BIO202 Final Term Past Papers by Muhammad Kashif

1) Chargaff’s Rules.

• Due to specific base pairing of DNA i.e A toT and G to C
• In any sample of double-stranded DNA the amount of adenine equals the amount of thymine.
• The amount of guanine equals the amount of cytosine
• the total amount of purines equals the total amount of pyrimidines ;
• i.e. A + G = T+ C.
• The base pairs are held together by hydrogen bonds
• Two between A and T
• Three between G and C
• These hydrogen bonds, plus the van der Waals and hydrophobic interactions between the adjacent stacked bases, stabilize the structure of the double helix. The base composition of DNA varies from one species to another, but is same if isolated from different tissues of same species. The base composition does not change with age, nutritional status and environment.

• 2)Types of Reversible Inhibitors..

The two most commonly encountered types of reversible inhibition are;

• competitive and
• noncompetitive.
• Competitive inhibitors resemble the substrate and compete for binding to the active site of the enzyme.
• Noncompetitive inhibitors do not bind at the active site. They bind either free enzyme at a site other than active site or the ES comcomple

3) Ribozymes

some RNA molecules have intrinsic catalytic activity these RNA enzymes are called ribozyme.

4)Condensation Reaction with Example

Chemical reaction which involves removal of a water molecule is called Condensation reaction.

• The N-glycosyl bond is formed by removal of the elements of water , a hydroxyl group from the pentose and hydrogen from the base, Thus it is a condensation reaction.
• Similar to O-glycosidic bond formation in carbohydrates.

5)Isozyme of hexokinase

The isozyme of hexokinase, called glucokinase, which is found in the liver has a much higher Km of approximately 5 to 6 mMhelpful in storing large amounts of “excess” glucose as glycogen or converting it to fat after a carbohydrate meal.

6)Reaction Velocity (v)

The rate or velocity of a reaction (v) is the number of substrate molecules converted to product per unit time; Velocity is usually expressed as µmol of product formed per minute. The rate of an enzyme-catalyzed reaction increases with substrate concentration until a maximal velocity (Vmax) is reachedreflecting the saturation with substrate of all available binding sites are occupied on the enzyme molecules present. (Vmax) is reachedreflecting the saturation with substrate of all available binding sites are occupied on the enzyme molecules presen

7)Unsual or Modified Bases of RNA

• t RNA molecules consist of 74-95 nucleotides in a particular sequence.
• The t RNA molecules contain not only the usual bases like adenine, guanine, cytosine, uracil but also contain unusual bases
• These unusual bases(also called modified bases) include
• Dihydrouracil • Pseudouridine