20) The most common sigma subunit in E. coli is? A) ?i C) o33 D) ?70 E) o\" 21)
ID: 256917 • Letter: 2
Question
20) The most common sigma subunit in E. coli is? A) ?i C) o33 D) ?70 E) o" 21) Transcription of a DNA template by a single RNA polymerase leads to how many strands of RNA? A) 1 B) 10 C) 50 D) Up to 1,000 E) None of the above 22) Several ribosomes translating a single mRNA at the same ti A) Episome B) Multisome C) Ribosome multiplex D) Translosome E) None of the above 23) Which of the following is/are true concerning tRNA? A) tRNA contains thymine instead of uracil. B) tRNA is a single strand of RNA with regions of double-strandedness. C) The anticodon is identical in sequence to the codon on mRNA. D) All of the above E) A and BExplanation / Answer
20) Sigma factors are part of RNA polymerase and required to initiate transcription in E.coli. They allow the RNA polymerase to bind to DNA. This results in formation of open complex and initiates transcription. The sigma factors can be divided into two classes: ?70 and ?54. The ?70 family is divided RpoD, RpoS, RpoH/FliA and RpoE/FecI. The most common sigma factor is ?70.
Right answer is D. ?70.
21) The length of DNA that extends from the promoter to the termination site is known as a transcription unit. A single RNA is obtained from one transcription unit, which may have one or several genes. RNA is single stranded, hence a single RNA polymerase will form one strand of RNA from DNA.
Right option is A.
22) Multiple ribosomes translating a single mRNA molecule at the same time are known as polysomes. These ribosomes begin at first codon on mRNA and move one at a time until they reach the stop codon. As a result, multiple strings called polypeptides are produced.
Right option is E. None of the above.
23) Mostly tRNA has uracil instead of Thymine. However, the T loop of tRNA has a thymine instead of uracil. The T loop (T?C-loop) contains thymine, and pseudouracil (?). The tRNA is single stranded like mRNA (70-95 nucleotides long). However, it forms three double stranded hairpin loops: T loop, D loop and anticodon loop. Some bases in the acceptor and three loops are complementary and hence base pair to form double stranded regions. Anticodon region is not identical but complementary to codon on mRNA.
Hence, right option is E. A and B.
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