CHAPTER 12 NOTES
Structure of DNA
· Describe the basic structure of DNA and how this structure enables DNA to function as the hereditary molecule that directs the production of RNA and proteins.
· Describe the proteins largely determine the traits of an organism.
DNA (Deoxyribonucleic Acid): DNA is a long, stringy, twisted molecule made up of nucleotides that stores genetic information.
Composition: DNA is composed of nucleotides. The nucleotide is the monomer.
Three parts of a nucleotide:
1. 5-Carbon Sugar: Deoxyribose
2. Phosphate Group
3. Nitrogen Base
There are four different types of nitrogen bases:
Purines: Composed of two rings
1. Adenine (A)
2. Guanine (G)
Pyrimidines: Composed of one ring
3. Thymine (T)
4. Cytosine (C)
NOTE: Uracil (U) is found only in RNA, NOT in DNA
Chargaff’s Rule: DNA contains equal percentages of A and T and equal percentages of G and C.
· A molecule of DNA is best described as a double helix because it consists of two long strands of nucleotides which are twisted together and when untwisted resemble a ladder.
· James Watson and Francis Crick along with Rosalind Franklin discovered the shape of the DNA molecule in 1953.
· The sides of the ladder are made of alternating deoxyribose sugars and phosphate groups held together by covalent bonds.
· The “rungs” of the ladder are made up of pairs of nitrogen bases. They follow the “base-pairing rule”.
Base Pairing Rule:
· A purine always pairs with a pyrimidine.
· A = T: Held together with two hydrogen bonds forming a double bond.
· C = G: Held together with three hydrogen bonds forming a triple bond.
· The “rungs” of the ladder are attached to the sugar part of the side (NOT the phosphate group).
· The nitrogen bases are held together by weak hydrogen bonds. These weak bonds break apart easily.
Functions of DNA:
· DNA codes for proteins and hormones to be made.
· Proteins result in genetic traits.
· Hormones result in your body functioning.
· The order or sequence of the 4 different nucleotide base pairs and the number of them determines what proteins are made and therefore, what genetic traits are present.