Biology: Structure of DNA and RNA

Structure of DNA and RNA

• Nucleic acids consist of polynucleotides formed by linking nucleotides via phosphodiester bonds.
• Nucleotides are composed of a phosphate group, a pentose sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base.
• Formation of a mononucleotide involves a condensation reaction resulting in the elimination of two water molecules.

Nitrogenous Bases

• DNA contains five nitrogenous bases: adenine (A), thymine (T), guanine (G), and cytosine (C).
• RNA replaces thymine with uracil (U), comprising adenine (A), uracil (U), guanine (G), and cytosine (C).
• Adenosine triphosphate (ATP) is a phosphorylated mononucleotide made from ribose, adenine, and can have one to three phosphate groups (AMP, ADP, ATP).

ATP Significance

• ATP is termed the universal energy currency for cells.
• It is small and water-soluble, facilitating easy diffusion between cell organelles.
• Acts as an immediate energy donor, easily hydrolyzed to ADP to release energy.

Uses of ATP

• Vital for cell division, muscle contraction, and maintenance of body temperature.
• Plays a role in anabolic reactions, such as protein synthesis and nerve impulse transmission.

Formation of Polynucleotide

• Occurs during interphase through condensation reactions creating phosphodiester bonds.
• Links the phosphate of one nucleotide to the 3' carbon of the sugar in another nucleotide, creating a sugar-phosphate backbone.

Structure of DNA Molecule

• Comprises two polynucleotide strands of deoxyribose, phosphate, and nitrogenous bases.
• Strands run in opposite directions (antiparallel).
• Hydrogen bonds between nitrogenous bases connect the strands, adhering to complementary base pairing (A=T, G≡C).

Importance of Hydrogen Bonding in DNA

• Hydrogen bonds hold the two polynucleotide strands together and contribute to the DNA's 3D structure.
• Provide stability, allowing for easier separation during replication and transcription.
• Specificity in base pairing reduces replication errors, ensuring faithful genetic replication.

Structural Features Contributing to DNA Stability

• Complementary base pairing and numerous hydrogen bonds enhance structural integrity.
• The sugar-phosphate backbone is reinforced with phosphodiester bonds.
• The double helix configuration protects bases from chemical attacks.

Importance of DNA Stability

• Prevents spontaneous changes in sequences, minimizing mutation risk and ensuring functional proteins.
• Ensures the preservation of genetic information across generations, maintaining integrity in daughter cells.
• Keeps DNA size manageable for storage within the nucleus.

Semi-Conservative Replication of DNA

• Semi-conservative replication generates two identical DNA molecules, each containing one original and one new strand.
• Occurs during the S phase of the cell cycle, using each old strand as a template for new complementary strand synthesis.

Steps of DNA Replication

• The double helix unwinds, and hydrogen bonds between complementary bases break, facilitated by the enzyme DNA helicase.
• Free activated nucleotides align along the separated strands, pairing with complementary bases (A=T, C≡G).
• DNA polymerase adds new nucleotides sequentially, while DNA ligase forms phosphodiester bonds between adjacent nucleotides.

Sense and Anti-sense Strands

• Sense strand encodes for protein manufacture, while anti-sense strand stabilizes the DNA and serves as a template for mRNA synthesis.
• A triplet of bases (codon) in the template strand codes for one amino acid, forming the genetic code.

Genetic Code Characteristics

• Each triplet of bases specifies one amino acid, resulting in 64 possible codons from four bases.
• The genetic code is universal across living organisms, suggesting a common ancestry.
• Degenerate nature allows multiple codons to code for the same amino acid, totaling 61 codons usable for amino acids and three stop codons.