Molecular Biology MCQ 2 - IIT JAM MCQ

One of the characteristics of the enzymes is that they are not consumed in the reaction they catalyse, but this is not true for ribozymes which get consumed in the reaction.

Rec BCD is an enzyme which initiates recombinational repair, wherever there is double stranded break in DNA. RecBCD acts both as a helicase, which separates the two strands of DNA and nuclease which makes single stranded nicks in DNA. RecA is then recruited on to these single stranded nicks which proceed with the repair.

According to the principle of degeneracy of genetic codons, more than one codon code for the same amino acid. Out of the given options, only codon GGU and GGC code for the same amino acid i.e. Glycine. On the other hand AUG codes for methionine, AUC codes for isoleucine, UAA and UAG are stop codons, and UAC codes for tyrosine.

One of the post- transcriptional modification of eukeryotic mRNA is the addition of cap at its 5' end. This cap consists of a guanine nucleotide connected  to mRNA via an unusual 5' to 5' triphosphate linkage. Also this guanine is methylated at position 7, thereby forming 7-methylguanosine cap.

Griffith first showed the process of transformation through different strains of Streptococcus pneumoniae. Avery- MacLeod showed that DNA is the transforming factor in Griffith’s experiment. Hershey and Chase experiment using a bacteriophage further proved that DNA and not protein is the genetic material. Watson and Crick described the double helical model of the DNA.

DNA polymerase cannot synthesize a new strand de novo. The enzyme functions by adding the nucleotides to the free 3’-OH groups of the preexisting nucleotide. Thus a primer, which is a short strand of RNA or DNA consisting of 18-20  bases. 

Small interfering RNAs are a class of double stranded RNA molecules, which play important roles in gene silencing through RNA interference pathway. Structurally, they are formed after long dsRNA is cleaved by enzyme Dicer. Once siRNA enters inside the cell through transfection, it is incorporated into other proteins to form RISC (RNA induced silencing complex). Inside the RISC, siRNA is unwound to form ssRNA. The ssRNA, which is a part of RISC scans and finds a complementary mRNA and induces its cleavage and thus silences the corresponding gene.

The Ames test is a widely employed biological assay which is used to check whether the chemical under study can induce the mutations in the test organism or not. This test is neither employed to check the potency of various antibiotics, nor to determine the rate of DNA replication.

Methyl directed repair is a type of mismatch repair system, whereby inserted, deleted and mismatched bases are detected and repaired. During this repair, the repair machinery recognises hemimethylated DNA (methylated placental strand and non-methylated daughter strand), removes the mismatched nucleotide from daughter strand and the gap so created is repaired by DNA polymerase III.

In eukaryotes, different RNA polymerases are specialised for the synthesis of different types of RNA for Eg. RNA polymerase I synthesizes 28S, 18S and 5.85 rRNAs. RNA polymerase II synthesizes mRNAs and RNA polymerase III synthesizes tRNAs and 5Sr RNAs.

Topoisomerases are the enzymes which are involved in underwinding or overwinding of DNA. All these enzymes bind to double stranded DNA, make an incision in one or both strands of DNA, pass the DNA strands through one another and then seal the breaks. These enzymes always require energy, but the source can be NAD or ATP.

Z-DNA is one of the biologically active double helical structures. It is a left handed helix and is formed under conditions of high salt concentration. Structurally, it consists of alternating purine and pyrimidine residues and is formed at a pH around 7.4 

Telomerase is an enzyme which adds TTAGGG repeact sequences at the 3' end of telomeres in eukaryotes and prevents telomere shortening. This enzyme has its own RNA, which serves as a template for telomere synthesis. Telomerase works in a cyclic fashion, after every cycle, it realigns the 3' end of telomeres to the template and repeat the process. It has no role in the synthesis of RNA primers    

Out of the given options, statements 1 and 3 are incorrect. All the parts of DNA are replicated equally, similarly, once the final mRNA is formed after post transcriptional modifications, the entire mRNA is translated equally. But, we can say that according to the needs of a cell, some parts of DNA are transcribed more, some transcribed less and some not transcribed at all.

RNA polymerase itself unwinds the DNA and does not need another helicase enzyme for this purpose.

The rRNA has complementary regions where it pairs, and the other regions that do not pair form loops. Thus, there are 4 complementary regions in the tRNA which pair up to form double stranded regions. One of the loops has an anticodon at one end which recognizes the codon of mRNA during the process of translation.

The heteronuclear RNA or hnRNA is the RNA before splicing has occurred. They are pre-mRNAs that contain both introns and exons, i.e. the total DNA information in a contiguous (adjacent) manner. UTRs or untranslated regions are present either upstream(5’-UTR) or downstream (3’-UTR) to the coding segment of the gene. These do not form part of the proteins but are very much part of the exons and are retained after splicing.

If the nucleotide is synthesized at a rate of 50  nucleotides per second, then the number of codons would be 50/3. This corresponds to 16 codons and 2 nucleotides. Hence, a peptide of 16 amino acids would form in one sec. Thus, the rate at which protein is synthesized would be 16 amino acids/sec

There are 390 bases in mRNA and they lead to the synthesis of 390/3 = 130 amino acids after translation. Average weight of an amino acid is 110 Daltons. Hence the weight of translated product is 130*110 = 14300 Daltons or ~14KDa.

Number of base pairs present per turn are 10. Hence in a stretch of 180 base pairs we get 180/10 = 18 turns. 1.5 turns makes one transcription bubble hence when there are 18 turns we have 18/1.5 = 12 transcription bubbles.

When DNA is relaxed

DNA gyrase underwinds the DNA by 2 turns in each cycle
So. it would underwind the DNA by 2 x 20 = 40 nuns in 20 cycles
Hence, the new linking number would be Lk = Lk₀ - 40 = 160