Structure of DNA

Core Standards:

·        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.