Base Pairing

Nucleotide Composition

• DNA is composed of building blocks known as nucleotides.
• Nucleotides consist of three components: deoxyribose sugar, phosphate group, and a nitrogenous base.
• The nitrogenous base is variable and can be one of four types: adenine (A), thymine (T), guanine (G), or cytosine (C).

Complementary Base Pairing

• The foundation of DNA's structure is complementary base pairing.
• Adenine (A) always pairs with thymine (T), forming an A-T base pair.
• Guanine (G) always pairs with cytosine (C), creating a G-C base pair.
• These pairs are held together by hydrogen bonds, ensuring a stable and specific connection.

Formation of DNA Strands

• Nucleotides in DNA strands are linked together by strong covalent bonds between the deoxyribose sugar of one nucleotide and the phosphate group of the next.
• This strong linkage creates a sugar-phosphate backbone that runs along the sides of the DNA molecule.

Directionality of DNA

• DNA strands exhibit a unique directional orientation.
• One strand runs in a 5' to 3' direction, while the other runs in a 3' to 5' direction.
• This antiparallel arrangement contributes to the twisting double helix structure of DNA.

---

Base Pairing

Genetic Information in DNA

• The base sequence of DNA contains the genetic code.
• All cells store their genetic information in the specific arrangement of these bases.
• The genotype, or genetic makeup of an organism, is determined by the sequence of bases in its DNA.

Summary

Base pairing in DNA involves complementary pairing of adenine (A) with thymine (T) and guanine (G) with cytosine (C), connected by hydrogen bonds. Nucleotides, consisting of sugar, phosphate, and base, are linked together to form strands with a sugar-phosphate backbone. DNA strands are antiparallel, creating the iconic double helix structure. The genetic code, which determines an organism's genotype, is encoded in the sequence of bases in DNA, making it a fundamental molecule for all living organisms.