Large Molecules Problem Set

Problem 6: Identification of a monomer unit of a biological macromolecule

Tutorial to help answer the question

The structure shown in the diagram is an example of a monomer unit used in the formation of:

A. RNA B. protein C. DNA D. polysaccharides E. lipids

Tutorial

Nucleotides

The molecule illustrated in the question is a nucleotide. Nucleotides, the monomer units of RNA and DNA, consist of a pentose sugar, either ribose in RNA or deoxyribose in DNA, a phosphate group, and a nitrogenous base. As the name implies, a pentose is a 5-membered, puckered ring. Attached to the ring is the phosphate group, which is a phosphorous atom with 4 covalently attached oxygen atoms. Nucleotides also have either a pyrimidine or purine base, attached to the pentose sugar.

Pyrimidines are planar, six-membered rings of 5 carbon and 1 nitrogen atom. Purines are planar, fused N-containing rings. The purine and pyrimidine bases of RNA and DNA are shown below.

RNA and DNA are polymers of nucleotides. The backbone of the polymer is a repeating chain of sugar-phosphate-sugar-phosphate- etc. These polymers have "polarity", meaning having a directionality. The phosphate of the 5'-end of one nucleotide is linked to the 3'-position of the next nucleotide. In the model of DNA (left), the left strand has a polarity of 5' (top) to 3' (bottom), and the other DNA strand has the opposite polarity. The two DNA strands are said to be "antiparallel".

In the RNA model, note the extra -OH of the pentose sugar, and the use of the uracil base (U) rather than the thymine (T) base of DNA.