Spiral staircase, secondary structure

In 1953, James Watson and Francis Crick made their classic proposal for the secondary structure of DNA. According to the Watson-Crick model, DNA under physiological conditions consists of two polynucleotide strands, running in opposite directions and coiled around each other in a double helix like the handrails on a spiral staircase. The two strands are complementary rather than identical and are held together by hydrogen bonds between specific pairs of... 

One type of secondary protein structure is like a spiral staircase and is called the a-helix. The spheres represent individual amino acids. 

One common type of secondary structure resembles a spiral staircase. This spiral stmcture is called an a-helix (see Figure 21.3). A spiral-like secondary stmcture gives the protein elasticity (springiness) and is found in the fibrous proteins in wool, hair, and tendons. Another type of secondary structure involves joining several different protein chains in an arrangement called a pleated sheet, as shown below. 

A new photo of a spiral staircase is added to illustrate the alpha helical secondary structure of proteins. 

---