Molecular Basis of Inheritance MCQs

Degeneracy = redundancy.

UGG is not a stop codon (Trp).

Friedrich Miescher (1869) was the first to identify DNA as an acidic substance inside the nucleus and named it "Nuclein". Later, Altmann renamed it "Nucleic Acid" due to its acidic properties. Maurice Wilkins and Rosalind Franklin produced the critical X-ray diffraction data of DNA. Based on this data, James Watson and Francis Crick proposed the famous Double Helix model of DNA structure in 1953.

The Phosphodiester bond connects nucleotides to form the sugar-phosphate backbone of the DNA strands. The N-Glycosidic bond links the nitrogenous base to the pentose sugar. Hydrogen bonds form between complementary bases (A-T, G-C) holding the two strands together. Stacking interactions between the stacked base pairs provide additional stability to the helical structure.

The Histone Octamer forms the core of the nucleosome, consisting of two molecules each of H2A, H2B, H3, and H4. The H1 Histone acts as a linker or plug, sealing the DNA as it enters and exits the nucleosome. DNA is negatively charged due to its phosphate groups, allowing it to wrap around the positively charged histones. Lysine and Arginine are the abundant basic amino acids that give histones their positive charge.

Injecting the virulent S-strain (Smooth) kills the mice. Injecting the non-virulent R-strain (Rough) allows the mice to live. Heat-killed S-strain alone is non-virulent, so mice live. However, mixing Heat-killed S-strain with Live R-strain causes the mice to die because the R-strain is transformed into the virulent S-strain by the DNA from the heat-killed bacteria.

Hershey and Chase used Radioactive Sulfur (S-35) to label the protein coat (capsid) because amino acids contain sulfur. Radioactive Phosphorus (P-32) was used to label DNA because DNA contains phosphate. Blending agitates the mixture to shear off empty viral coats. Centrifugation spins the mixture to separate the heavy bacterial pellet (containing DNA) from the lighter viral supernatant.

DNA Helicase unzips or unwinds the DNA double helix. Topoisomerase (like DNA Gyrase) relieves the tension or supercoiling that builds up ahead of the replication fork. Primase synthesizes short RNA primers needed to start replication. DNA Ligase joins the discontinuous Okazaki fragments on the lagging strand.

The Leading Strand is synthesized continuously in the 5 to 3 direction towards the replication fork. The Lagging Strand is synthesized discontinuously away from the fork. Okazaki Fragments are the short DNA segments produced on the lagging strand. Single-Strand Binding (SSB) proteins stabilize the separated strands and prevent them from snapping back together.

The Promoter is located upstream at the 5-prime end of the coding strand. The Terminator is located downstream at the 3-prime end of the coding strand. The Template Strand has 3-prime to 5-prime polarity and guides synthesis. The Coding Strand has the same sequence as the RNA (except Thymine is replaced by Uracil) and runs 5-prime to 3-prime.