13 2 The Structure of DNA Learning Objectives
13. 2 The Structure of DNA
Learning Objectives § Identify the chemical components of DNA. § Describe the clues that helped scientists determine the structure of DNA. § Explain what the double-helix model shows about DNA. Vocabulary § Base pairing-Erwin Chargaff discovered that the ratios of adenine (A) and thymine (T) and cytosine (C) and guanine were always the same. “Chargaff’s Rule” A=T and C=G, because A pairs with G and C pairs with G.
Nucleotide Structure • DNA is made up of nucleotides joined into long strands or chains by covalent bonds. • Nucleic acids are made up of building blocks called nucleotides. Phosphate group Base 5 -carbon sugar • There are five types of nucleotides: adenine, thymine, cytosine, guanine and uracil.
Nitrogenous Bases Adenine Guanine Cytosine Thymine
Nucleic Acid Structure One nucleotide Covalent bond between nucleotides
![Chargaff’s Rule [A] = [T] and [C] = [G]](http://slidetodoc.com/presentation_image_h2/78ca2fd6e213016ef2cd92f851be5ab0/image-6.jpg "Chargaff’s Rule [A] = [T] and [C] = [G]")
Chargaff’s Rule [A] = [T] and [C] = [G]
Franklin’s X-rays • DNA is a helix. • Likely two strands to the molecule • Nitrogenous bases near the center of the molecule
The Work of Watson and Crick DNA is a double helix, in which two strands of nucleotide sequences are wound around each other.
The Double Helix: Antiparallel Strands The two strands in a DNA molecule run in opposite directions.
The Double Helix: Hydrogen Bonding Hydrogen bonds
The Double Helix: Base Pairing The two strands of DNA are held together by hydrogen bonds between the nitrogenous bases adenine and thymine and between guanine and cytosine.
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