DNA Composition and Structure

DNA Composition

• Sugars in DNA are deoxyribose and ribose, both 5-carbon sugars, but ribose has an extra hydroxyl (-OH) group attached to the 2' carbon.
• Nitrogenous bases in DNA are pyrimidines (Cytosine, Thymine, and Uracil) and purines (Adenine and Guanine).
• Phosphate in DNA is a negatively charged inorganic ion consisting of one phosphorus atom and four oxygen atoms.
• Phosphate forms phosphodiester bonds with the 5'-carbon of deoxyribose sugar and the 3'-carbon of the preceding deoxyribose sugar.

Nucleosides and Nucleotides

• A nucleoside is a compound composed of a nitrogenous base linked to a 5-carbon deoxyribose or ribose sugar.
• A nucleotide is a nucleoside attached to a phosphate group.

DNA Structure

• Nucleic acids are polymers, and monomers are called nucleotides.
• Nucleotides consist of a base, sugar, and phosphate.
• Bases are either purine (adenine, guanine) or pyrimidine (thymine, cytosine, uracil).
• Sugar is either deoxyribose or ribose.
• Phosphate links nucleotides together.
• The 3' carbon of one nucleotide is linked to the phosphate of another nucleotide by a phosphodiester bond through the 5' carbon of the other nucleotide.

Important Experiments

Chargaff's Rule

• Chargaff's rule states that in DNA, the amount of purine bases is equal to the amount of pyrimidine bases, i.e., [A] = [T] and [G] = [C] but [A + G] ≠ [T + C].

The Hershey-Chase Experiment

• The Hershey-Chase experiment showed that DNA carries genetic information, not protein.

DNA Double Helix

• The double helix has anti-parallel strands with a sugar-phosphate backbone.
• Two hydrogen bonds connect T to A, and three hydrogen bonds connect G to C.
• The sugar-phosphate backbones run anti-parallel to each other, aligning the 3' and 5' ends of the two strands.

DNA Conformations

• The A-form is a short, wide, right-handed helix.
• The B-form is the most common conformation.
• The Z-form is a left-handed helix.

The Meselson-Stahl Experiment

• The Meselson-Stahl experiment showed that DNA replication is semi-conservative, meaning each new strand contains one original and one new strand.