Lesson Overview 12.2 The Structure of DNA

Functions of DNA DNA has three roles/purposes/functions: 1. Storing information – genes are segments of DNA that carry messages to make proteins. 2. Copying information – so that when cells divide, all cells get a complete copy of the genetic material. 3. Transmitting information – DNA is passed from parents to offspring.

Structure of DNA Deoxyribonucleic acid is a slightly acidic molecule originally identified in cell nuclei. Nucleotides are the building blocks of DNA. Each nucleotide contains a phosphate group, a five carbon sugar, and a nitrogen base. The five carbon sugar is called deoxyribose. Covalent bonds hold the sugar of one nucleotide to the phosphate group of another nucleotide to form chains.

Nitrogenous Bases DNA has four kinds of nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Adenine and Guanine are called purines. Purines have 2 rings of carbon and nitrogen atoms. Thymine and Cytosine are called pyrimidines. Pyrimidines have a single ring of carbon and nitrogen atoms. The nitrogenous bases stick out sideways from the nucleotide chain.

Chargaff’s Rules Erwin Chargaff discovered that the percentages of adenine [A] and thymine [T] bases are almost equal in any sample of DNA. The same thing is true for the other two nucleotides, guanine [G] and cytosine [C]. The observation that [A] = [T] and [G] = [C] became known as one of “Chargaff’s rules.” However, the amount of each nucleotide was not the same among different organisms.

Race to Determine the Structure Rosalind Franklin and Maurice Wilkins used X ray diffraction to take first picture of DNA. Determined a two dimensional picture of DNA’s structure. James Watson and Francis Crick – 3-D shape of DNA being 2 strands of nucleotides that form a spiral staircase or double helix.

The Double-Helix Model DNA is a double helix which looks like a twisted ladder. The phosphates & sugars make the sides of the ladder. The rung is a purine/pyrimidine pair held together by hydrogen bonds.

Antiparallel Strands The base pair rules tell us what the rungs can be, A and T or G and C. Each strand of the double helix is complementary to each other; the sequence of 1 strand determines the sequence of the other. The two strands of DNA in the double-helix are antiparallel – they run in opposite directions. This arrangement enables the nitrogenous bases on both strands to come into contact at the center of the molecule. It also allows each strand of the double helix to carry a sequence of nucleotides, arranged almost like letters in a four-letter alphabet.