ring) Fig. 12.4: Strucutre of Riboflavin 200 Unit 12 Vitamins.......................................................................................................................................................................... Active form Flavin adenine dinucleotide (FAD) is a riboflavin derivative (Fig. 12.5). Fig. 12.5: Structure of Flavin Adenine Dinucleotide Biochemical functions : Riboflavin involves in the metabolism of carbohydrates, lipids and amino acids. FAD is a coenzyme which play a role in the transferring of electrons in many metabolic reactions. It acts as electron carrier in oxido-reduction reactions of oxidative phosphorylation, fatty acid and amino acid catabolism and citric acid cycle.FAD-dependent enzymes are pyruvate dehydrogenase complex, - ketoglutarate dehydrogenase complex, succinate dehydrogenase, acyl CoA dehydrogenase, and glycine oxidase. Deficiency diseases and symptoms: Lack of riboflavin in the body leads to sympotms like fatigue, slow growth; cheilosis, glossitis dry and scaly skin (seborrheic dermatitis), abnormal vision and corneal inflammation. RDA: 1.7 mg/day for male and 1.3 mg/day for female. 3) Niacin (Vitamin B3) The term ‘Niacin’ is referred collectively to nicotinamide and nicotinic acid. It is known as vitamin B3. Niacin is the non-toxic derivative of the toxic tobacco alkaloid, nicotine. The amide derivative (CO-NH2) of nicotinic acid is called nicotinamide. Niacin was discovered as nutrient while studying pellagra COOH CONH 2 disease. It is synthesized from tryptophan (an amino acid). Peanuts, legumes, meat, eggs and milk are rich sources of niacin. Nicotinic acid Nicotinamide Active forms: Nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) are active forms of niacin. Fig. 12.6 shows the structure of NAD and NADP in which nicotinamide group is attached to adenine dinucleotide. NADP differs from NAD in the presence of additional phosphate group on 2' position of the Ribose ring that carries adenine moiety. 201 Block 4 Vitamins and Nucleic Acids.......................................................................................................................................................................... + + Fig. 12.6: Structures of NAD and NADP Biochemical functions : NAD and NADP as a coenzyme primarily acts as hydrogen acceptor and electron carrier in wide variety of oxidation and reduction reactions of metabolism. At least 200 reactions of cellular metabolism are known where role of these coenzymes is well defined. Oxidized NAD is an electron acceptor and its reduced form NADPH is electron donor in many biochemical reactions. FAD, NAD-dependent enzymes are dehydrogenase enzymes such as lactate dehydrogenase, pyruvate dehydrogenase complex, -ketoglutarate dehydrogenase etc. Besides, NAD plays an important role in DNA repair and mobilization of calcium ions in the body. Deficiency disease and symptoms: Pellagra disease results from the deficiency of niacin. The early symptoms are vague, weakness, anorexia, indigestion and dizziness. Photosensitive dermatitis, skin infections and digestive problems which are associated with niacin or tryptophan deficiency. RDA: 18 mg/day for male and 14 mg/day for female. SAQ 1 a) Choose the correct option. i) Vitamins are essential nutrient compounds. [True/False] ii) TPP participates in the carboxylation reactions. [True/False] iii) Niacin vitamin contains Isoalloxazine ring. [True/False] iv) Niacin deficiency causes pellagra disease [True/False] v) Riboflavin has a thiazol ring. [True/False] vi) Thiamin is a precursor of FAD. [True/False] 202 vii) Thiamine deficiency causes beriberi disease. [True/False] Unit 12 Vitamins.......................................................................................................................................................................... b) Fill in the blanks with correct answer(s). i) The first vitamin in the group of Vitamin B complex is ……………. ii) Thiamin consists of …………………. iii) ………………………is the coenzyme form of thiamin. iv) The deficiency disease of thiamin is................... v) The structure of riboflavin is composed of………… and ………… vi) …………………and ………. are active forms of riboflavin. vii) Niacin synthesized from …………………. viii) The coenzyme of niacin are ……………. and ………………… 4)P antothenic acid (Vitamin B5) Pantothenic acid is also known as vitamin B5 which was identified by Roger J. Williams in 1931. It is the peptide of  -alanine and pentoic acid (Fig.12.7). Pantothenic acid has a central role in energy-yielding metabolism.Human body cannot synthesize pantothenic acid, we must obtain it from dietary sources like whole grains, broccoli, avocados, cereals and mushrooms. Peptide linkage Pantoic acid -Alanine Fig. 12.7: Structure of Pantothenic acid Biochemical functions : Pantothenic acid involes in the formation of coenzyme A (A for acyl). Coenzyme A (CoA) consists of cysteine, pantothenic acid and phospho-adenosine diphosphate (Fig. 12.8). Pantothenic acid is one of the functional moiety of CoA. It helps to carry acyl groups for many biochemical reactions such as fatty acid synthesis, metabolism of carbohydrate and amino acids. CoA is a component of acyl carrier protein (a component of fatty acid synthase) involves in fatty acid synthesis. CoA requires for the formation of acetyl-CoA and succinyl-CoA in the oxidative decarboxylation of glucose and in the Crebs cycle respectively. It also required for the acetylation of choline to form the neurotransmitter acetylcholine in the brain. 203 Block 4 Vitamins and Nucleic Acids.......................................................................................................................................................................... Fig. 12.8: Strucutre of Coenzyme A Deficiency disease and symptoms: Deficiency of pantothenic acid is often related to low energy-level symptoms including fatigue and weakness. RDA: The dietary intake of pantothenic acid is 5 mg/day for male and female. 5. Pyridoxine (Vitamin B6) This vitamin exists in three different forms -pyridoxine, pyridoxamine and pyridoxal. It is found in eggs, liver, yeast, cereals, legumes and milk. The structure of vitamin B6 possess single pyridine ring having hydroxyl goup in pyridoxine, an amino group in the pyridoxamine and aldehyde group in the pyridoxal as shown in Fig. 12.9. P)yridoxine Pyridoxamine Pyridoxal Fig. 12.9: Strcutre of vitamin B6 Active form: The biologically active form of vitamin B6 is Pyridoxal-5- phosphate (PLP) ( Fig.12.10) 204 Fig. 12.10: Structure of Pyridoxal-5-phosphate Unit 12 Vitamins.......................................................................................................................................................................... Biochemical functions: Vitamin B6 plays an important role in the metabolism of amino acids, fatty acids, glycogen and also as a steroid hormone. Pyridoxal- 5-phosphate serves as a coenzyme in many enzymatic reactions of amino acid metabolism e.g. transamination, deamination, decarboxylation, racemization and aldol reactions. Pyridoxal-5-phosphate required for transferring of amino group from one molecule to another in the transamination reaction of amino acid metabolism. Pyridoxal-5-phosphate dependent enzymes are aspartate transaminase, alanine transaminase, glutamic decarboxylase and gamma-aminobutyric acid transaminase involved in many metabolic reactions. Deficiency diseases and symptoms: The clinical signs and symptoms of pyridoxine deficiency include anemia, dermatitis, peripheral neuropathy, gastrointestinal disorders, nausea and vomiting. RDA: The diatary intake of vitamin B6 is about 2.0 mg/day for male and female and 2.5 mg/day for pregnant women. 6) Biotin Biotin is a water soluble vitamin. It was isolated from the egg by Dutch biochemist Fritz Kogl in 1935 later named it as biotin. It is necessary for cell growth, metabolism of lipids and amino acids. Yeast, nuts, tomato, animal meat, and egg yolk are some of the well-known sources of biotin. The structure of biotin consists of imidazole and thiophene ring along with a fatty acid side chain (Fig. 12.11). It is widely found in many food sources in the form of biocytin. It is linked through a peptide bond with the amino acid lysine. Fig. 12.11: Structure of Biotin Active form: Biocytin Biochemical functions: Biotin serves as a coenzyme for many carboxylase enzymes such as pyruvate carboxylase, acetyl-CoA carboxylase and propionyl-CoA carboxylase. It involves in transferring carbondioxide (CO 2) in carboxylation reactions. It is covalently bound to the enzyme via an amide linkage with a lysine residue to form biotincytin (Fig 12.12). 205 Block 4 Vitamins and Nucleic Acids.......................................................................................................................................................................... Fig. 12.12: Strcutre of Biocytin Deficiency diseases and symptoms: The general deficiency symptoms of biotin include loss of appetite and growth; dermatitis, hair loss and thickening of bones. RDA: About 30 g/day is recommended for male and female. 7) Folic acid The term, Folic acid is derived from Latin word ‘folium’ which stands for ‘leaf’. It is also known as pteroylglutamic acid or folacin. Plants are a rich source of folic acid especially, spinach leaves. Green vegetables, whole grains, cereals, yeast, soybean and eggs are some of the other important sources of this vitamin. Folic acid consists of a pteridine ring, para-amino benzoic acid (PABA), and glutamic acid (Fig. 12.13). Pterindine P-Aminobezoic acid Glutamate Fig. 12.13: Strucutre of Folic acid Active forms: Tetrahydrofolate is active form of dietary folic acid (Fig. 12.14). It exits in two forms: 5,10-methylene tetrahydrofolate (THF) and 5-methyltetrahydrofolate in the human body: 206 Unit 12 Vitamins.......................................................................................................................................................................... Fig. 12.14: Strucutre of Tetrahydrofolate Biochemical functions: Tetrahydrofolic acid is a coenzyme in many biochemical reactions, especially in the synthesis of amino acids and nucleic acids (DNA). It works closely with Vitamin B12 in the synthesis of blood cells (RBC) and the amino acid homocysteine. 5,10-methylene tetrahydrofolate required for the methylation and repair of nucleic acids while 5-methyltetrahy-drofolate required for the formation of amino acid methionine. In addition, folate involved acid the formation of healthy red RBCs and prevent anemia especially in infants and pregnant women. Deficiency diseases and symptoms: The deficiency of folic acid affects the synthesis of nucleic acids and amino acids. The deficiency of folic acid is most common in pregnant women. The deficiency in either folic acid or Normal blood cells in the blood smear. The vitamin B12 can lead to megaloblastic anemia which cause clinical size, shape, and color of symptoms like weakness, fatigue, depression, breathing problems and the red blood cells-show fetal neural tube defect in pregnant womens. hat they are normal. Mature red blood cells RDA: About 200 g/day is recommended dose for adult male and female. have lost their nuclens. 8) Cobalamin (Vitamin B12) Vitamin B12 contains cobalt metal ion in the center of coring ring hence known as cobalamin. It is the heaviest (molecular weight = 1355.4kd) and most complex of all the vitamins. Vitamin B12 consists of a class of chemically related compounds known as vitamers (exhibiting vitamin activity). Neither plants nor animals are capable to synthesizing vitamin B12. Bacteria have the enzymes required for the synthesis of vitamin B 12. Dietary sources of vitamin B12 are animal meat (espically liver), milk Megaloblastic blood cells products, eggs and shellfish. Vitamin B12 is required for the normal in the bone marrow (imma- ture stage of development). functioning of the brain, nervous system and formation of red blood cells They still have their nuclei (RBCs). Vitamin B12 requires the specific protein, intrinsic factor for its and are slightly larger than absorption in the human body. Due to lack of this factor causes vitamin normal red blood cells. B12- deficiency in the human body. Strucutre: Vitamin B12 is found as hydroxocobalamin in bacteria but it is converted into methyl cobalamin and 5’-deoxyadenosylcobalamin in human body. The chemical structure of cyanocobalamin was determined 207 Block 4 Vitamins and Nucleic Acids.......................................................................................................................................................................... by Dorothy Crowfoot Hodgkin in which 5-deoxyadenosyl group replaces the cyano group attached to cobalt in the sixth coordination position. Vitamin B12 is composed of Cobalt ion (Co3+), corrin ring system, dimethylbenzimidazole (DMB) and 5 deoxy-adenosine as shown in Fig. 12.15. Corrin stands for “core” component of vitamin B12. A Corrin ring is heterocyclic compound and its structure is similar to the porphyrin ring of hemoglobin. Fig. 12.15: Strcture of Vitamin B12 Active form: Methyl cobalamin and adenosyl cobalamin Biochemical functions: Methyl cobalamin and adenosyl cobalamin are two coenzymes of Vitamin B12 which are involved in metabolism of amino acids and synthesis of hemoglobin. Vitamin B12 acts as a coenzyme for two enzymes: methionine synthase and L-methylmalonyl-CoA mutase. (Fig. 12.16). Vitamin B12 participates in the transfer and rearrangement of the methyl group in biochemical reactions. 208 Unit 12 Vitamins.......................................................................................................................................................................... Fig. 12.16: Synthes is to methionine via transfer of methly group form Vit B12 to homocysteine Deficiency diseases and symptoms: The major signs of vitamin B12 deficiency are pernicious anemia and neuropathy. The pernicious anemia disease is characterized by low hemoglobin levels, decreased number of erythrocytes and neurologic manifestation. RDA : The recommended doses are 3 g/day for both male and female and up to 0.5-1.5 g/day for children. 12.3.2 Vitamin C (Ascorbic Acid) It is also known as vitamin C. Unlike B-complex vitamins, vitamin C acts as an antioxidant molecule which also falls in the group of water soluble vitamins. Citrus fruits, peanuts, tomatoes and green vegetables are rich sources of this vitamin. The structure of vitamin C has six corbon atoms with hydroxyl groups that resemble a glucose to some extent. It is synthesized in most of the animals and plants but it cannot be synthesized by human body due to lack of an enzyme L-gulonolactone oxidase. Hence it is essential to take this vitamin through diet. This vitamin occurs in two forms; L-ascorbic acid (reduced form) and L-dehydro ascorbic acid (oxidized form) in the cells (Fig. 12.17). L-Ascorbic acid Dehydroascorbic acid Fig. 12.17: Structure of Vitamin C 209 Block 4 Vitamins and Nucleic Acids.......................................................................................................................................................................... Biochemical functions: Vitamin C serves as a water soluble antioxidant that involve in neutralising free radicals in the cells. It protects cells from oxidative damage. Besides, vitamin C is required for the hydroxylation of proline and lysine for the formation of collagen in the skin. It also involved in the metabolism of folic acid, tyrosine and tryptophan. Vitamin C helps in wound healing, bone and teeth formation, improving immunity and absorption of iron from the diet. It is also widely used as a food additive to prevent oxidation. Deficiency diseases and symptoms: Vitamin C-deficiency causes scurvy disease in humans. Fragility of blood capillaries, bleeding from gums and tooth loss are common symptoms of Scurvy. RDA :Human body needs vitamin C about 90 mg/day for male and 75 mg/day for female. Appearance of teeth and SAQ 2 gums in scurvy. a) Write the active forms of following vitamins S.No. Vitamins Active forms 1) Vitamin B6 2) Biotin 3) Folic acid 4) Vitamin B12 b) Match the following Column A Column B a) CoA i) vitamins B6 b) Blood cell formation ii) biotin c) transfer of amino group iii) pantothenic acid d. transfer of CO2 group iv) vitamin C e) collgen formation and antioxidant v) folic acid and vitamins B12 12.4 STRUCTURE, ACTIVE FORMS AND FUNCTIONS OF FAT-SOLUBLE VITAMINS Unlike water soluble vitamins, fat-soluble vitamins are soluble in fat. Vitamins A, D, E and K are collectively known as fat soluble vitamins. These vitamins stored in liver and adipose tissue and their requirement is something like “demand and supply” i.e., whenever there is a demand. These vitamins accumulate in the body due to over consumption which can cause toxic effects. This group of vitamins play an important role in performing various 210 Unit 12 Vitamins.......................................................................................................................................................................... physiological functions. Human body needs fat soluble vitamins to maintain healthy skin, hair, eyes, heart and bone. Let us study fat soluble vitamins one by one. 12.4.1 Vitamin A Vitamin A is found in two forms; retinoids and carotenoids. All forms of vitamin A consists of a -ionone cyclic ring and an isoprenoid side chain that is unsaturated and participates in the visual cycle. Carrot, papaya and fish Isoprene liver oil are the best sources of vitamin A.  -ionone ring i) Retinoids comprise of three isomers of vitamin A: retinol, retinaldehyde, retinoic acid. Vitamin A is a collective term for biologically active isomers as shown in Fig. 12.18. Fig. 12.18: Vitamin A isomers 1. Retinol: It is primary alcohol (OH) containing -ionone ring with an unsaturated side chain. 2. Retinal: It is an aldehyde (CHO) containing vitamin A which is formed by the oxidation of retinol. Retinal plays a crucial role in the visual system. 3) Retinoic acid: It is formed by the oxidation of retinol. It serves as an intracellular messenger that affects gene expression. -Carotene is one of the carotenoids found in the plants. When ii) Carotenoids: we consume -carotene it converts to retinol in the human body. Hence, it is called as provitamin A -Carotene has two -ionone rings at both ends of the molecule. Biochemical functions: Vitamin A plays many vital roles for the proper functioning of our body. It is crucial for vision. Let us discuss in brief about few important functions of vitamin A: 211 Block 4 Vitamins and Nucleic Acids.......................................................................................................................................................................... 1. Role of vitamin A in vision: Role of vitamin A and rhodopsin in visual cycle was elucidated by George Wald. Hence this cycle is also known Wald’s visual cycle or rhodopsin cycle. In eye, retina has two photosentisitve rod and cone cells. In retina, retinal (Vitamin A) binds to the photon-sensitive opsin protein to form visual pigment rhodopsin (in rod cells) and iodopsin (in cone cells). Rhodopsin is made up of a protein called opsin which firmly bound to 11-cis-retinal. Rhodopsin absorbs light that induces isomerization when 11-cis retinal converts into all-trans retinal and opsin is released (Fig. 12.19a and b). This isomerization leads to the dissociation of all trans-retinaldehyde, that triggers an electrical signal into the brain (nerve impulse in neurons) which processes the signals into an image. Cis-trans photo isomerization: When light strikes retina of eyes, the 11-cis-retinal of rhodopsin absorbs a photon, and it isomerizes into all- trans –retinal in which cis configuration state change into trans state. This configuration occurs at carbon number 11 of retinal in which interconversion of atoms occur. Fig. 12.19(a): Visual cycle 212 Fig. 12.19(b): Photoisomerization of 11-cis retinal to all-trans retinal in retinal Unit 12 Vitamins.......................................................................................................................................................................... 2. Regulation of tissue differentiation and gene expression: Vitamin A (Trans-retinoic acid and cis-retinoic acid) has a major role in regulation of tissue differentiation process and expression of certain genes at the transcriptional level. 3. Antioxidant: -carotene is a pro-vitamin A that shows antioxidant like Vitamin A deficiency property to scavange free radicals in the body. diseases in human eye Deficiency diseases and symptoms: Vitamin A-deficiency (VAD) leads to impaired vision which has been recognized as major global problem specially in India and developing countries. The most common cause is inadequate dietary intake of vitamin A, different signs of vitamin A deficiency are: i) Night blindness: It is one of the most common and primary clinical symptom of vitamin A deficiency in which a person cannot see in dim light. It can affect children as well as pregnant and lactating women. ii) Bitot spots: Abnormal squamous cell proliferation and keratinization of the conjunctiva affected in the eye due to VAD. iii) Xerophthalmia (dryness in eyes): It is also known as dry-eyes syndrome. It results from keratinization of the cornea and blindness due to sudden acute deficiency of vitamin A. It is generally present in children and can occur in any age group. iv) Keratomalacia (dryness in corner): The most severe form of xerophthalmia is keratomalacia in which one third part of cornia is Source: Clate Gilbert. The affected due to chronic deficiency of vitamin A resulting in necrosis. eye signs of vitamin A deficiency. Community eye Health, 2013; 26; 66-67. RDA: 400 g/day for childern, 600 g/day for male and female, 800 g/ PMCIP : PMC393668 day for pregnant women. 12.4.2 Vitamin D Vitamin D is also known as calciferol. It is found in two active forms like calciferol (vitamin D2) and cholecalciferol (vitamin D3) (Fig. 12.20). Vitamin D is derived from sterol compounds which occur in plants and animals. Natural exposure of skin to sunlight (UV-B radiations) leads to the formation of this vitamin from its exposure to provitamins, ergosterol in plants and 7-dehydrocholesterol in human. When the exposure to sunlight is inadequate, dietary intake of vitamin D is required. Cod liver oil is the rich source of this vitamin. Besides, sunlight exposure is also required to meet the recommended daily intake of vitamin D in the human body. 213 Block 4 Vitamins and Nucleic Acids.......................................................................................................................................................................... Fig. 12.20: Structure of Vitamin D Biochemical function Vitamin D functions like a hormone. It acts as a transcription factor which promotes the expression of gene encoding for calcium binding protein. It regulates the concentration of calcium and phosphate ions in the blood stream, thereby promoting growth and remodeling of bone. It also enhances absorption of calcium, iron and magnesium ions in the intestine. Deficiency diseases and symptoms Vitamin D deficiency leads to poor bone mineralization in the body resulting in osteomalacia in old age people and rickets in children. The common symptoms are muscle weakness, softening of tissues and fragility of the bones. Rickets in Indian child (Vitamin-D Deficiency) RDA: 15 µg/day for male and female 12.4.3 Vitamin E (Tocopherols) Vitamin E is an essential nutrient because human body cannot make its own. In 1936, Evans and his colleagues isolated vitamin E from wheat-germ oil later named it as tocopherol. Tocopherol also known as vitamin E. It includes eight naturally occurring compounds of two classes: (1) tocopherols and (2) tocotrienols. Tocopherols is synthesized in plants and photosynthetic organisms. The rich sources of vitamin E are corn oil, soyabean oil 214 Wheat and germ oil. Unit 12 Vitamins.......................................................................................................................................................................... Vitamin E is a derivative of 6-hydroxy chromane ring with phytal side chain The isomers of four-tocopherols and four tocotrienols are homologues which are designated as α (alpha), β (beta),  (gamma) and  (delta) based on number and position of methyl groups on the chromane rings (Table 1.1). Tocopherols and tocotrienols have saturated side chain and unsaturated fytal side chain along with chroman ring respectively as shown in Fig. 12.21 a and b. Among these, α-tocopherol is the most active form of tocopherol which is widley available form of vitamin E in foods. (a) Tocopherol (b) Tocotrienols Fig. 12.21: Strucutre of Vitamin E Table 12.1: Position of methyl group in alpha, beta, gamma and delta tocopherol R1 R2 -Tocopherol CH3 CH3 -Tocopherol CH3 H -Tocopherol H CH3 -Tocopherol H H Biochemical functions Most of the biochemical functions of vitamin E relate to its anti-oxidative property. especially in cell membranes where it helps to prevent oxidation of membrane 215 Block 4 Vitamins and Nucleic Acids.......................................................................................................................................................................... lipids. It acts as an anti-aging molecule, that reduces oxidative stress in the cells. Vitamin E is essential in maintaining redox homeostasis in the body. Scientifc studies that vitamin E helps to reduce cardiovascular diseases and is essential for normal reproduction. Deficiency disease and symptoms: Animals with Vitamin E deficiency causes infertility. The other symptoms of vitamin-E deficiency are muscular dystrophy in addition to this liver kidney different times modified. RDA: About 7.5-10 mg/day (α-tocopherol) recommended for female and male. 12.4.4 Vitamin K Vitamin K is known as blood clotting vitamin. It was discovered by H. Dam in 1929. The structure of vitamin K consists of a quinine ring that linked to the isoprenoid side chain. Spinach, cabbage and green leafy vegetables are rich sources of this vitamin. Vitamin K exits in two forms: 1. Vitamin K1 (phyllo Quinone): It is synthesized by plants. The isoprenoid side chain is saturated as shown in Fig. 12.22. Fig. 12.22: Strucutre of Vitamin K1 2) Vitamin K2 (menaquinone): It is synthesized by intestinal bacteria. It is the main storage form in animals. Fig. 12.23 shows the structure of vitamin K2 in which isoprenoid chain is unstaturated (containing double bonds) as compared to saturated isoprenoids in vitamin K1. 216 Fig. 12. 23: Structure of Vitamin K2 Unit 12 Vitamins.......................................................................................................................................................................... Biochemical functions Vitamin K is essential for the carboxylation of glutamic acid to produce -carboxy glutamic acid in blood coagulation. Prothrombin and several other proteins of blood clotting system contain -carboxy glutamic acid residues. Such modified residues can bind Ca2+, which is required for blood clotting cascade. Deficiency diseases and symptoms Deficiency of Vitamin K causes delayed blood cogulation, that results into prolonged clotting time. RDA: 55 g/day for males and females. SAQ 3 a) Write the functions of following vitamins. S. No. Vitamins Functions 1) Vitamin A.................................................................. 2) Vitamin D.................................................................. 3) Vitamin E.................................................................. 4) Vitamin K.................................................................. b) Fill in the blanks with correct answer(s). i) The first sign of vitamin A deficieny..................................... ii) In the presence of light, rhodopsin is convereted to......................... from.......................... which is essential reaction for human vision. iii) The structure of tocotrienols differ from tocopherols due to........... presence of.............................................................. iv)........................... Vitamin acts as hormone. v) Exposure to.............................. stimulates synthesis of Vitamins D vi)................................... is Anti-aging vitamin. vii) Vitamin required for the carboxylation of glutamic acid.................. is.......................... 12.5 HYPERVITAMINOSIS The term hyper-vitaminosis means that presence of higher amount of vitamins than that of required. This condition leads to toxicity. Hypervitaminosis is primarily associated with fat soluble vitamins. Water soluble vitamins are not 217 Block 4 Vitamins and Nucleic Acids.......................................................................................................................................................................... toxic because they do not store in the body except vitamin B 6. When the levels water soluble vitamins exceeds they get excreted through urine. The excess amount of fat soluble vitamins in the body results from over dose of vitamin- supplements. As you know vitamin supplements are readily available in many different formulations (pills, syrup and tablates) which are recommended for the treatment of vitamin-deficiency diseases. Hence, taking multivitamins for longer periods may lead to metabolic abnotmalities that consequently affect the vital organs and tissues as these organs are storage sites for fat soluble vitamins. Let us know some the common hypervitaminosis: Hypervitaminosis A: High intake of vitamin A shows significant toxicity. Storage of vitamin A may cause visual changes, congenital birth defects, dizziness, nausea, headache, discoloration of skin, pain in joints and bones, coma, and even death. Hypervitaminosis D: Overdose of this vitamin causes adverse health effects such as anorexia, weight loss, polyuria, heart arrhythmias and also vascular and tissue calcification. Higher levels of vitamin D damages heart, blood vessels and kidneys. Prolonged exposure to sunlight does not result in vitamin D toxicity. Hypervitaminosis E: Higher intake of vitamin E-supplements leads to weakness, headache and gastrointestinal upset. Research studies reported that abnormal storage of vitamins E can affect blood clotting, inhibit platelet aggregation and cause haemorrhage. Hypervitaminosis K: Higher doses of vitamin K can cause allergic reactions such as skin rashes, itching and redness on the body. Its higher dose can cause hemolysis in newborns and hyperbilirubinemia. Hyper vitaminosis B6: Though it is a water soluble vitamin, it shows some adverse effects in the body. High intake of vitamin B6 or pyridoxine supplements lead to common symptoms such as headache, severe fatigue, nerve damage and mood changes. 12.6 VITAMINS AS COENZYMES AND ANTIOXIDANTS So for you have studied about the active forms and biochemical functions of vitamins. It is noted that most vitamins serve as coenzymes and antioxidants. Most coenzymes are derivatives of water soluble vitamins (B-complex vitamins) which are required for enzyme-catalyzed reactions. The major functions of coenzymes are to transport chemical groups between substrates and products. Coenzymes are specific to specific enzyme that serve unique function during cellular metabolism (Table 12.2). 218 Unit 12 Vitamins.......................................................................................................................................................................... NAD+ and NADH involve in Oxido-reduction reactions (Fig. 12.24) and B6 in conversion of -amino acid to -ketoacid (Fig. 12.25). Fig. 12.24: Interconversion of NAD+ to NADH COO COO COO + C=O  H3 N  C=O COO +  CH 2 + H3 N  C  H + CH2 C=O   Transaminase CH 2 R CH 2 R COO COO -Ketoguterate -Amino acid L-Glutamate -Keto acid Fig. 12.25: Transamiation reaction Table 12.2: Water-soluble Vitamins, Coenzymes their and Biochemical Functions Vitamins Coenzyme forms Biochemical function Thiamin (B1) Thiamin-pyrophosphate (TPP) Participates in the Oxidative decarboxylation reactions Riboflavin (B2) Flavin mono nucleotide (FMN) Participates in the Redox & Flavin di nucleotide (FAD) reactions Niacin (B3) Nicotinamide adenine Participates in the Redox dinucleotide (NAD+) reactions Nicotinamide adenine dinucleotide phosphate (NADP ) Pantothenic Coenzyme A Transfer of acyl group acid (B5) Pyridoxine (B6) Pyridoxal phosphate Participates in the Transamination reactions Biotin Biocytin Participates in the Carboxylation reactions Folic acid Tetrahydrofolate Transfer of one carbon unit Vitamin B12 methylcobalamin and Methyl group transfer and 5- deoxyadenosyl cobalamin its rearrangements *All water-soluble vitamins (with the exception to vitamin C) are coenzymes or precursors of coenzymes. 219 Block 4 Vitamins and Nucleic Acids.......................................................................................................................................................................... Vitamins as antioxidants: Vitamins that are considered as antioxidants include vitamin A, vitamin C, and vitamin E. You are aware that vitamin C is a water soluble vitamin and vitamins A and E are fat soluble vitamins. Recall the structure of these vitamins having hydroxyl and oxygen groups that play an important role in the scavenging of free radicals. These vitamins serve as a antioxidant molecule to mitigater the oxidative damage in the biological system. Antioxidant molecules neutralize free radicals by accepting or donating electron(s). -tocopherol is one of the effective form among other forms of vitamin E that has an antioxidant property. Vitamin E involves in protecting of membrane lipids from oxidation. It terminates the formation of free radicals producing through chain reaction process of lipid peroxidation. -carotene is a precursor of vitamin A has been shown to have antioxidant property. It is considered the most efficient “quencher” of singlet oxygen. Vitamin C and vitamin E can directly react with or neutralize free redicals such as hydroxyl, alkoxyl and lipid peroxyl (ROO*) and convert them into non-reactive molecules like water molecule , alcohol and lipid hydroperoxides, respectively. Since, we are discussing about antioxidants let us know in brief about free radicals.Free radicals are highly reactive atoms or group of atoms bearing unpaired electrons. Human body naturally produces free radicals through metabolic reactions involving oxygen.These free radicals are also known as reactive oxygen species(ROS) such as hydroxyl radical (OH ), superoxide anion radical, singlet oxygen, hydrogen peroxide (H2O2), hypochlorite and nitric oxide. Free radicals react with lipoproteins and unsaturated fatty acids in cell membranes, removing an electron from these molecules and thus generating a new free radical which can start a chain reaction process. Free radicals generated by environmental pollutants, sun light, smoking, stress and exposure to heavy metals and synthetic drugs etc. SAQ 4 Tick ( 3 ) the correct option given in the brackets i) (Vitamin A/vitamin E) Protects membrane lipid peroxidation from free radicals ii) Water soluble antioxidant is (vitamin E/vitamin A/ vitamin C iii) Reactive oxygen species in cells are also know as antioxidant/ free radicals iv) Coenzymes are derivatives of water soluble vitamins/fat soluble vitamins 220 v) Provitamin of vitamin A is (Rotinone /-carotene. Unit 12 Vitamins.......................................................................................................................................................................... Table 12.3 : Vitamins at glance: Vitamin Functions Deficiency *Recommended Dietary diseases/symptoms Dietary sources Allowances (RDA) Water soluble vitamins Thiamin Coenzmye in Beri-beri Cereals, 1.4 mg/day for male decarboxylation groundnuts, and 1.1 mg/day for reactions soybean and female. capsicum. Riboflavin Serve as electron Fatigue Yeast, milk, eggs 1.6 mg/day for male donor and Cheilosis and green and 1.3 mg/day for acceptor in redox vegetables female. reactions Niacin Formation of Pellagra Peanuts, legumes, 18 mg/day for male and NAD & NADP meat, eggs and milk 14 mg/day for female. Pantothenic Functional part of Peripheral nerve Whole grains and 5 mg/day acid CO-A and transfer damage vegetables acyl group Pyridoxin Involves in amino group Trnsfer in Disorders in amino eggs, liver, yeast, 2.0 mg/day for male transamination metabolism cereals, legumes and female reactions and milk Biotin Transfer of CO2 in Impired lipid and Liver, kidney, yeast, 30 g/day for male and biotin dependent carbohydrate nuts, tomatoes and female enzymes for metabolism egg yolk carboxylation Folic acid or Transferring of Megaloblastic anemia Green vegetables, 200 g/day for male folate one carbon group whole grains, and female, 500 ìg/day in biochemical cereals, for pregnant women reactions Vitamin B12 Formation of Pernicious anemia animal foods, meat, 1.0 g/day for male or Hemoglobin milk, egg and fish. female and 1.2 ìg/day for pregnant women Vitamin C Required for Scurvy Citrus fruits, 40 mg/day for male collagen peanuts, tomatoes and female, 60 mg/day biosynthesis and and green for pregnant women serve as an vegetables antioxidant Fat Soluble vitamins Vitamin A Essential for Night blindness, Carrot, papaya and 400 g/day for child, proper vision xeropthalemia fish liver oils 600 ìg/day for male and female, 800 ìg/day for pregnant women Vitamin D Help to body to Rickets in children Cod liver oil 10 g/day for male utilize calcium and osteomalacia in and female and phosphate adults from food. Essential for bone formation. Vitamine E Is essential for Infertifity Wheat, seed oil and 8-10 mg/day reproduction and fish oil serves as an antioxidant Vitamin K Participate in Prolonged blood Spinach, cabbage Blood clotting coagulation and green leafy 55 g/day for adulsts mechanism vegetables 221 Block 4 Vitamins and Nucleic Acids.......................................................................................................................................................................... 12.7 SUMMARY l Vitamins are essential nutrients required for development and maintenance of the healthy body. They perform various functions in the body. Their inadequate levels in body lead to the various deficiency dieseases. l They broadly devided into two groups like water soluble and fat soluble. l B complex vitamins primarily serves as coenzymes to transfer functional groups and assist enzymes to relase energy from food stuffs. Vitamin C serve as an anti-oxidant molecule and essential for collagen formation. l Vitamin A is important for vision. Vitamin D is for absorption of calcium and phosphorus, bone and teeth formation; vitamin E for neutralizing free radicals and improve immune system. Vitamin K for blood clotting. l Over storage of vitamins for longer period in the body results in toxicity. This medical condition is known as hypervitaminosis. l Vitamins serves as coenzymes and antioxidants. Most B-complex vitamins primarily function as coenzymes and vitamins A, C and E serve as antioxidants to protect the cell from oxidative stress. 12.8 TERMINAL QUESTIONS 1) Define vitamins? 2) Name the deficiency diseases or symptoms of the water-soluble and fat- soluble vitamins. 3) Provide the rich sources of the following vitamins? Thiamine, Niacin, Folic acid, riboflavin, ascorbic acid and vitamin A 4) Write RDA values the following vitamins. A, B1, B6, Biotin, D and K. 12.9 ANSWERS Self Assessment Questions a) (i) True (ii) True (iii) False (iv) True (v) False (vi) False (vii) True b) i) Thiamin ii) pyrimidine ring and a thiazole ring iii) Thiamin pyrophosphate iv) beriberi v) Flavin and ribose vi) FMN and FAD vii) tryptophan 222 viii) NAD and NADP Unit 12 Vitamins.......................................................................................................................................................................... 2) a) 1. Pyridoxin phosphate 2. Biocytin 3. 5, 10 methylene tetrahydrofolate 5-Methyl tetrahydrofolate 4. Adenosyl cobalamin and methyl cobalamin b) a) iii b) v c) i d) ii e) iv 3) a) 1. Visual cycle 2. Teeth and bon formation 3. antioxidant and improving immune system 4. Blood clotting formation b) i) Night blindness ii) 11-cis retinal to all-trans retinal iii) Unsaturated fytal side chain iv) Vitamin D v) Sunlight vi) Vitamin E vii) Vitamin K 4) i) vitamin E ii) vitamin C iii) free radicals iv) water soluble vitamins v) carotene Terminal questions 1) Refer to section 12.1 for definition of vitamins 2) Refer to section 12.3 3) Vitamins Sources Thiamine Sunflower seeds and brewer’s Yeast Niacin Rice and wheat Folic acid Peanuts, tomato juice and green beans Riboflavin Spinach Vitamin C Citrus juice and lemon Vitamin A Carrots 223 Block 4 Vitamins and Nucleic Acids.......................................................................................................................................................................... 4) RDA of the following vitamins. Vitamins Dietary requirement Vitamin A 400 g/day for child, 600 g/day for male and female, 800 g/day for pregnant women Vitamin B1 1.4 mg/day for male and 1.1 mg/day for female. Vitamin B6 2.0 mg/day for male and female Biotin 30 g/day for male and female Vitamin D 15 g/day for male and female. Vitamin K 55 g/day for adults 12.10 FURTHER READING 1. Albert L. Lehninger: Principles of Biochemistry, Worth Publishers, Inc. New York, 1984. 2. Harper’s Illustrated Biochemistry, 29e. Robert K. Murray, David A Bender, Kathleen M. Botham, Peter J. Kennelly, Victor W. Rodwell, P. Anthony Weil, USA. 3. Donald J Voet Principles of Biochemistry, Jophn Wiley and Sons, Inc, USA. 4. J. L. Jain: Fundamentals of Biochemistry, S. Chand & Company Ltd. India. 5. U. Satyanarayana and U. Chakrapani: Biochemistry, UBS Publishers Distributors Pvt Ltd. Kolkatta, India. 6. Thomas M. Devlin: Textbook of Biochemistry, John Wiley and Sons, Inc.Danvers, MA, USA. 7. Pearson Global Edition - Appling/Anthony - Cahill/Mathews - Biochemistry: Concepts and Connections. Edinburgh Gate, Harlow, Essex CM20 2IE, England, 2015. 224  UNIT 13 NUCLEIC ACIDS Structure 13.1 Introduction 13.4 Structure of DNA Expected Learning Outcomes Watson-Crick Model of DNA 13.2 Nucleic acids as genetic Other forms of DNA material: Experimental evidences 13.5 Structure of major species of RNA 13.3 Constituents of nucleic acids: structure and properties 13.6 Summary Nitrogenous bases 13.7 Terminal Questions Nucleosides, Nucleotides 13.8 Answers 13.9 Further Reading 13.1 INTRODUCTION In the pervious Blocks you have studied about the significance, structure and functional aspects of water and buffers, amino acids, proteins, carbohydrates and lipids. In this unit you will learn about nucleic acids both deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). You must have noticed that we resemble either of our parents. Have you ever asked yourself why? Because genetic information is transferred from one generation to another, i.e. from parents to the off springs. This flow of information is a vital condition for life and for the entire cellular processes. The genetic information is stored in the form of nucleotide sequences in the cells nucleus. You will study the structural as well as functional aspects of both DNA and RNA in detail in this unit. Objectives ____________________________________ After studying this Unit, you should be able to: v explain how nucleic acids act as genetic material; v define and draw structures of nitrogenous bases, purines and pyrimidines; v explain the significence phosphodiester linkage; v differentiate between DNA and RNA; v illustrate the structure of DNA double helix; and v describe various types of RNA and their functions. 225 Block 4 Vitamins and Nucleic Acids.......................................................................................................................................................................... 13.2 NUCLEIC ACIDS AS GENETIC MATERIAL: EXPERIMENT EVIDENCES Earlier it was believed In biology course at intermediate level you have studied about nucleic acids that protein is the briefly. Let us recall and build upon that simultaneously. Nucleic acids genetic material because of its vast constitute about 7 % of the dry weight of a cell. Both the nucleic acids are diversity while nucleic linear polymers of repeating units called nucleotides. Nucleotides are made acid was considered to up of sugar, phosphate and nitrogenous bases which you will study in detail in be a static molecule. the next section. Sugar and phosphate form the backbone while sequence of Friedrich Miescher, a Swiss physician in 1869 bases linked to sugar moiety characterize the uniqueness of the nucleic acid. isolated and identified a macromolecular Let’s go further. Nucleic acids are very appropriately referred as information substance from the pus storage molecules of a cell. This information is organized in the form of genes, cells and from salmon the fundamental unit of genetic information. Information flows from DNA to RNA sperm which he termed to protein i.e. DNA directs the synthesis of RNA, which in turn directs the as nuclein. synthesis of proteins. This flow of information is known as central dogma of Meischer’s student molecular biology (Fig. 13.1). You will study about these processes in detail in Richard Altmann in 1899 later courses. used the term nucleic acid for phosphorus containing nuclein, which was later proved to be the hereditary material in 1950s. Fig. 13.1: Central dogma of life Now the question which generally comes to our mind is what qualifies DNA as the genetic material? Why not protein or lipid or some other biomolecule? Today, we know that DNA is the genetic blueprint of life. It functions as the molecule that carries on the genetic information from one generation to another, lets understand how. Nitrogenous bases linked to sugar phosphate backbone in a Friedrich Meischer sequential manner stores and transfer the genetic information. Bases have a property to form pairs with the help of hydrogen bonds. Because of this base pairing nucleic acids tend to form a double stranded helical structure, and also base pairing provides a mechanism for copying this genetic information. If any error occurs in any of the steps involved in expressing the genetic information (replication, transcription and translation) stored in DNA, a genetic disease may occur. Thus, an understanding of how nucleic acids store and deliver genetic information within the cells and from one generation to another becomes necessary for understanding diseases and also to devise strategies for treatment, for example gene therapy. DNA Carries Genetic Information: Experimental Evidences DNA is the genetic material in all prokaryotic and eukaryotic organisms. In 1869 Miescher isolated DNA from the pus cells but the first experimental evidence that DNA is the genetic material came much later. Griffith’s experiment on transforming principle in the year 1928 suggested that bacteria are capable of transferring genetic information through a process known as transformation. However, in the year 1944, Avery, Macleod and McCarty using Griffith’s experiment on bacterial transformation provided the evidence that the transforming agent is DNA and it is the genetic material (Fig. 13.2). Griffith found that DNA extracted from a virulent (smooth, disease causing) strain of the bacterium Streptococcus pneumoniae, genetically transformed a non-virulent (rough, non-disease causing) 226 strain of this organism into a virulent form. Unit 13 Nucleic Acids.......................................................................................................................................................................... They observed that: Stage 1 - Living ‘S’ strain causes death in mouse. Stage 2 - Living ‘R’ strain causes no-death in mouse. Stage 3 - Heat killed ‘S’, strain can not cause death in mouse. Stage 4 - Mixture containing both heat killed ‘S’, strain and live ‘R’, strain will cause death in mouths. Hence, they concluded that in stage 4, some substance causing virulence in ‘S’, strain is transfered into ‘R’, strain and is responsible virulence. Fig. 13.2: Griffith’s experiment on bacterial transformation Second independent experimental evidence came in the year 1952. Hershey and Chase devised an experiment with T2 bacteriophage (virus) using radio- labeled sulpher and phosphorus (S35 and P32). Since T2 phage contains both DNA and proteins, in one experiment the protein part was made radioactive with S35 and in the other the DNA was made radioactive with P32.They infected E. coli with labeled T2 bacteriophage. After infection, culture was gently agitated in a blender to loosen the phage particles and culture was centrifuged. It was found that when T2 phage containing radioactive protein was used as the infecting agent, the bacterial pellet contained very little radioactivity and most of the radioactivity was in the supernatant. But if T2 phage with radioactive DNA was used, the heavier bacterial pellets were radioactive (Fig. 13.3). This experiment indicated that during infection with the virus, it is the DNA that actually entered the bacteria. This experiment is also known as Blender experiment. Fig. 13.3: Blenders experiment of Hershey and Chase 227 Block 4 Vitamins and Nucleic Acids.......................................................................................................................................................................... Let us now study in detail about the constituents of nucleic acids. 13.3 CONSTITUENTS OF NUCLEIC ACIDS: STRUCTURE AND PROPERTIES Nucleic acids (both DNA and RNA) are composed of pentose sugar, phosphate and nitrogenous bases. Sugar is deoxyribose in DNA (deoxyribonucleic acid) while ribose in RNA (ribonucleic acid). Deoxyribose means the 2' ca