Explain the terms primary and secondary structure of proteins. What is the difference between $\alpha$-helix and $\beta$-pleated sheet structure of proteins?
Primary structure of proteins Proteins consist of one or more polypeptide chains. Each polypeptide is a protein contains amino acids joined with each other in a specific sequence. Secondary structure of proteins It refers to the shape in which a long polypeptide chain can exist.
| $\alpha$-helix structure | $\beta$-pleated sheet structure |
|---|---|
| A structure of twisting of all a polypeptide chain formed by possible H-bonds into a right handed screw (helix) with the $-$NH group of each amino acid, and residue hydrogen bonded to the $-$ CO $-$ of an adjacent turn of the helix. Hence, called $\alpha$-helix. | All peptide chains are stretched out to nearly maximum extension and then laid side by side which are held together by intermolecular hydrogen bonds. This structure resembles the pleated folds of the drapery. Hence, called $\beta$-pleated sheet structure. |
Write the structures of fragments produced on complete hydrolysis of DNA. How are they linked in DNA molecule? Draw a diagram to show pairing of nucleotide bases in double helix of DNA.
On complete hydrolysis of DNA, following fragments are formed a pentose sugar (3-D-2-deoxyribose) phosphoric acid $\left(\mathrm{H}_3 \mathrm{PO}_4\right)$ and bases (nitrogen containing heterocyclic compounds).
(iii) Nitrogen bases DNA contains four bases Adenine (A), Guanine (G), Cytosine (C) and thymine (T).
A unit formed by the attachment of a base to 1-position of sugar is called nucleoside. When nucleoside links to phosphoric acid at 5'-position of sugar moiety, a nucleotide is formed. Nucleotides are joined together by phosphodiester linkage between $5^{\prime}$ and 3' carbon atoms of the pentose sugar. In DNA two nucleic acid chains are wound about each other and held together by hydrogen bonds between pairs of bases.
The two strands are complementary to each other because hydrogen bonds are formed between specific pair of base adenine form hydrogen bonds with thymine whereas cytosine form hydrogen bonds with guanine.
