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Biology LibreTexts

Part 1: DNA Structure

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  • DNA Structure

    In 1953, James Watson and Francis Crick established the structure of deoxyribonucleic acid (DNA). The structure is a double helix, which is like a twisted ladder. The sides of the ladder are made of alternating sugar (deoxyribose) and phosphate molecules.

    The rungs of the ladder are pairs of 4 types of nitrogen bases. Two of the bases are purines - adenine and guanine. The pyrimidines are thymine and cytosine. The bases are known by their coded letters A, G, T, C. These bases always bond in a certain way: adenine will only bond to thymine and guanine will only bond with cytosine. This is known as the base-pair rule. The bases can occur in any order along a strand of DNA. The order of these bases is unique and codes for specific genes.

    The combination of a single base, a deoxyribose sugar, and a phosphate make up a nucleotide. DNA is actually a molecule of repeating nucleotides.

    The two strands of DNA are held together loosely by hydrogen bonds. The strands are also antiparallel, meaning that they run parallel but in opposite directions. One strand will go in the \(5'\rightarrow3'\) direction andone will go \(3'\rightarrow5'\). The 5’ end has a phosphate group attached and the 3’ end does not.


    • DNA Puzzle Kits

     **Before beginning this lab, please confirm that your kit contains the correct materials listed in the table below.

    Number Component Color
    24 Phosphate Yellow
    24 Deoxyribose (sugar) Red
    12 Ribose (sugar) Pale Pink
    4 Adenine (base) Light Green
    4 Thymine (base) Light Blue
    8 Guanine (base) Dark Blue
    8 Cytosine (base) Dark Green
    2 Uracil (base) White
    2 Alanine (Amino Acid) Tan
    2 Glycine (Amino Acid) Brown
    2 Alanine activating enzyme Tan
    2 Glycine activating enzyme Brown
    2 Alanine-specific tRNA Tan
    2 Glycine-specifc tRNA Brown
    1 Ribosome sheet White worksheet


    1. Using the materials in your kit, please assemble the longest complete DNA segment you can. In other words, all of the deoxyriboses should be used up! (You may use the photo below as a reference.)

    Keep these factors in mind:

    a. DNA is a double stranded molecule

    b. The double helix is antiparallel

    c. DNA includes deoxyribose and thymine

    d. Base-pairing rules apply

    2. Once you have assembled your molecule, record it in the space below:

    5'     3'
    3'     5'


    3. Disassemble your DNA molecule.