DNA Structure, Replication, and Repair: Study Notes

DNA Structure, Replication, and Repair

DNA Structure

DNA (deoxyribonucleic acid) is the hereditary material in almost all living organisms. Its structure is essential for its function in storing and transmitting genetic information.

• Nucleotide: The basic building block of DNA, consisting of three components:

  • Nitrogenous Base: Can be a purine (adenine, guanine) or a pyrimidine (cytosine, thymine in DNA; uracil in RNA).

  • Pentose Sugar: Deoxyribose in DNA, ribose in RNA.

  • Phosphate Group: Links nucleotides together via phosphodiester bonds.

• Double Helix: DNA consists of two antiparallel strands forming a double helix, with sugar-phosphate backbones on the outside and nitrogenous bases paired on the inside.

• Base Pairing: Complementary base pairing occurs via hydrogen bonds: A-T (2 bonds), G-C (3 bonds).

• Directionality: Each DNA strand has a 5' (phosphate) end and a 3' (hydroxyl) end.

Nucleotide Structure and Polymerization

Nucleotides are joined together to form nucleic acids through condensation reactions.

• Phosphodiester Bond: The linkage between the 3' hydroxyl group of one nucleotide and the 5' phosphate group of the next.

• Polymerization: DNA polymerases add nucleotides to the 3' end of a growing DNA strand.

• ATP (Adenosine Triphosphate): A nucleotide that serves as an energy source for many cellular processes, including DNA synthesis.

$\text{Phosphodiester bond:} \quad \text{3'-OH} + \text{5'-phosphate} \rightarrow \text{DNA backbone}$

Key Features of DNA Strands

• Each nucleic acid strand has only one free 5' phosphate and one free 3' hydroxyl group.

• DNA synthesis always proceeds by adding nucleotides to the 3' end.

• Antiparallel arrangement: The two strands run in opposite directions (5' to 3' and 3' to 5').

Example: Nucleotide Components

Component	Description	Example
Nitrogenous Base	Purine or Pyrimidine	Adenine, Thymine, Guanine, Cytosine
Pentose Sugar	5-carbon sugar	Deoxyribose (DNA), Ribose (RNA)
Phosphate Group	Links nucleotides	Forms backbone

Additional info:

• DNA's structure was first described by Watson and Crick in 1953, based on X-ray diffraction data from Rosalind Franklin.

• The stability and specificity of base pairing are crucial for accurate DNA replication and repair.