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Translation

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This quiz is based on the process of translating mRNA (produced in the nucleus) into protein.

1. The process of translating mRNA into protein occurs...
• within the nucleus
• inside the Lysosomes
• inside the Peroxisomes
• at the ribosomes
Answer: at the ribosomes
A ribosome is an organelle found in the cytosol of all cells. It is composed of RNA and ribosomal proteins which translate mRNA into protein (i.e a polypeptide chain).


2. Prokaryotes use 70S ribosomes which consist of :
• a (small) 30S and a (large) 50S subunit
• a (small) 40S and a (large) 60S subunit
• a (small) 40S and a (large) 30S subunit
• a (small) 10S and a (large) 60S subunit
Answer: a (small) 30S and a (large) 50S subunit
Ribosomes consist of two subunits that fit together and work as one to translate the mRNA into a protein. The size of the ribosome and its subunits are expressed in terms of Svedberg units (S) which is a measure of the rate of sedimentation of a particle in a centrifuge, where the sedimentation rate is associated with the size of the particle and its 3D shape. Note : Svedberg units are not additive ! !


3. The triplet base sequence of tRNA nucleotides that is complementary to an mRNA codon (for a particular amino acid) is called :
• a transposon
• a retroposon
• a cistron
• an anticodon
Answer: an anticodon
The three-nucleotide sequence at the end of a transfer RNA molecule that is complementary to, an amino acid specifying codon in messenger RNA is called an anticodon.



4. Every polypeptide chain formed in translation starts with the amino acid :
• methionine
• lysine
• serine
• alanine
Answer: methionine
The leading codon of the messenger RNA molecule is the sequence AUG which encodes the amino acid, methionine. The initial methionine unit is usually split off the finished polypeptide. In bacteria, the first aminoacyl-tRNA to initiate translation is always a formyl derivative of methionine called FMet-tRNA.


5. tRNA molecules are linked to their respective amino acids by enzymes called :
• phenylalanine hydroxylases
• aminoacyl-tRNA synthetases
• beta-galactosidases
• Ornithine decarboxylases
Answer: aminoacyl-tRNA synthetases

6. Some tRNAs can recognize more than one codon because there is a relaxation of the complementation rule of base pairing between the anticodon and codon in the third position (of the codon sequence). This relaxation is called :
• the Wobble Hypothesis
• Sutton-Boveri Hypothesis
• the Levene's Hypothesis
• the Monkey-Man Hypothesis
Answer: the Wobble Hypothesis
The Wobble Hypothesis says that a single tRNA can decode more than one codon because there is a relaxation of the complementation rule of base pairing between the anticodon and codon in the third position (of the codon sequence).


7. Which of the following is not a stop codon ?
• UAA
• UAG
• GUA
• UGA
Answer: GUA
AUG is a start codon and encodes the protein methionine. In some organisms, GUG is also used as a start codon for some proteins.


8. Most amino acids are encoded by more than one codon except tryptophan and :
• glycine
• lysine
• leucine
• methionine
Answer: methionine
Codons are triplets of four possible bases (A, U, G and C). Basic math tells us that we can have 43 or 64 triplets in all. Since there are only 20 amino acids, each of them is encoded by more than one codon except tryptophan and methionine.


9. In prokaryotic mRNA, a particular base sequence (AGGAGGU) exists near the AUG start codon. This sequence which is sometimes referred to as the ribosome binding site is known as :
• a promoter sequence
• a Shine Dalgarno sequence
• a Kozak's consensus sequence
• a trailer sequence
Answer: a Shine Dalgarno sequence
The Shine Dalgarno sequence is a section of nucleotides on a prokaryotic mRNA molecule upstream of the translational start site, that serves to bind to ribosomal RNA and thereby bring the ribosome to the start codon on the mRNA. It is complementary to the 3' end of 16S rRNA.


10. If the mammalian genetic code were a four letter code, then it would have the potential to encode...
• 256 amino acids
• 128 amino acids
• 64 amino acids
• 216 amino acids
Answer: 256 amino acids
If the genetic code were a four letter code, then the number of amino acids that could be encoded would be equal to 44 = 256.


11. A sequence of about 5 to 20 bases seperating one stop codon from the next start codon in a polycistronic prokaryotic mRNA molecule is called :
• a Shine Dalgarno sequence
• a poly-A-tail
• a transposon
• a spacer
Answer: a spacer
A polycistronic prokaryotic mRNA molecule must possess a series of start and stop codons since it encodes more than one protein. A spacer simply seperates two successive cistrons (genes).


12. The movement of peptidyl tRNA from the A site to the P site (of the larger ribosomal subunit) and the movement of mRNA in relation to the ribosome is called :
• RNA processing
• dislocation
• elongation
• translocation
Answer: translocation
A tRNA molecule enters the large ribosomal subunit through the A site. Follwing this, a peptide bond forms between its amino acid and the one being held in the P site and the tRNA molecule in the P site releases its hold on its amino acid, while the tRNA molecule in the A site moves to the P site. At the same time, the mRNA molecule moves a distance of three bases in order to position the next codon at the A site. This whole process is called translocation.


13. Which of the following statements is correct ?
• Methionine is found at the N-terminus of nearly all newly synthesised eukaryotic proteins.
• Each mRNA molecule is usually translated into, hundreds of copies of a single polypeptide chain.
• In prokaryotes, both transcription and translation take place in the same part of the nucleus.
• All of the above.
Answer: All of the above.
In prokaryotes, both transcription and translation take place in the cytoplasm.


14. The enzyme that forms a peptide bond between adjacent amino acids in the ribosome during translation is called :
• Deiodinase
• peptidyl transferase
• Diastase
• Phospholipase
Answer: peptidyl transferase
Peptidyl transferase catalyses peptide bond formation between adjacent amino acids in the ribosome during translation.


15. Synthesis of proteins can take place quickly by multiple ribosomes being able to attach themselves to one mRNA chain. An mRNA chain with multiple ribosomes is called :
• a peroxisome
• a polysome
• a lysosome
• a centrosome
Answer: a polysome
A cluster of several ribosomes simultaneously translating the same mRNA is called a polysome (short form for polyribosome !).


16. The difference in the products translated from a tricistronic mRNA molecule by the ribosomes of prokaryotes and eukaryotes is that :
• the prokaryotic ribosome translates only one cistron while the eukaryotic ribosome translates all cistrons.
• the eukaryotic ribosome will produce one polyprotein instead of three smaller proteins.
• None of the above.
Answer: the eukaryotic ribosome translates only one cistron while the prokaryotic ribosome translates all cistrons.
In eukaryotes, reinitiation of polypeptide synthesis following an encounter of a ribosome with a stop codon does not occur. Eukaryotic mRNA is always polycistronic.


17. The base inosine is usually found in :
• mRNA
• rRNA
• tRNA
• hnRNA
Answer: tRNA
The base inosine is found in the anticodons of several tRNA molecules.


18. Which of the following pairs of codons encode the same protein ?
• AUG and AUC
• UAA and UAC
• GUA and GUG
• UAG and UAC
Answer: GUA and GUG
UAA, UAG are stop codons and do not encode any proteins. AUG is the only codon that encodes methionine. This eliminates all options except GUA and GUG.


19. The triplet base sequence in DNA that is translated into a stop codon, whose corresponding tRNA anticodon, is AUU is given by :
• TAA
• ATT
• CGG
• GCC
Answer: ATT
The tRNA anticodon corresponding to the codon in question is AUU. Hence, the base sequence of the corresponding codon is UAA (a stop codon). The DNA sequence that corresponds to the RNA sequence UAA, is ATT.


20. How many amino acids are encoded by the following mRNA sequence : UAUCAUCCACUUGGUUGA ?
• 5
• 6
• 4
• 7
Answer: 5
The last codon UGA is a stop codon. All the other codons in the sequence encode specific amino acids. Since there are five codons other than the stop codon, the correct answer is 5.



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