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2. What is this picture, and how did it help us to understand the shape of DNA? 3. Label the nucleotide and double helix pictured below with the three parts of a nucleotide deaxyribose suger, phosphate group, nitrogenous base. ΝΗ, H-C CH -O-P-O-CHO O=0 H TH H H CH 4. If we think of a DNA molecule as a ladder.... make Alternating and make up the sides of the ladder up the "rungs" (middle bars) of the ladder. The "rungs" are held together by bonds. 5. Purines and pyrimidines are two types of nitrogen bases. Explain the difference between purines and pyrimidines, and list the nitrogen bases that fall under each category. Difference: Purines Pyrimidines Display Settings D Focus

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  1. The picture you are referring to is likely an image of the DNA double helix, which is often represented using a twisted ladder diagram. This structure was famously elucidated by James Watson and Francis Crick in 1953, and it helped us understand that DNA is composed of two long strands that twist around each other to form a double helix. The rungs of the ladder are made up of pairs of nitrogenous bases, which connect the two strands, while the sides of the ladder are made up of sugars and phosphate groups.

  2. In the diagram of a nucleotide, there are three main components:

    • Deoxyribose sugar (the sugar molecule that forms part of the DNA backbone)
    • Phosphate group (which links the sugar molecules together in the backbone)
    • Nitrogenous base (which pairs with complementary bases on the opposite strand)

    To label the components in the nucleotide diagram:

    • The sugar would be depicted with a five-carbon structure called deoxyribose.
    • The phosphate group is typically shown as a phosphorus atom bonded to four oxygen atoms.
    • The nitrogenous base is attached to the sugar and can be adenine (A), thymine (T), cytosine (C), or guanine (G).

  3. Thinking of a DNA molecule as a ladder:

    • The sides of the ladder are made up of alternating deoxyribose sugars and phosphate groups.
    • The rungs of the ladder are formed by pairs of nitrogenous bases. The pairs of bases are connected by hydrogen bonds: adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G).

  4. The difference between purines and pyrimidines lies in their structure:

    • Purines are larger nitrogenous bases characterized by a double-ring structure. The two purines in DNA are:

      • Adenine (A)
      • Guanine (G)

    • Pyrimidines are smaller nitrogenous bases with a single-ring structure. The two pyrimidines in DNA are:

      • Cytosine (C)
      • Thymine (T)

To summarize:

  • Purines: Adenine (A), Guanine (G)
  • Pyrimidines: Cytosine (C), Thymine (T)