NEET Practice Questions (MCQs) with Answers & Solutions

Mast cells are large irregular cells with granular cytoplasm, present in the areolar connective tissue. These cells store inflammation producing substances such as histamine in dense granules. Mast cells defend themselves which in turn helps in body defence by attracting phagocytes to the injured tissue.

(b) B-DNA is helical structure with 20Å and the distance between the two base pairs is 3.4Å and there are 10 base pairs in each turn or pitch (one round). Hence, one turn of the helix is approximately 34Å or 3.4 nm (10Å = 1.0 nm). Z-DNA (in comparison to B-DNA) is left handed double helical structure in which double helix winds to left in zig-zag pattern (instead of right, like B-DNA). 

(b) A competitive inhibitor competes with substrate molecule for occupying the active site of an enzyme. These inhibitors have structural resemblance with substrate molecules due to which they easily get active site of an enzyme and form an enzyme-inhibitor complex.

              E        +          I          $\to$   EI complex

       (enzyme)        (inhibitor)

(c) Total DNA (100)= A + T + C + G

                         A = 20% (given)

                         A = T (base pairing rule)

                     100 = 20+ 20 + C + G

                  C + G = 100-40 = 60

                         C = G = 30

(c) Length of one helix or the pitch per turn of helix is 34 Å in B-DNA, 25 Å in A-DNA and 46 Å in Z-model of DNA. In Z-DNA sugar molecules are seen in opposite direction. So the 3-5 diester bond forms zig-zag structure in Z-DNA.

(d) In DNA molecule the adjacent deoxyribonucleotides are joined in a chain by phosphodiester bridges or bonds, which link the 5' carbon of deoxyribose of one mononucleotide unit with 3' carbon of deoxyribose of next mononucleotide unit.

According to Watson and Crick DNA molecule consists of two such polynucleotide chains wrapped helically around each other, with the sugar phosphate chain on the outside and purine and pyrimidine on the inside of helix.

The two strands run antiparallely, i.e. one strand has phosphodiester linkage in 3'$\to$5' direction while other strand has phosphodiester linkage in 5'$\to$3' direction.

Chargaff (1950) suggested that despite wide compositional variations exhibited by different types of DNA the total amount of purines equalled the total amount of pyrimidines (A+G = T+C).   

(b) The major portion of the dry weight plants comprises carbon, nitrogen, oxygen, hydrogen.

(b) is incorrect with respect to enzyme action because addition of a lot of succinate reverse the inhibition of succinic dehydrogenase by malonate. If substrate succinate concentration is increased, it will remove malonate from its active site and thus succinate reverses the inhibition of dehydrogenase.

Inhibition of succinic dehydrogenase by malonate is an example of competitive inhibition.Competitive inhibition occurs when enzyme and inhibitor both have more or less similar structure and are present in higher concentration. Thus, both enzyme and inhibitor for compete for the active site of enzyme resulting in the decrease of the enzymatic activity. 

Two free ribonucleotide units are linked together by a phosphodiester bond. This bond is formed by the condensation reaction between the 5' phosphate group of one nucleotide and the 3' hydroxyl group of another nucleotide, with the elimination of a water molecule.

(b) Bacteria are prokaryotic organisms. They possess a double stranded DNA molecule as it's genetic material. Polyamines like spermidine and cadaverine (instead of histones) are associated with DNA packaging in bacteria.