1.6 - Nucleic Acids

Nucleic Acids

• DNA Structure: DNA (deoxyribonucleic acid) is a double-stranded helix composed of two polynucleotide chains.

  • Polynucleotide: A long chain of nucleotides linked by phosphodiester bonds.

  • Phosphodiester Bond: A covalent bond that links the sugar of one nucleotide to the phosphate group of the next, forming the backbone of the DNA strand.

• Antiparallel Strands: The two DNA strands run in opposite directions: one strand runs 5' to 3', and the other runs 3' to 5'. This refers to the carbon numbers in the deoxyribose sugar.

• Complementary Base Pairing: Nucleotides on opposite strands pair up specifically due to hydrogen bonding:

  • Adenine (A) always pairs with Thymine (T) (or Uracil (U) in RNA)

  • Guanine (G) always pairs with Cytosine (C)

• Purines and Pyrimidines:

  • Purines: Adenine (A) and Guanine (G) have two-ring structures.

  • Pyrimidines: Cytosine (C), Thymine (T), and Uracil (U) have one-ring structures.

DNA Replication

• 5' to 3' Direction: New DNA strands are synthesized in the 5' to 3' direction, meaning new nucleotides are added to the 3' end of the growing strand.

• Antiparallel Synthesis: Because the two DNA strands are antiparallel, replication occurs continuously on one strand (leading strand) and in fragments on the other (lagging strand).

Key Point: The complementary and antiparallel nature of DNA allows for precise replication and transmission of genetic information.

DNA vs. RNA

DNA Structure

• Double Helix: DNA consists of two antiparallel polynucleotide strands twisted into a double helix.

• Sugar-Phosphate Backbone: The backbone of each strand is formed by alternating sugar (deoxyribose) and phosphate groups linked by phosphodiester bonds.

• Nitrogenous Bases: The nitrogenous bases (A, T, G, C) extend inward from the backbone and pair up via hydrogen bonds, forming the "rungs" of the ladder.

RNA Structure

• Single Strand: RNA (ribonucleic acid) molecules are usually single-stranded, but they can fold into complex shapes due to complementary base pairing within the strand.

• Ribose Sugar: RNA contains ribose sugar, which has an extra hydroxyl (-OH) group on the 2' carbon compared to deoxyribose.

• Uracil (U): RNA uses uracil instead of thymine, which pairs with adenine (A).