The Structure of DNA -Identify the components of DNA and how they pair up. -Discuss the scientists responsible for the identification of DNA’s structure and bonding. -Describe the steps leading to the development of the double-helix model of DNA.

Lesson Overview Lesson Overview The Structure of DNA Nucleic Acids and Nucleotides The Nucleic acids are DeoxyriboNucleic Acid (DNA) and RiboNucleic Acid (RNA). Nucleic acids are composed of nucleotides.

Lesson Overview Lesson Overview The Structure of DNA The Nucleotides that make up DNA are: 1. Adenine (A), 2. Guanine (G), 3. Cytosine (C), 4. Thymine (T).

Lesson Overview Lesson Overview The Structure of DNA What are Nucleotides made of??? DNA’s 4 nucleotides are made up of 3 basic components: 1. a 5-carbon sugar called Deoxyribose; 2. a phosphate group; 3. a nitrogenous base.

Lesson Overview Lesson Overview The Structure of DNA Chargaff’s Rules? Erwin Chargaff discovered that the percentages of adenine (A) and thymine (T) bases are always equal in any sample of DNA. The same thing is true for the other two nucleotides as well, guanine (G) and cytosine (C). The observation that A = T and G = C became known as one of “Chargaff’s rules.”

Lesson Overview Lesson Overview The Structure of DNA Franklin’s X-Rays In the 1950s, British scientist Rosalind Franklin used a technique called X-ray diffraction to try to figure out the structure of the DNA molecule. X-ray diffraction revealed an pattern showing that the strands in DNA are twisted around each other like the coils of a spring.

Lesson Overview Lesson Overview The Structure of DNA Along come Watson and Crick… At the same time, James Watson, an American biologist, and Francis Crick, a British physicist, were also trying to understand the structure of DNA. Early in 1953, Watson was shown a copy of Franklin’s X-ray pattern. The clues in this picture enabled them to build an accurate model of DNA.

Lesson Overview Lesson Overview The Structure of DNA The Double-Helix Model A double helix looks like a twisted ladder. The double helix accounted for Franklin’s X-ray pattern and explains Chargaff’s rule of base pairing and how the two strands of DNA are held together.

Lesson Overview Lesson Overview The Structure of DNA Antiparallel Strands In the double-helix model, the two strands of DNA 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.

Lesson Overview Lesson Overview The Structure of DNA Purines and Pyrimidines

Lesson Overview Lesson Overview The Structure of DNA How Purines and Pyrimidines Bond

Lesson Overview Lesson Overview The Structure of DNA Hydrogen Bonding H ydrogen bonds form between nitrogenous bases, providing just enough force to hold the two DNA strands together. Hydrogen bonds are weak chemical bonds that allow for the two strands of the DNA to separate during replication.

Lesson Overview Lesson Overview The Structure of DNA *While the nitrogenous bases (A,T,C,G) are held together by hydrogen bonds  the nucleotides that make up the backbone of the DNA molecule are held together by covalent bonds.