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Nucleotide DNA and RNA

1. NUCLEOTIDES ( DNA & RNA) Presented by – SHRIKA DUTTA BDS 1 ST YEAR

2. Nucleotides are composed of a nitrogenous base, a pentose sugar and a phosphate. Nucleotides perform a wide variety of functions in the living cells, besides being the building blocks or monomeric units in the nucleic acid ( DNA and RNA ) structure . Nucleoside – This term refers to base + sugar. Thus, nucleotide is nucleoside + phosphate. What are nucleotides ?

3. Structure of Nucleotides: Nitrogenous Base : These are aromatic heterocyclic compounds The bases are of two types – Purines and Pyrimidines. Purines are numbered in the anticlockwise direction while pyrimidines are numbered in the clockwise direction. Purines - Adenine, Guanine Pyrimidines - Cytosine, Thymine, Uracil The structure of DNA consists of Adenine, guanine, Cytosine and Thymine . The structure of RNA consists of Adenine , Guanine Cytosine and Uracil .

4. Structure of Nucleotides: Tautomeric forms of Purines and Pyrimidines: The existence of a molecule in a keto (lactam) and enol ( lactim) form is known as tautomerism. The heterocyclic rings of purine and pyrimidines with oxo functional groups exhibit tautomerism . Tautomeric forms of cysteine are Lactam and Lactim. At physiological pH, Lactam is predominantly present. Minor Bases found in Nucleic Acid: Several minor and unusual bases are often found in DNA and RNA. These include 5-methylcytosine, N4-acetylcytosine , pseudouracil etc. These help in recognition of specific enzymes. 2. Sugars Of Nucleic Acid: The five carbon monosaccharides (pentoses) are found in nucleic acid structure. RNA contains D-Ribose and DNA contains D- Deoxyribose

5. Biologically important nucleotides and their function: Metabolic Function: Energy Metabolism, protein synthesis, control of enzyme Nucleotides are structural components of some co-enzymes of B-complex vitamins e.g. FAD. CDP – acylglycerol ( a lipid derivative ) is an intermediate in lipid biosynthesis. cAMP and cGMP are second messengers for some hormonal activity. GTP and GDP are involved in cascade events of single transduction pathways.

6. Structure of DNA: DNA is a polymer of deoxyribonucleotides (or simply deoxynucleotides). It is composed of monomeric units namely deoxyadenylate (dAMP), deoxyguanylate (dGMP), deoxycytidylate (dCMP) and deoxythymidylate (dTMP) . The monomeric deoxynucleotides in DNA are held together by 3’ ,5’ -phosphodiester bridges. Features : Chargaff’s rule of DNA Composition DNA Double Helix Model Other Types of DNA structures

7. Chargaff’s Rule of DNA Composition: Erwin Chargaff in late 1940s quantitatively analysed the DNA hydrolysates from different species. He observed that in all the species he studied, DNA had equal numbers of adenine and thymine residues (A = T) and equal numbers of guanine and cytosine residues (G = C). This is known as Chargaff’s rule of molar equivalence between the purines and pyrimidines in DNA structure. The significance of Chargaff’s rule was not immediately realised. The double helical structure of DNA derives its strength from Chargaff’s Rule. Single-stranded DNA, and RNAs which are usually single-stranded, do not obey Chargaff’s rule. However, double-stranded RNA which is the genetic material in certain viruses satisfies Chargaff’s rule.

8. DNA Double Helix: The double helical structure of DNA was proposed by James Watson and Francis Crick in 1953 The DNA is a right handed double helix. It consists of two polydeoxyribonucleotide chains (strands) twisted around each other on a common axis. The two strands are antiparallel , i.e., one strand runs in the 5c to 3c direction while the other in 3c to 5c direction. The width (or diameter) of a double helix is 20 A° (2 nm). Each turn (pitch) of the helix is 34 A° (3.4 nm) with 10 pairs of nucleotides , each pair placed at a distance of about 3.4 A °. Each strand of DNA has a hydrophilic deoxyribose phosphate backbone (3c-5c phosphodiester bonds) on the outside (periphery) of the molecule while the hydrophobic bases are stacked inside (core ). The two polynucleotide chains are not identical but complementary to each other due to base pairing . The two strands are held together by hydrogen bonds formed by complementary base pairs ( Fig.5.10 ). The A-T pair has 2 hydrogen bonds while G-C pair has 3 hydrogen bonds. The G triple bond C is stronger by about 50% than A=T. The hydrogen bonds are formed between a purine and a pyrimidine only.

9. Other Types Of DNA Structures: It is now recognized that besides double helical structure, DNA also exists in certain unusual structures. It is believed that such structures are important for molecular recognition of DNA by proteins and enzymes. This is in fact needed for the DNA to discharge its functions in an appropriate manner. Some selected unusual structures of DNA are Bent DNA, triple – stranded DNA and Four Stranded DNA.

10. Structure of RNA: RNA is a polymer of ribonucleotides held together by 3 c,5c-phosphodiester bridges. Although RNA has certain similarities with DNA structure , they have specific differences: Pentose : The sugar in RNA is ribose in contrast to deoxyribose in DNA. Pyrimidine : RNA contains the pyrimidine uracil in place of thymine (in DNA). Single strand : RNA is usually a single stranded polynucleotide. However, this strand may fold at certain places to give a double stranded structure, if complementary base pairs are in close proximity. 4. Chargaff’s rule—not obeyed : Due to the single-stranded nature, there is no specific relation between purine and pyrimidine contents. Thus the guanine content is not equal to cytosine (as is the case in DNA).

11. Structure of RNA: Susceptibility to alkali hydrolysis : Alkali can hydrolyse RNA to 2c,3c-cyclic diesters. This is possible due to the presence of a hydroxyl group at 2c position. DNA cannot be subjected to alkali hydrolysis due to lack of this group. Orcinol colour reaction : RNAs can be histologically identified by orcinol colour reaction due to the presence of ribose.

12. Types Of RNA:

13. Mutations in DNA: Some Disorders due to mutations in DNA are: Cystic Fibrosis Sickle Cell Anaemia Haemophilia Thalessemia etc.

14. Mutations in RNA: Some disorders due to mutations in RNA are: Prader Willi Syndrome Spinal Muscular Atrofy Myotonic Dystrophy etc.

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