solved MCQs for Protein Synthesis

Slide 1 : SOLVED MCQs FOR PROTEIN SYNTHESIS By Dr. Ichha Purak 10/14/2011 1

Slide 2 : 1 What is added to the 3'-end of many eukaryotic mRNAs after transcription? A. Intron B A poly A tail   C. A cap structure consisting of a modified G nucleotide ide D. The trinucleotide 5’-CCA Answer B A poly A tai Explanation In Eukaryotes mRNA formed just after transcription is in precursor form . It is further processed in the nucleus before transport to cytoplasm Following Processings take place Addition of a cap of Modified Guanine Nucleotide at 5’end Addition of a poly adenine (50-150-200) at 3’ end Splicing of Introns to give continuous stretch of coding region by joining the exons  Cap and tail provide stability to m RNA 10/14/2011 2

Slide 3 : 2 Which of the following molecules functions to transfer information from the nucleus to the cytoplasm? A. DNA B. mRNA   C. tRNA   D.   Proteins Answer B. mRNA Explanation mRNAs after transcription move to cytoplasm where they translated on ribosome. So carry genetic information from nucleus to cytoplasm . 10/14/2011 3

Slide 4 : 3 Three types of RNA involved in comprising the structural and functional core for protein synthesis, serving as a template for translation, and transporting amino acid, respectively, are: A mRNA, tRNA,rRNA B rRNA, tRNA,mRNA   C tRNA,mRNA,rRNA   D rRNA,mRNA,tRNA   Answer D rRNA, mRNA ,tRNA Explanation Three types of RNA • mRNA- messenger RNA. The template for protein synthesis. • tRNA- transfer RNA. The "adapter" molecule that converts nucleic acid sequence to protein sequence. • rRNA- ribosomal RNA. The structural and sometimes catalytic molecule of the ribosome. rRNA's serve a structural and functional role in protein synthesis as a component of the ribosome. 10/14/2011 4

Slide 5 : 4 With what mRNA codon would the tRNA with anticodon ‘ UAC’ will be able to to form a codon-anticodon base pairing interaction A 3’-AUG-5’   B. 3’-GUA-5’   C. 3’-CAU-5’   D. 3’-UAC-5’   Answer A 3'-AUG-5’ Explanation The codon is a sequence of three bases on mRNA that determines an amino acid. The anticodon is a sequence of tRNA that is complementary to the codon. In this example,the anticodon is 5'UAC. Thus, the codon would be 3'AUG which is identical to answer A or 3'-AUG-5'. 10/14/2011 5

Slide 6 : 5 The Process of making RNA from DNA is A Transcription B Translation C Transduction D Peptide Synthesis Answer A Transcription Explanation Transcription is synthesis of RNA using a segment of one strand of DNA as template with the help of RNA polymerase enzyme and Transcription factors 10/14/2011 6

Slide 7 : 6 A polypeptide is assembled on a DNA molecule Nuclear membrane Nuclear pore Ribosome Answer D-Ribosome Explanation Ribosome is nucleoprotein particle made of rRNAs and Proteins. Ribosome provide space for protein synthesis and accommodate mRNA and loaded tRNAs in the tunnel like space between two ribosomal sub units 10/14/2011 7

Slide 8 : 7 The first event in translation is the binding of the mRNA to the A Smaller subunit of Ribosome B Larger subunit of Ribosome C Polysomal core D tRNA Answer A Smaller subunit of Ribosome Explanation mRNA can bind to only dissociated smaller subunit of ribosome. This binding is helped by IF3 (also known as anti-association factor ) as it prevents binding of larger sub unit of ribosome 10/14/2011 8

Slide 9 : 8 The signal to start translation is the initiator codon , usually AUG, the codon for A Tyrosine B Methionine C Leucine D Glycine Answer B Methionine Explanation It is the codon for Methionine in Eukaryotes both at the start as well as intercalary position 10/14/2011 9

Slide 10 : 9 The enzyme that catalyzes peptide bond between two amino acids is located in the A Smaller subunit of ribosome B Larger subunit of ribosome C Leader region of the mRNA D Central part of tRNA Answer B Larger subunit of ribosome Explanation Before 1989 it was considered as protein enzyme of larger subunit of ribosome but after studies made by Harry Noller (1992) ,it was described that 23SrRNA of larger sub unit of ribosome called as RNA enzymes or Ribozymes 10/14/2011 10

Slide 11 : 10 Which one of the following statement is true about tRNA ? A It binds to DNA initiating translation B It has a greater molecular weight than mRNA C It transfers the code from nucleus to cytoplasm D There is at least one tRNA for each kind of protein amino acid Answer D There is at least one tRNA for each kind of protein amino acid Explanation tRNA is the smallest RNA molecule having only 75-95 nucleotides .Due to presence of unusual bases it is folded on itself and acquires3-D structure with helical regions and loops .Anticodon loop bear three central unpaired bases which act as anticodon which complementary to codon present on mRNA. As each Amino Acid is recognized by atleast one specific codon ,so there is atleast one tRNA for each specific amino acid 10/14/2011 11

Slide 12 : 11 Translation is the A Movement of mRNA through nuclear membrane B Transfer of genetic code to an mRNA molecule C Transfer of genetic code to a tRNA molecule D Transfer of the genetic code from an mRNA to a sequence of amino acids in a polypeptide Answer D Transfer of the genetic code from an mRNA to a sequence of amino acids in a polypeptide Explanation During formation of polypeptide chain , amino acids are polymerized as per codon ( 3 ribonucleotides in sequence) present on mRNA .so the language of ribonucleotides is changed into language of amino acids as in case of translation (English to Hindi ) and as mRNA is single stranded and carries genetic code which is read in 5’ to 3’ direction . 10/14/2011 12

Slide 13 : 12 Out of 64 codons, how many code for amino acid ? A 20 B 22 C 43 D 61 Answer D 61 Explanation There are 20 protein Amino acids. Some AAs have one, some two, some four and some six codons. Only 3 codons are not recognized by any amino acid ,these are termination codons. 61 codons are assigned to different AAs 10/14/2011 13

Slide 14 : 13 The function of tRNA is to A Form a site for protein synthesis B Transcribe the genetic code C Transport specific amino acid to specific site on the mRNA D Synthesize amino acids Answer C Transport specific amino acid to specific site on the mRNA Explanation Because of having anticodon tRNA behaves as adapter molecule,it can pick up specific amino acid and drop it on correct position by recognizing specific codon on mRNA 10/14/2011 14

Slide 15 : 14 The second event in translation after mRNA binding to smaller subunit of ribosome is A Synthesis of Amino Acids B Base pairing between Initiator codon and anticodon of tRNA loaded with Methionine C Joining together of two ribosomal sub units D Peptide bond formation Answer B Base pairing between Initiator codon and anticodon of tRNA loaded with Methionine Explanation Binding of Initiator tRNAMet at the peptidyle site with the help GTP takes place by H bonds between I.codon and I anticodon. Only after this binding larger subunit unites to form complete Initiation complex. 10/14/2011 15

Slide 16 : 15. tRNA bound at Acceptor oe Active site is A Acceptor tRNA B Donar tRNA C Initiator tRNA Met D None of the above Answer A Acceptor tRNA Explanation tRNA at Acceptor site accepts amino acid from donar tRNA present at P site and a peptide bond is formed between COOH group of donated amino acid and NH2 group of Acceptor amino acid. 10/14/2011 16