DNA Structure and Complementary Base Pairing Quiz

DNA Structure

• Two chains are joined by weak hydrogen bonds, with specific pairing: A = T, G = C.
• The H atom of one base is linked to the O2 or N2 atom of another base.
• The two strands are antiparallel, with 3' and 5' phosphor links in opposite directions.
• The pairing is specific, and the sequence of N2 bases in one chain determines the sequence in the other chain.
• The diameter of DNA is 20 A°.

Erwin Chargaff's Rule

• Regardless of the source, purine and pyrimidine components occur in equal amounts in a DNA molecule.
• This rule is based on the following relationships:
  • A = T, G = C
  • A + C = T + G
  • A + G = T + C
  • A + T ≠ G + C (not always true)

James Watson and Francis Crick's Model

• The model is based on X-ray crystallography technique.
• DNA consists of two right-handed helices.
• Each turn has 10 nitrogen base pairs.
• One spiral has a length of 3.4 A°.
• The distance between two nitrogen bases is 3.4 A°.

Denaturation and Renaturation of DNA

• Denaturation: When DNA solution is heated or exposed to alkaline or acidic pH, H bonds break, and the two strands uncoil.
• Renaturation: When the solution is gradually cooled or neutralized, new base pair formation begins, and the double-stranded DNA is formed.

DNA Types

• Linear DNA with free ends (eukaryotes) has histones.
• Circular DNA with covalently linked ends (prokaryotes) has no histones.

Properties of Proteins

• There are thousands of proteins, varying in length, number, and type of polypeptides.
• Proteins have high specificity in individuals but are shared with related species or groups.
• Molecular weight ranges from 4,500 daltons (ACTH) to 4,600,000 daltons (Pyruvate Dehydrogenase).
• Solubility: Some proteins are insoluble due to large size, while many form colloidal solutions with water.
• Amphoteric nature: Proteins show both acidic and basic properties.
• Electrical reaction: Proteins have an isoelectric point at which pH is neutral.
• Denaturation: Proteins can undergo permanent or temporary loss of three-dimensional structure due to UV, heat, strong acid, and alkali, or high salt concentration.