Describe the α-helix and the β-pleated sheet of the secondary structure of protein.

Ans : The secondary structure of protein is one of the four levels of organization in a protein. It refers to the region(s) of a polypeptide chain which is highly regular, and is the result of intrachain H bond between nearby amino acid residues. There are two common types of secondary structure : α-helix and β-pleated sheet.

α-helix

• a region where the polypeptide chain forms a uniform helical coil
• due to H bonds between the backbones of the aa in successive turns of the helical coil
• each H bond is formed b/w oxygen of the carboxyl gp of one aa and the hydrogen of the amine gp of the 4th aa down the chain
• 3.6 aa forms a complete turn of the helix

β-pleated sheet

• diff regions of a polypep chain turn back on itself
• each region has a zigzag struc that the polypep has an overall pleated conformation

Each polypeptide chain can include both α-helical and β-pleated sheet conformations. 

(Note that β-pleated sheet can also form b/w diff polypep chains as part of the quaternary struc of the protein)

Source: http://en.wikipedia.org/wiki/File:Main_protein_structure_levels_en.svg

H1 or H2 student: A very good site to view the different levels of organization of the protein is : http://www.ncbi.nlm.nih.gov/books/NBK21811/ (Fig 9-4 to 9-8). I will also recommend you read the write-up of Protein Structure.

P.S. If you are a H3 or advanced student, go ahead and read the rest of the page including Protein motif, Determing protein sequence, etc.