Molecular Basis of Inheritance MCQs

The Histone Octamer is the core of the nucleosome, formed by two copies each of H2A, H2B, H3, and H4. DNA is the negatively charged polymer (due to phosphate groups) that wraps around the octamer. H1 Histone is the linker histone that binds to the linker DNA where it enters and leaves the nucleosome. Lysine and Arginine are the positively charged basic amino acids abundant in histones.

Euchromatin is loosely packed, stains lightly, and is transcriptionally active. Heterochromatin is densely packed, stains darkly, and is transcriptionally inactive. The Nucleosome is the repeating unit of chromatin, giving the "beads-on-string" appearance. Non-histone Chromosomal (NHC) proteins are required for the packaging of chromatin at higher levels.

Radioactive Sulfur (S-35) labels the protein coat because amino acids (Methionine, Cysteine) contain sulfur, but DNA does not. Radioactive Phosphorus (P-32) labels DNA because the backbone contains phosphates. Blending is the agitation step to shear off viral coats. Centrifugation separates the heavier bacterial pellet (containing DNA) from the lighter viral supernatant.

Helicase unwinds the DNA double helix by breaking hydrogen bonds. Topoisomerase relieves the torsional strain/supercoiling ahead of the replication fork. Primase synthesizes the short RNA primer needed for DNA polymerase to start. DNA Ligase joins the discontinuous Okazaki fragments on the lagging strand.

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

In eukaryotes, RNA Polymerase I transcribes large rRNAs (28S, 18S, 5.8S). RNA Polymerase II transcribes hnRNA, which is processed into mRNA. RNA Polymerase III transcribes tRNA, 5S rRNA, and snRNAs. Mnemonic: I-rRNA, II-mRNA, III-tRNA (RMT).

Capping adds a 7-methylguanosine cap to the 5-prime end. Tailing adds a Poly-A tail (Adenylate residues) to the 3-prime end. Splicing removes non-coding Introns. Exons are the coding sequences retained in the mature mRNA.

Degeneracy means some amino acids are coded by more than one codon. Unambiguous means a codon codes for one specific amino acid only. Universal means the code is shared by almost all organisms. Comma-less means the code is read contiguously.

Aminoacyl-tRNA synthetase charges tRNA with amino acids. 23S rRNA acts as a ribozyme (peptidyl transferase) in bacteria. Release factors bind to stop codons to end translation. UTRs (Untranslated Regions) improve the efficiency of translation.

The i gene codes for the Repressor. The z gene codes for Beta-galactosidase (hydrolyzes lactose). The y gene codes for Permease (increases permeability). The a gene codes for Transacetylase.