Fig. 1.5. The amino acid alanine

of the pair of chains results in a serious and sometimes - fatal form of anemia known as sickle cell anemia.

Both globular and fibrous proteins serve as structural molecules. Microtubules, which are important cell components, are composed of globular protein molecules assembled into a very long helix. Collagen, hair, silk, wool, horns, nails, and feathers are all fibrous structural proteins.

The other important molecules found in cells are nucleic acids: DNA and RNA. Nucleic acids are composed of units known as nucleotides, each of which is made up of nitrogen base, a five-carbon sugar (ribose or deoxyribose), and a phosphate group. DNA (deoxyribonucleic acid) is found principally in the chromosomes of cells and is the carrier of the genetic information and contains instructions for building new cells (Fig. 1.6).

One early hypothesis was that the DNA somehow formed a template for protein production. The relationship between DNA and protein had to be a more complicated one. If the proteins with their 20 amino acids, were the “language of life “, to extend the metaphor of the 1940s, the DNA molecules, with its four nitrogen bases, could be envisioned as a sort of code for this language. So the term “genetic code” came into being.

A molecule of DNA is large and it consists of two strands each made of repeating units called nucleotides.Each DNA nucleotide is made of a molecule of deoxyribose sugarjoined to a phosphate group and a base. Since DNA possesses four different kinds of bases: adenine, cytosine, thymine, guanine (these bases are called A, C, T and G for short), it has four different types of nucleotide. A single DNA molecule may be composed of many thousands of such nucleotides.