Macromolecules: Nucleic Acids and Proteins Animations

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DNA and RNA are nucleic X acids (polymers of nucleotides). Two polymers with complementary nucleotide sequences can pair with each other. This pairing endows nucleic acids with the ability to store, transmit, and retrieve genetic information.

Two strands of DNA pair by S hydrogen bonding. A component of one nucleotide, called a base, forms a hydrogen bond with a complementary base on the opposite strand. The base cytosine (C) pairs with guanine (G), and adenine (A) pairs with thymine (T).

The monomer of the nucleic I acid—the nucleotide—is composed of three parts. The central component is a pentose sugar.

A nitrogen-containing base is covalently linked to the 1' carbon on the sugar. This base can either be a purine with two fused rings (as in adenine and guanine) or a pyrimidine with a single ring (as in cytosine and thymine). In RNA, the thymine base is replaced by uracil.

A phosphate group is covalently attached to the 5' carbon of the sugar molecule. The phosphate group, the base, and the sugar make up a complete nucleotide. Note that the orientation of the sugar provides DNA with a 5' to 3' directionality.

Base pairing only occurs between a pyrimidine and a purine. More specifically, a guanine only pairs with a cytosine (forming three hydrogen bonds, indicated by the green circle), and an adenine only pairs with a thymine (forming two hydrogen bonds, indicated by the red circle).

A macromolecule's structure is intimately connected with its function. Consider a nucleic acid. This type of polymer is made up of a chain of nucleotides that are strung together in a precise sequence. The nucleotide sequence provides a code that stores genetic information and that the cell can copy and pass on to the next generation of cells. In a similar way, the precise sequence of monomers (amino acids) in a protein also acts as a kind of code. In this case, the amino acid sequence determines the protein's 3-dimensional shape and chemical reactivity, which, in turn, endow a protein with its specific function. Some proteins, for example, have shapes that allow them to grab molecules and speed chemical reactions. Others, such as strong cables of collagen, provide structural support to cells and tissues.

Proteins are chains of amino acids linked by peptide bonds. The 20 different amino acids used to make all proteins differ only in their side chains, and the properties of these side chains account for the great diversity of protein structure and function.

Collagen is an example of how a protein's amino acid sequence determines its structure and function. Amino acid sequences are encoded in the DNA of genes.

Collagen is the most abundant mammalian protein. It is the main component of skin, bones, and teeth. This protein is composed of three helical polypeptide chains that form a stiff, supercoiled cable. The type of helix in each polypeptide is unique to collagen and results from collagen's unusual amino acid composition.

A collagen helix forms largely from the influences of two types of amino acids: proline, which introduces sharp twists in the polypeptide, and glycine, which has a small side chain (H) that doesn't interfere with packing in the helix. Collagen contains many of these amino acids, but few bulky ones (e.g., phenylalanine).

Each helix contains three amino acids per turn, with glycine located at every third position. The three polypeptides in a collagen molecule associate with their glycines all facing collagen's center. Glycine is the only amino acid small enough to allow the polypeptides to pack (by hydrogen bonding) into a tight cable.

Individual collagen molecules cross link to other collagen molecules to form tough collagen fibrils. Collagen, from which gelatin is derived, is a poor source of essential amino acids the amino acids that the body cannot manufacture and must receive from the diet.