Structure of DNA

Overview

• DNA is a polymer made up of monomers called DNA nucleotides.
• DNA nucleotides consist of:
  • A phosphate group
  • Deoxyribose sugar (a pentose sugar with five carbons)
  • A nitrogenous base (Adenine [A], Thymine [T], Cytosine [C], or Guanine [G])
• DNA affects our characteristics such as appearance and can influence traits like height and skin color.

DNA Molecule

• DNA forms a double helix structure.
• Composed of two strands that spiral along each other, not directly touching, but connected by bases.
• James Watson and Francis Crick conceptualized the structure of DNA, leading to a Nobel Prize in Physiology and Medicine.

Nitrogenous Bases

• Two types: Purines (Adenine, Guanine) and Pyrimidines (Thymine, Cytosine).
• Purines are larger with two-ring structures, while pyrimidines have a single-ring structure.
• Base pairing:
  • Adenine pairs with Thymine (A-T), forming two hydrogen bonds.
  • Cytosine pairs with Guanine (C-G), forming three hydrogen bonds.
• This pairing is universal across all life forms, with rare exceptions due to mutations.

Complementary Base Pairing

• The principle that bases must match: A with T, and C with G.
• Example sequence: If one strand is ACGGC, the complementary strand will be TGCCG.

DNA Strands and Bonds

• DNA strands are made of nucleotides joined by phosphodiester bonds.
• Phosphodiester bond is a covalent bond linking the sugar of one nucleotide to the phosphate of the next.
• DNA strands are antiparallel, meaning they run in opposite directions.

Directionality: 5' and 3'

• DNA has directional ends: 5' (five prime) and 3' (three prime).
• 5' end: Carbon 5 of the sugar is closest to the phosphate group.
• 3' end: Carbon 3 is closest to the blank space or to where bonds form.

Importance of Directionality

• New nucleotides are added in a 5' to 3' direction.
• During replication, nucleotides are added to the 3' end of the strand.

Summary

1. DNA consists of nucleotides connected by phosphodiester bonds forming single strands.
2. Two strands are antiparallel and connect via hydrogen bonds through complementary base pairing.
3. The double helix is formed by the coiling of two strands around each other.
4. Understanding the structure is vital for explaining DNA replication and function.