Molecular Basis of Inheritance Test - 46

RNA polymerase is the enzyme of transcription and synthesizes RNA using DNA template strand. It is a complex enzyme with five core subunits and a sixth sigma subunit which binds transiently to the core enzyme and positions it to promoters on the DNA. DNA polymerase III is major polymerizing enzyme with ten types of subunits; polymerization and proofreading activities reside in its alpha and epsilon subunits, respectively. The theta, alpha and epsilon form a core polymerase with polymerization activity but lack processivity which is imparted by clamp-loading complex (two copies of tau, gamma, delta and delta prime subunits). DNA and RNA ligases have single subunit. Correct answer is A.

As we know that genetic code show degeneracy and multiple codons can code for same amino acid. This can be explained by Wobble hypothesis. According to Wobble hypothesis, first two bases of an mRNA codon form strong Watson-Crick base pairs with the corresponding tRNA anticodon and confer most of the coding specificity. The first base of the anticodon that pairs with the third base of the codon determines the number of codons recognized by the tRNA. The presence of C/A as first base of the anticodon confers specific base pairing which means only one codon is recognized by that tRNA. The presence of U or G means less specific pairing and two different codons may be read. The presence of inosine (I) as the first nucleotide of an anticodon means that three different codons can be recognized. The triplet "UGC" codes for cysteine. Other triplets that code for cysteine is " UGU". "UGA and UAG" are stop codons that cause termination of translation. "UGG" codes for arginine. It can be observed that multiple codons for same amino acids have same bases in first two positions and determine the specificity. 

DNA replication is the bidirectional, semiconservative process of synthesis of daughter DNA strand by polymerisation of deoxyribonuclotides using parental DNA strand as template. The process is carried out by DNA polymerase. DNA polymerase cannot synthesize daughter strand de novo but can only add monomers to already present DNA strand. This requirement is fulfilled by a primer which is a strand segment (complementary to the template). It has a free 3-hydroxyl group to which a nucleotide can be added. Most primers are oligonucleotides of RNA rather than DNA. A RNA polymerase, known as primase, synthesizes the primer using DNA strand as template. Thus, RNA polymerase with primase activity control DNA replication.

The start codon specifies the first codon of mRNA transcript that is translated by a ribosome. It codes for methionine in eukaryotes and a modified Met in prokaryotes. "AUG" is the most common start codon; option A is correct. The codons, different from the standard AUG codon, but are translated as Met when present at the start of a protein are called as alternative start codons. These codons code a different amino acid otherwise. GUG, UUG, AUU and CUG are the prokaryotic alternative start codons; option B is incorrect as GUG is not primary start codon. UGA and UAA are stop codons that cause termination of translation. 

Two deoxyribonucleotides are joined together by phosphoryl group transfer. The 3’ hydroxyl group of the nucleotide at the 3’ end of the growing strand serve as nucleophile. Nucleophilic attack occurs at the $$\alpha$$-phosphorus of the incoming deoxynucleotide with release of inorganic pyrophosphate. Nucleotides have phosphate group at their 5’ carbon, the 3’ carbon has hydroxyl group; thus, the phosphoester bond cannot be formed between 5’ hydroxyl and 3’ phosphate group, option B is incorrect. Correct answer is A.

Meselson and Stahl cultured E. coli in a 15N medium to get the DNA of heavy density. Then bacteria were shifted to a 14N medium, DNA of replication cycles 0, 1, and 2 was isolated. The DNA was all of intermediate density after one replication cycle and ruled out the conservative replication model, which predicts that both heavy density DNA and light density DNA will be present. This result proved the semi-conservative replication model, according to which all DNA molecules will consist of one 15N-labeled DNA strand and one 14N-labeled DNA strand. The result of second replication cycle were also in consistent with semi-conservative replication model. So, the correct answer is 'Escherichia coli'.

Codons in option A codes for valine (GUU) and alanine (GCU). Alanine is coded by GCU, GCA, GCC, GCG. Codons in option B (UAG, UGA) do not code for any amino acid and thus cause termination of translation; hence called stop codons. Codons in option C code for methionine (AUG) and threonine. "AUG" also serves as start codon and encodes the first amino acid to be incorporated in any polypeptide of all organisms. But ACG is not a start codon. Codons in option D codes for leucine (UUA) and serine (UCA). Correct answer is option B.

A transcription unit is a sequence of nucleotides in DNA that carries the information for encoding a single RNA molecule as well as the sequences required for its transcription namely a promoter, an RNA-coding sequence, and a terminator. The promoter is a DNA segment that serves to initiate transcription of a particular gene. They are present upstream near the transcription start sites of genes. The gene that codes for any RNA / protein product which does not serve as regulatory protein is known as structural genes. The lac z, lac y and lac genes in lac operon are the structural genes. The inducer is a molecule that binds to the repressor and causes conformational changes in repressor which in turn can no longer bind to the operator. Thus, inducer induces the expression of the structural gene. It is not part of an operon. Termination sites are present downstream the structural genes which serve to terminate the transcription. The correct option is A.

Rifampicin serves as the potent inhibitor of bacterial RNA synthesis by binding to the beta-subunit of bacterial RNA polymerases thereby preventing the promoter clearance step of transcription. The active site of the enzyme is present in the cleft of beta and beta prime subunit. Transcription initiation includes binding of polymerase to the promoter, forming initiation complex, followed by transcription initiation within the complex. This, in turn, leads to a conformational change to convert the complex to the elongation form that allows movement of the transcription complex away from the promoter. It is called promoter clearance. Rifampicin does not affect any enzyme of DNA synthesis, making option B and D incorrect. Since the toxin affects the subunit responsible for promoter clearance step which is fundamental for elongation phase of protein synthesis; it is a potent inhibitor. The correct answer is A.

Marshall Nirenberg and Heinrich Mathaei incubated synthetic polyuridylate, poly(U), GTP, ATP,  E. coli extract and a mixture of the 20 amino acids in 20 different tubes containing a different radioactively labelled amino acid. The poly(U) mRNA has many successive UUU triplets and thus the resultant polypeptide should contain only the amino acid encoded by the triplet UUU. One out of 20 tubes synthesized a radioactive polypeptide that contained radioactive phenylalanine. It was concluded that the triplet codon UUU encodes phenylalanine. Khorana used same approach to reveal that polycytidylate, encodes a polypeptide containing only proline and polyadenylate encodes polylysine. Correct option is D.