Unit 3: Nucleic Acids - Subjective Questions

Purines and pyrimidines are nitrogen-containing heterocyclic bases found in nucleic acids.

• Purines: These are larger, double-ring structures consisting of a six-membered pyrimidine ring fused to a five-membered imidazole ring. The major purines are adenine and guanine.
• Pyrimidines: These are smaller, single-ring structures containing a six-membered nitrogenous ring. The major pyrimidines are cytosine, thymine, and uracil.
• Occurrence: Adenine, guanine, and cytosine occur in both DNA and RNA. Thymine is mainly present in DNA, whereas uracil is mainly present in RNA.
• Chemical significance: The arrangement of nitrogen and oxygen atoms in these bases allows them to form specific hydrogen bonds during nucleic acid pairing.

The numbering of nitrogenous bases is important for describing their bonding and reactions.

• Purine ring: Purines contain fused six-membered and five-membered rings. The atoms are numbered consecutively around the fused ring system, with the ring nitrogens designated as , , , and .
• Pyrimidine ring: Pyrimidines contain a six-membered ring with nitrogen atoms at positions and . Carbon atoms are numbered as , , , and .
• Glycosidic attachment: In nucleosides, the sugar attaches to of a purine and of a pyrimidine.
• Functional groups: The presence of amino, keto, and methyl groups determines the identity and hydrogen-bonding properties of each base.

A nucleoside consists of a nitrogenous base linked to a pentose sugar, whereas a nucleotide consists of a nitrogenous base, a pentose sugar, and one or more phosphate groups.

• Nucleoside: Base + sugar. Examples include adenosine, guanosine, cytidine, thymidine, and uridine.
• Nucleotide: Base + sugar + phosphate. Examples include adenosine monophosphate, guanosine triphosphate, and deoxythymidine monophosphate.
• The base is linked to the carbon of the sugar by a -N-glycosidic bond.
• The phosphate group is commonly attached to the carbon of the sugar.
• Therefore, the addition of phosphate to a nucleoside produces a nucleotide.

Ribose and deoxyribose are five-carbon sugars that form the structural framework of nucleotides.

• Ribose: It is present in RNA and contains a hydroxyl group at the carbon.
• Deoxyribose: It is present in DNA and contains hydrogen instead of a hydroxyl group at the carbon.
• Both sugars have carbon atoms numbered with prime symbols: , , , , and .
• The nitrogenous base attaches to the carbon.
• Phosphate groups commonly attach to the carbon.
• The hydroxyl group forms a phosphodiester bond with the next nucleotide.
• The absence of the hydroxyl group makes DNA more chemically stable than RNA and helps DNA function as a long-term storage molecule.

A phosphodiester bond links adjacent nucleotides in a nucleic acid strand.

• The hydroxyl group of the sugar in one nucleotide reacts with the phosphate group attached to the carbon of the next nucleotide.
• This produces a bond connecting the carbon of one sugar to the carbon of the next sugar through phosphate.
• The bond is called a phosphodiester bond because the phosphate group forms ester linkages with two sugar hydroxyl groups.
• The reaction is associated with the release of water during chemical condensation, although biological polymerization uses activated nucleotide triphosphates.
• Repeated formation of these bonds produces the sugar-phosphate backbone of DNA or RNA.
• As a result, nucleic acid strands have a definite direction, from the end to the end.

ATP and GTP are purine nucleotides with major roles in cellular metabolism.

• ATP: Adenosine triphosphate contains adenine, ribose, and three phosphate groups. It acts as the major energy currency of the cell.
• ATP hydrolysis can be represented as:
  
• ATP supplies energy for biosynthesis, active transport, muscle contraction, and phosphorylation reactions.
• GTP: Guanosine triphosphate contains guanine, ribose, and three phosphate groups.
• GTP provides energy for protein synthesis and participates in signal transduction through GTP-binding proteins.
• Both molecules transfer phosphoryl groups and couple energy-releasing reactions to energy-requiring cellular processes.

Cyclic nucleotides are nucleotide derivatives in which the phosphate group forms a cyclic phosphodiester linkage with two hydroxyl groups of the same sugar.

• Cyclic AMP (cAMP): It is formed from ATP by the enzyme adenylate cyclase.
• cAMP acts as a second messenger for hormones such as adrenaline and glucagon.
• It activates protein kinase A and helps regulate glycogen metabolism, lipid metabolism, and gene expression.
• Cyclic GMP (cGMP): It is formed from GTP by guanylate cyclase.
• cGMP participates in phototransduction, smooth muscle relaxation, and nitric oxide signaling.
• The actions of cyclic nucleotides are terminated by phosphodiesterases, which hydrolyze them to non-cyclic nucleotides.

Several biologically important coenzymes contain nucleotide components.

• NAD⁺: Nicotinamide adenine dinucleotide contains two nucleotides joined through phosphate groups. Its nicotinamide ring accepts and donates hydrogen during oxidation-reduction reactions.
• The reduction reaction is represented as:
  
• FAD: Flavin adenine dinucleotide contains an adenine nucleotide linked to a flavin group. It functions as a tightly bound or mobile electron carrier in oxidation-reduction reactions.
• Coenzyme A: It contains an adenosine diphosphate component and a reactive sulfhydryl group. The sulfhydryl group forms high-energy thioester bonds with acyl groups.
• These compounds are essential for energy production, oxidation of nutrients, and transfer of chemical groups.

DNA structure can be described at several levels.

• Primary structure: It is the linear sequence of deoxyribonucleotides joined by - phosphodiester bonds.
• Secondary structure: DNA generally consists of two antiparallel polynucleotide strands forming a double helix. The bases face inward and the sugar-phosphate backbones face outward.
• Adenine pairs with thymine through two hydrogen bonds, while guanine pairs with cytosine through three hydrogen bonds.
• Tertiary structure: The double helix undergoes supercoiling and associates with histone proteins to form chromatin in eukaryotic cells.
• The most common cellular form is B-DNA, which is a right-handed helix with approximately ten base pairs per turn.
• DNA structure allows accurate replication and stable storage of genetic information.

The Watson-Crick model describes DNA as a double-helical molecule.

• DNA contains two polynucleotide strands that run in opposite directions, or antiparallel directions.
• One strand runs from to , while the complementary strand runs from to .
• The sugar-phosphate backbones are located on the outside, and the nitrogenous bases project toward the interior.
• Adenine pairs specifically with thymine, and guanine pairs specifically with cytosine.
• Hydrogen bonds provide specificity: A-T has two hydrogen bonds and G-C has three.
• Base stacking interactions between adjacent aromatic bases contribute substantially to stability.
• Hydrophobic interactions and ionic interactions involving the phosphate groups and surrounding cations also support the structure.

The major structural forms of DNA differ in helical direction, geometry, and biological conditions.

• A-DNA:
  • Right-handed helix.
  • Shorter and wider than B-DNA.
  • Contains approximately eleven base pairs per turn.
  • May occur under conditions of low hydration and in DNA-RNA hybrids.
• B-DNA:
  • Right-handed helix and the most common form under physiological conditions.
  • Contains approximately ten base pairs per turn.
  • Has a prominent major groove and a narrower minor groove.
• Z-DNA:
  • Left-handed helix.
  • Has a zigzag-shaped sugar-phosphate backbone.
  • Often forms in sequences rich in alternating purines and pyrimidines.
  • May be involved in regulation of gene expression and relief of torsional strain.

DNA strands possess two important organizational features: complementarity and antiparallel orientation.

• Complementarity: Adenine on one strand pairs with thymine on the other, while guanine pairs with cytosine.
• Because of this pairing rule, the sequence of one strand determines the sequence of the other.
• Antiparallel orientation: One strand has a to direction, and the opposite strand has a to direction.
• Complementary base pairing maintains a uniform width of the double helix because a larger purine always pairs with a smaller pyrimidine.
• Complementarity permits accurate DNA replication and transcription.
• It also allows repair mechanisms to use the undamaged strand as a template for correcting the damaged strand.

RNA is generally a single-stranded nucleic acid containing ribose sugar and the bases adenine, guanine, cytosine, and uracil.

• The hydroxyl group of ribose makes RNA more reactive and less chemically stable than DNA.
• RNA strands can fold into complex secondary and tertiary structures through intramolecular base pairing.
• Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes for protein synthesis.
• Transfer RNA (tRNA): Carries amino acids to ribosomes and contains an anticodon that recognizes an mRNA codon.
• Ribosomal RNA (rRNA): Combines with proteins to form ribosomes and participates in peptide bond formation.
• Other types include microRNA, small nuclear RNA, and small nucleolar RNA, which participate in gene regulation and RNA processing.

DNA is optimized for stable information storage, whereas RNA is structurally more versatile and can function as an information carrier, adaptor, regulator, or catalyst.

RNA structure is organized into primary, secondary, and tertiary levels.

• Primary structure: The linear sequence of ribonucleotides linked by phosphodiester bonds.
• Secondary structure: Local folding caused by complementary base pairing within the same strand. Examples include hairpin loops, stem-loop structures, bulges, and internal loops.
• Tertiary structure: Further three-dimensional folding stabilized by hydrogen bonds, base stacking, ionic interactions, and interactions with metal ions or proteins.
• Transfer RNA has a cloverleaf secondary structure and an L-shaped tertiary structure.
• Ribosomal RNA forms extensive secondary and tertiary structures that create the functional core of the ribosome.
• RNA folding enables some RNA molecules to act as catalysts, known as ribozymes.

Nucleic acids possess several important physical properties.

• Solubility: They are generally soluble in water because of their charged phosphate groups. They may be precipitated from aqueous solutions using alcohol in the presence of salts.
• Ultraviolet absorption: Nitrogenous bases absorb ultraviolet radiation strongly near . This property is used to estimate nucleic acid concentration.
• Viscosity: High-molecular-weight DNA solutions are viscous because long DNA molecules occupy large volumes and become entangled. Viscosity decreases when DNA is fragmented or denatured.
• Density: DNA density depends on base composition, molecular structure, and the presence of associated proteins or ions.
• These properties are useful in the isolation, identification, quantification, and analysis of nucleic acids.

DNA denaturation is the separation of the two strands of the double helix without breaking the covalent phosphodiester bonds.

• Denaturation can be caused by heat, extreme pH, organic solvents, or low ionic strength.
• Hydrogen bonds between complementary bases and base-stacking interactions are disrupted.
• The absorbance at increases during denaturation, a phenomenon called the hyperchromic effect.
• Renaturation, or annealing, is the reassociation of complementary single strands when favorable conditions are restored.
• Renaturation requires appropriate temperature, ionic strength, and sufficient time.
• DNA with a high percentage of G-C pairs generally has a higher melting temperature because G-C pairs form three hydrogen bonds and have stronger stacking interactions.
• These principles are used in hybridization techniques and molecular biology experiments.

The melting temperature, denoted by , is the temperature at which half of the DNA molecules in a sample are denatured into single strands.

Factors influencing include:

• G-C content: A higher proportion of G-C base pairs generally increases because of stronger hydrogen bonding and base stacking.
• Ionic strength: Positively charged ions shield the negative phosphate groups and stabilize the double helix, increasing .
• DNA length: Longer DNA molecules usually require more heat for complete separation.
• pH: Extreme pH disrupts base pairing and lowers the stability of DNA.
• Mismatched bases: Sequence mismatches reduce helix stability and lower .
• is important in designing primers and controlling nucleic acid hybridization reactions.

Nucleic acids show chemical properties derived from their sugars, phosphate groups, and nitrogenous bases.

• Acidic behavior: Phosphate groups can release protons, giving nucleic acids a negative charge at physiological pH.
• Hydrolysis: Phosphodiester bonds can be hydrolyzed chemically or enzymatically by nucleases.
• RNA is more susceptible to alkaline hydrolysis because its hydroxyl group can attack the adjacent phosphodiester bond.
• DNA is relatively resistant to alkaline hydrolysis because it lacks the hydroxyl group.
• Base reactions: Nitrogenous bases can undergo protonation, deamination, oxidation, and methylation.
• Such modifications can alter base-pairing properties and may affect genetic information.
• Nucleotides can also undergo phosphorylation and dephosphorylation reactions in metabolism.

The hyperchromic effect is an increase in ultraviolet absorption by DNA when the double helix is denatured.

• In double-stranded DNA, base stacking restricts the absorption of ultraviolet light.
• When the strands separate, the bases become more exposed and absorb more strongly near .
• This increase in absorbance is called hyperchromicity and is used to monitor DNA melting.
• The hypochromic effect is the lower ultraviolet absorbance of bases when they are arranged in a well-stacked, double-helical structure.
• Hypochromicity results mainly from interactions between neighboring aromatic bases.
• These effects provide useful information about nucleic acid conformation, strand separation, and hybridization.