Section I: Fundamentals of Genetic Code
1. What is meant by the genetic code?
A. The sequence of amino acids in a protein
B. The set of rules by which nucleotide sequences specify amino acids
C. The sequence of nucleotides in DNA only
D. The mechanism of DNA replication
Correct Answer: B
Explanation:
The genetic code is the set of rules that establishes the correspondence between nucleotide sequences, particularly mRNA codons, and amino acids during protein synthesis. It provides the molecular link between nucleic acids and proteins.
2. What is the basic unit of the genetic code?
A. Nucleotide
B. Amino acid
C. Codon
D. Anticodon
Correct Answer: C
Explanation:
The basic unit of the genetic code is the codon, which consists of three consecutive nucleotides in mRNA and specifies an amino acid or a termination signal.
3. A codon consists of how many nucleotides?
A. 1
B. 2
C. 3
D. 4
Correct Answer: C
Explanation:
The genetic code is a triplet code, meaning that each codon contains three nucleotides.
4. Which molecule contains codons?
A. DNA
B. mRNA
C. tRNA
D. rRNA
Correct Answer: B
Explanation:
Codons are three-nucleotide sequences present on messenger RNA (mRNA). During translation, ribosomes read these codons to determine the amino acid sequence of the protein.
5. Which molecule contains an anticodon?
A. DNA
B. mRNA
C. tRNA
D. rRNA
Correct Answer: C
Explanation:
A transfer RNA (tRNA) molecule contains an anticodon that recognizes a complementary codon on mRNA during translation.
6. How many different codons are possible in the standard genetic code?
A. 20
B. 32
C. 61
D. 64
Correct Answer: D
Explanation:
RNA contains four different bases: A, U, G and C. Three bases make one codon, so:
4³ = 64 codons
7. How many of the 64 codons specify amino acids?
A. 20
B. 61
C. 62
D. 64
Correct Answer: B
Explanation:
There are 61 sense codons that specify the 20 standard amino acids. The remaining three—UAA, UAG and UGA—are stop codons.
8. How many stop codons are present in the standard genetic code?
A. 1
B. 2
C. 3
D. 4
Correct Answer: C
Explanation:
The three standard stop codons are:
UAA, UAG and UGA
They signal termination of translation.
9. Which of the following is NOT a stop codon?
A. UAA
B. UAG
C. UGA
D. AUG
Correct Answer: D
Explanation:
AUG is the principal start codon and codes for methionine. UAA, UAG and UGA are the three standard stop codons.
10. Which codon usually functions as the start codon?
A. UAA
B. AUG
C. UAG
D. UGA
Correct Answer: B
Explanation:
AUG is the principal translation initiation codon. It codes for methionine in eukaryotes and initiating fMet in bacteria.
Section II: Properties of the Genetic Code
11. The genetic code is described as a triplet code because:
A. Three amino acids form a codon
B. Three nucleotides form a codon
C. Three codons form an amino acid
D. Three tRNAs recognize each codon
Correct Answer: B
Explanation:
Each codon consists of three nucleotides, making the genetic code a triplet code.
12. The term "degenerate genetic code" means that:
A. One codon specifies several amino acids
B. Several codons can specify the same amino acid
C. Codons overlap with one another
D. Codons contain three amino acids
Correct Answer: B
Explanation:
The genetic code is degenerate because most amino acids are specified by more than one codon.
13. Which statement best describes the unambiguous nature of the genetic code?
A. One amino acid has only one codon
B. One codon specifies only one amino acid or stop signal
C. All codons specify multiple amino acids
D. Each amino acid has three codons
Correct Answer: B
Explanation:
Although several codons may specify the same amino acid, a particular codon has only one meaning in the standard genetic code.
14. The genetic code is considered nearly universal because:
A. All organisms have identical DNA
B. Most organisms use essentially the same codon assignments
C. All organisms have identical proteins
D. All organisms contain the same number of chromosomes
Correct Answer: B
Explanation:
The codon assignments are remarkably conserved among organisms. However, some exceptions occur, especially in mitochondrial and certain other genetic systems, so the code is described as nearly universal.
15. The genetic code is called non-overlapping because:
A. Codons have no nucleotides
B. A nucleotide is generally read as part of only one codon in a given reading frame
C. Codons cannot be translated
D. Amino acids overlap
Correct Answer: B
Explanation:
In a standard reading frame, codons are read as consecutive triplets without sharing nucleotides.
16. The genetic code is described as comma-less because:
A. Codons contain no uracil
B. There are no punctuation marks between successive codons
C. Codons do not contain three bases
D. Translation does not require ribosomes
Correct Answer: B
Explanation:
mRNA is read continuously in successive triplets. There are no commas or other punctuation marks separating codons.
17. In which direction is mRNA read during translation?
A. 3′ → 5′
B. 5′ → 3′
C. N → C
D. C → N
Correct Answer: B
Explanation:
The ribosome moves along mRNA in the 5′ → 3′ direction.
18. In which direction is a polypeptide synthesized?
A. C-terminus to N-terminus
B. N-terminus to C-terminus
C. 5′ to 3′
D. 3′ to 5′
Correct Answer: B
Explanation:
Protein synthesis proceeds from the N-terminal end toward the C-terminal end.
19. Which property allows several different codons to specify the same amino acid?
A. Universality
B. Degeneracy
C. Ambiguity
D. Overlapping
Correct Answer: B
Explanation:
Degeneracy means that multiple codons can encode the same amino acid.
20. Which of the following is NOT a characteristic of the standard genetic code?
A. Triplet
B. Degenerate
C. Unambiguous
D. Highly overlapping
Correct Answer: D
Explanation:
The standard genetic code is non-overlapping, not highly overlapping.
Section III: Codons and Amino Acids
21. Which amino acid is encoded by AUG?
A. Alanine
B. Methionine
C. Valine
D. Leucine
Correct Answer: B
Explanation:
AUG codes for methionine and commonly functions as the start codon during translation.
22. Which amino acid is encoded by UGG?
A. Tryptophan
B. Tyrosine
C. Tryptophan and stop
D. Cysteine
Correct Answer: A
Explanation:
UGG is the only standard codon for tryptophan.
23. Which amino acid has only one standard codon?
A. Leucine
B. Serine
C. Methionine
D. Arginine
Correct Answer: C
Explanation:
Methionine is encoded only by AUG. Tryptophan is also encoded by only one codon, UGG.
24. Which other amino acid, besides methionine, has only one standard codon?
A. Glycine
B. Tryptophan
C. Serine
D. Valine
Correct Answer: B
Explanation:
UGG is the only codon for tryptophan, while AUG is the only codon for methionine.
25. Which amino acid has six codons?
A. Methionine
B. Tryptophan
C. Leucine
D. Tyrosine
Correct Answer: C
Explanation:
Leucine has six codons:
UUA, UUG, CUU, CUC, CUA and CUG.
Serine and arginine also have six codons each.
26. Which group contains amino acids that each have six codons?
A. Leucine, serine and arginine
B. Alanine, glycine and valine
C. Methionine, tryptophan and lysine
D. Tyrosine, histidine and cysteine
Correct Answer: A
Explanation:
The three amino acids with six standard codons are leucine, serine and arginine.
27. Which amino acid is encoded by GCU?
A. Glycine
B. Alanine
C. Valine
D. Arginine
Correct Answer: B
Explanation:
GCU is one of four codons for alanine:
GCU, GCC, GCA and GCG.
28. Which amino acid is encoded by GGG?
A. Glycine
B. Glutamate
C. Proline
D. Arginine
Correct Answer: A
Explanation:
The four glycine codons are:
GGU, GGC, GGA and GGG.
29. Which amino acid is encoded by AAA?
A. Asparagine
B. Lysine
C. Glutamine
D. Arginine
Correct Answer: B
Explanation:
AAA and AAG encode lysine.
30. Which amino acid is encoded by UUU?
A. Leucine
B. Phenylalanine
C. Tyrosine
D. Serine
Correct Answer: B
Explanation:
UUU and UUC encode phenylalanine.
Section IV: Stop Codons and Translation
31. Which of the following is a stop codon?
A. AUG
B. UGG
C. UAA
D. GCU
Correct Answer: C
Explanation:
UAA is one of the three standard stop codons.
32. The stop codons are:
A. AUG, UGG, UAA
B. UAA, UAG, UGA
C. UUU, UUC, UUA
D. GCU, GCC, GCA
Correct Answer: B
Explanation:
The three standard termination codons are UAA, UAG and UGA.
33. Stop codons are also called:
A. Sense codons
B. Anticodons
C. Nonsense codons
D. Initiator codons
Correct Answer: C
Explanation:
Stop codons are traditionally called nonsense codons because they do not specify an amino acid in the standard genetic code.
34. Stop codons are recognized during translation primarily by:
A. tRNA
B. DNA polymerase
C. Release factors
D. Aminoacyl-tRNA synthetase
Correct Answer: C
Explanation:
Stop codons are recognized by protein release factors, which promote termination and release of the completed polypeptide.
35. Which of the following is NOT normally recognized by a conventional tRNA carrying an amino acid?
A. AUG
B. UAA
C. GCU
D. UGG
Correct Answer: B
Explanation:
UAA is a stop codon. Stop codons are recognized by release factors rather than ordinary aminoacyl-tRNAs.
36. What happens when a ribosome encounters a stop codon?
A. DNA replication begins
B. Translation terminates
C. Transcription begins
D. The ribosome synthesizes another codon
Correct Answer: B
Explanation:
A stop codon triggers termination of translation and release of the newly synthesized polypeptide.
37. Which of the following is an initiation codon?
A. AUG
B. UAA
C. UAG
D. UGA
Correct Answer: A
Explanation:
AUG is the principal initiation codon and specifies methionine.
38. In bacteria, the initiating amino acid is generally:
A. Methionine only
B. N-formylmethionine
C. Valine
D. Glycine
Correct Answer: B
Explanation:
In bacteria, the initiator tRNA generally carries N-formylmethionine (fMet).
39. What is the major role of the start codon?
A. Stop DNA replication
B. Establish the reading frame and initiate translation
C. Terminate transcription
D. Degrade mRNA
Correct Answer: B
Explanation:
The start codon identifies the site of translation initiation and establishes the correct reading frame.
40. Translation of a typical coding sequence terminates at:
A. AUG
B. The promoter
C. A stop codon
D. The anticodon
Correct Answer: C
Explanation:
Translation terminates when the ribosome encounters one of the stop codons: UAA, UAG or UGA.
Section V: Wobble Hypothesis
41. The wobble hypothesis was proposed by:
A. James Watson
B. Francis Crick
C. Marshall Nirenberg
D. Har Gobind Khorana
Correct Answer: B
Explanation:
The wobble hypothesis was proposed by Francis Crick to explain flexibility in codon-anticodon pairing.
42. Wobble mainly involves flexibility at which position of the codon?
A. First position
B. Second position
C. Third position
D. All positions equally
Correct Answer: C
Explanation:
Greater flexibility in base pairing occurs at the third position of the codon, often called the wobble position.
43. The wobble hypothesis helps explain:
A. DNA replication
B. Why one tRNA can recognize more than one codon
C. Why DNA contains thymine
D. Protein folding
Correct Answer: B
Explanation:
Wobble allows certain tRNAs to recognize multiple synonymous codons, reducing the number of distinct tRNAs needed.
44. Which codon position is generally most tolerant of variation?
A. First
B. Second
C. Third
D. None
Correct Answer: C
Explanation:
The third codon position frequently permits greater flexibility because of wobble pairing.
45. Wobble contributes to the:
A. Degeneracy of the genetic code
B. Replication of DNA
C. Synthesis of RNA primers
D. Formation of DNA double helix
Correct Answer: A
Explanation:
Wobble helps explain how different codons can be recognized by a smaller number of tRNA species and contributes to the functional degeneracy of the code.
Section VI: Codon-Anticodon Relationship
46. The anticodon is complementary to:
A. DNA template
B. mRNA codon
C. Ribosomal RNA
D. Amino acid
Correct Answer: B
Explanation:
The anticodon of tRNA base-pairs with the complementary codon on mRNA.
47. If the mRNA codon is 5′-AUG-3′, the complementary anticodon is:
A. 5′-AUG-3′
B. 3′-UAC-5′
C. 5′-UAC-3′ only
D. 3′-TAC-5′
Correct Answer: B
Explanation:
The anticodon pairs antiparallel to the codon:
mRNA: 5′-AUG-3′
tRNA: 3′-UAC-5′
48. Which molecule acts as an adaptor between mRNA codons and amino acids?
A. mRNA
B. rRNA
C. tRNA
D. DNA
Correct Answer: C
Explanation:
tRNA functions as an adaptor molecule, carrying a specific amino acid and recognizing the corresponding mRNA codon through its anticodon.
49. Amino acids are attached to tRNA by:
A. DNA polymerase
B. RNA polymerase
C. Aminoacyl-tRNA synthetase
D. Peptidyl transferase
Correct Answer: C
Explanation:
Aminoacyl-tRNA synthetases catalyze the attachment of specific amino acids to their corresponding tRNAs.
50. The correct amino acid-tRNA pairing is primarily determined by:
A. DNA ligase
B. Aminoacyl-tRNA synthetases
C. Ribosomal DNA
D. RNA polymerase
Correct Answer: B
Explanation:
Aminoacyl-tRNA synthetases recognize particular tRNAs and amino acids and charge the correct tRNA with its amino acid.
Section VII: Mutations and Genetic Code
51. A mutation that does not change the amino acid is called:
A. Missense mutation
B. Nonsense mutation
C. Synonymous mutation
D. Frameshift mutation
Correct Answer: C
Explanation:
A synonymous mutation changes a codon but does not change the encoded amino acid. Such mutations are often called silent mutations, although they can sometimes have biological effects.
52. Which mutation changes one amino acid into another?
A. Synonymous
B. Missense
C. Nonsense
D. Silent
Correct Answer: B
Explanation:
A missense mutation changes a codon so that a different amino acid is incorporated into the protein.
53. A mutation that converts an amino acid codon into a stop codon is called:
A. Missense mutation
B. Synonymous mutation
C. Nonsense mutation
D. Neutral mutation
Correct Answer: C
Explanation:
A nonsense mutation produces a premature termination codon, potentially resulting in a truncated protein.
54. Which mutation is most directly associated with premature termination of translation?
A. Synonymous
B. Nonsense
C. Silent
D. Conservative missense
Correct Answer: B
Explanation:
A nonsense mutation creates a stop codon within a coding region, causing premature termination.
55. Which is an example of a synonymous codon change?
A. UUU → UUC
B. UAU → UAA
C. GAG → GUG
D. AUG → UAG
Correct Answer: A
Explanation:
UUU and UUC both encode phenylalanine, so the change does not alter the amino acid.
56. GAG → GUG is an example of:
A. Synonymous mutation
B. Missense mutation
C. Nonsense mutation
D. Frameshift mutation
Correct Answer: B
Explanation:
GAG codes for glutamate, whereas GUG codes for valine. Therefore, the amino acid changes, making this a missense mutation.
57. A mutation that inserts or deletes nucleotides in numbers not divisible by three may cause:
A. Wobble
B. Frameshift
C. Degeneracy
D. Transcription
Correct Answer: B
Explanation:
Insertions or deletions that are not multiples of three shift the reading frame and can alter all downstream codons.
58. Why can some nucleotide substitutions have no effect on protein sequence?
A. DNA has no genetic information
B. The genetic code is degenerate
C. Translation does not use mRNA
D. All mutations are repaired
Correct Answer: B
Explanation:
Because multiple codons can specify the same amino acid, some substitutions are synonymous and do not alter the amino acid sequence.
59. A mutation changing UAU to UAA would most likely be:
A. Synonymous
B. Missense
C. Nonsense
D. Frameshift
Correct Answer: C
Explanation:
UAU codes for tyrosine, whereas UAA is a stop codon. The mutation therefore creates a premature termination signal.
60. A mutation from GCU to GCC is:
A. Nonsense
B. Missense
C. Synonymous
D. Frameshift
Correct Answer: C
Explanation:
Both GCU and GCC encode alanine. Therefore, the mutation is synonymous.
Section VIII: Genetic Code and Translation
61. Translation converts the information in mRNA into:
A. DNA
B. RNA
C. Protein
D. Lipid
Correct Answer: C
Explanation:
Translation is the process through which the nucleotide sequence of mRNA is used to synthesize a polypeptide/protein.
62. Which sequence correctly represents the flow of genetic information in the central dogma?
A. Protein → RNA → DNA
B. DNA → RNA → Protein
C. RNA → DNA → Protein only
D. Protein → DNA → RNA
Correct Answer: B
Explanation:
The classical central dogma is:
DNA → RNA → Protein
Transcription produces RNA from DNA, and translation produces protein from mRNA.
63. Which organelle or molecular complex reads mRNA codons?
A. Ribosome
B. Lysosome
C. Peroxisome
D. Centrosome
Correct Answer: A
Explanation:
The ribosome reads mRNA codons and coordinates tRNA binding and peptide bond formation during translation.
64. Which process directly interprets the genetic code?
A. DNA replication
B. Translation
C. DNA repair
D. Transcription
Correct Answer: B
Explanation:
Translation directly uses the genetic code to convert an mRNA nucleotide sequence into an amino acid sequence.
65. During translation, the ribosome moves along mRNA in which direction?
A. 3′ → 5′
B. 5′ → 3′
C. N → C
D. C → N
Correct Answer: B
Explanation:
Ribosomes read mRNA in the 5′ → 3′ direction.
66. During translation, the growing polypeptide chain is extended at its:
A. N-terminus
B. C-terminus
C. Both ends equally
D. Middle
Correct Answer: B
Explanation:
New amino acids are added to the C-terminal end of the growing polypeptide.
67. Which ribosomal site generally receives an incoming aminoacyl-tRNA during elongation?
A. E site
B. P site
C. A site
D. R site
Correct Answer: C
Explanation:
The A (aminoacyl) site is where an incoming aminoacyl-tRNA generally binds during elongation.
68. Which ribosomal site contains the tRNA carrying the growing polypeptide chain during elongation?
A. A site
B. P site
C. E site
D. T site
Correct Answer: B
Explanation:
The P (peptidyl) site holds the tRNA carrying the growing polypeptide chain.
69. Which site is associated with the exit of deacylated tRNA?
A. A site
B. P site
C. E site
D. C site
Correct Answer: C
Explanation:
The E (exit) site is associated with the departure of deacylated tRNA from the ribosome.
70. Peptide bond formation during translation occurs between:
A. Nucleotides
B. Amino acids
C. Sugars
D. Fatty acids
Correct Answer: B
Explanation:
The ribosome catalyzes peptide bond formation between amino acids, producing the growing polypeptide.
Section IX: Historical Discovery
71. Who performed the famous poly-U experiment that identified UUU as a phenylalanine codon?
A. Francis Crick
B. Marshall Nirenberg and Heinrich Matthaei
C. Watson and Crick
D. Robert Holley
Correct Answer: B
Explanation:
In 1961, Marshall Nirenberg and Heinrich Matthaei used a cell-free translation system and synthetic poly-U RNA to demonstrate that UUU specifies phenylalanine.
72. The codon UUU was found to specify:
A. Tyrosine
B. Phenylalanine
C. Leucine
D. Serine
Correct Answer: B
Explanation:
The landmark poly-U experiment established:
UUU → Phenylalanine
73. Har Gobind Khorana made major contributions to deciphering the genetic code using:
A. X-rays only
B. Chemically synthesized RNA sequences
C. Protein crystallization only
D. DNA sequencing alone
Correct Answer: B
Explanation:
Khorana and colleagues used synthetic, defined polynucleotide sequences to determine additional codon assignments.
74. Who determined the structure of a tRNA molecule, contributing to understanding translation?
A. Robert Holley
B. Francis Crick
C. George Gamow
D. Meselson
Correct Answer: A
Explanation:
Robert W. Holley determined the nucleotide sequence and structure of a tRNA, providing important evidence for the adaptor role of tRNA.
75. The Nobel Prize for work on the interpretation of the genetic code was awarded in 1968 to:
A. Watson, Crick and Franklin
B. Nirenberg, Khorana and Holley
C. Jacob, Monod and Lwoff
D. Sanger, Gilbert and Berg
Correct Answer: B
Explanation:
Marshall Nirenberg, Har Gobind Khorana and Robert Holley received the 1968 Nobel Prize in Physiology or Medicine for their contributions to interpreting the genetic code and its function in protein synthesis.
Section X: Codon Usage and Exceptions
76. Codon usage refers to:
A. The number of chromosomes
B. The frequency with which synonymous codons are used
C. The number of amino acids in a protein
D. The rate of DNA replication
Correct Answer: B
Explanation:
Different organisms, and even different genes within the same organism, may preferentially use certain synonymous codons. This is called codon usage bias.
77. Codon optimization is commonly used to:
A. Change the amino acid sequence randomly
B. Improve expression of a gene in a particular host
C. Stop transcription
D. Remove all introns from DNA
Correct Answer: B
Explanation:
Codon optimization redesigns a coding sequence using synonymous codons that are better suited to the codon preferences of the expression host, while preserving the intended protein sequence.
78. Codon usage bias can influence:
A. Translation efficiency
B. Protein expression
C. mRNA stability
D. All of the above
Correct Answer: D
Explanation:
Codon usage can influence translation efficiency, mRNA behavior and protein expression. Its effects depend on the organism and sequence context.
79. The genetic code is not absolutely universal because:
A. DNA has four bases
B. Some organisms and organelles have variant genetic codes
C. All codons are different in every organism
D. Amino acids cannot be translated
Correct Answer: B
Explanation:
Although highly conserved, the standard genetic code has exceptions, particularly in mitochondrial genomes and some microorganisms.
80. Which organelle is particularly well known for having genetic-code variations?
A. Golgi apparatus
B. Lysosome
C. Mitochondrion
D. Nucleus only
Correct Answer: C
Explanation:
Mitochondria in various organisms have genetic codes that differ from the standard nuclear genetic code.
Section XI: Advanced Concepts
81. Selenocysteine is often called the:
A. 19th amino acid
B. 20th amino acid
C. 21st amino acid
D. 25th amino acid
Correct Answer: C
Explanation:
Selenocysteine is commonly called the 21st amino acid. Its incorporation can involve recoding of UGA under specialized cellular conditions.
82. Pyrrolysine is commonly referred to as the:
A. 18th amino acid
B. 20th amino acid
C. 21st amino acid
D. 22nd amino acid
Correct Answer: D
Explanation:
Pyrrolysine is commonly called the 22nd genetically encoded amino acid and occurs in certain archaea and bacteria.
83. In specialized contexts, UGA can encode:
A. Selenocysteine
B. Methionine only
C. Lysine
D. Glycine
Correct Answer: A
Explanation:
UGA normally functions as a stop codon, but in specialized genetic contexts it can direct incorporation of selenocysteine through a recoding mechanism.
84. Which amino acid is encoded by the unusual codon UAG in certain organisms?
A. Pyrrolysine
B. Selenocysteine
C. Methionine
D. Tryptophan
Correct Answer: A
Explanation:
In certain organisms, particularly some methanogenic archaea and related systems, UAG can be reassigned to encode pyrrolysine.
85. The genetic code is considered highly conserved because:
A. It changes every generation
B. Most organisms share very similar codon assignments
C. Every organism has identical genomes
D. All proteins are identical
Correct Answer: B
Explanation:
The remarkable conservation of codon assignments across diverse organisms indicates that the genetic code was established early in evolutionary history.
86. Which amino acid is encoded by both UAU and UAC?
A. Histidine
B. Tyrosine
C. Phenylalanine
D. Cysteine
Correct Answer: B
Explanation:
UAU and UAC both encode tyrosine, illustrating degeneracy.
87. Which pair of codons both encodes phenylalanine?
A. UUU and UUC
B. UUA and UUG
C. UAU and UAC
D. UGU and UGC
Correct Answer: A
Explanation:
The two phenylalanine codons are:
UUU and UUC.
88. Which group contains only synonymous codons for alanine?
A. GCU, GCC, GCA, GCG
B. GUU, GUC, GUA, GUG
C. CCU, CCC, CCA, CCG
D. GGU, GGC, GGA, GGG
Correct Answer: A
Explanation:
All four codons—GCU, GCC, GCA and GCG—encode alanine.
89. Which group contains only glycine codons?
A. GCU, GCC, GCA, GCG
B. GGU, GGC, GGA, GGG
C. CCU, CCC, CCA, CCG
D. ACU, ACC, ACA, ACG
Correct Answer: B
Explanation:
The four glycine codons are:
GGU, GGC, GGA and GGG.
90. Which amino acid has the codons CUU, CUC, CUA and CUG?
A. Proline
B. Leucine
C. Arginine
D. Valine
Correct Answer: B
Explanation:
CUU, CUC, CUA and CUG are four of the six codons that specify leucine.
Section XII: Application and Problem-Solving Questions
91. What amino acid sequence is produced from the mRNA sequence 5′-AUG-GCU-AAA-UGG-UAA-3′?
A. Met-Ala-Lys-Trp
B. Met-Gly-Lys-Trp
C. Ala-Met-Lys-Trp
D. Met-Ala-Arg-Trp
Correct Answer: A
Explanation:
AUG → Methionine
GCU → Alanine
AAA → Lysine
UGG → Tryptophan
UAA → Stop
Therefore:
Met-Ala-Lys-Trp
92. What is the peptide encoded by 5′-AUG-GGU-UUU-UAG-3′?
A. Met-Gly-Phe
B. Met-Gly-Leu
C. Gly-Phe-Met
D. Met-Val-Phe
Correct Answer: A
Explanation:
AUG → Methionine
GGU → Glycine
UUU → Phenylalanine
UAG → Stop
Therefore, the peptide is:
Met-Gly-Phe
93. If an mRNA contains 300 nucleotides in its coding sequence, approximately how many codons does it contain?
A. 50
B. 100
C. 150
D. 300
Correct Answer: B
Explanation:
Since each codon contains three nucleotides:
300 ÷ 3 = 100 codons
If the 300 nucleotides include a stop codon, then 99 codons specify amino acids and one is a termination codon.
94. An mRNA contains the sequence 5′-AUG-GCU-GAA-UAA-3′. How many amino acids are incorporated before termination?
A. 2
B. 3
C. 4
D. 1
Correct Answer: B
Explanation:
AUG → Methionine
GCU → Alanine
GAA → Glutamate
UAA → Stop
Thus, three amino acids are incorporated.
95. If the codon GCU changes to GCC, what is the most likely consequence?
A. Alanine changes to glycine
B. Alanine changes to valine
C. No change in the encoded amino acid
D. Translation terminates
Correct Answer: C
Explanation:
Both GCU and GCC encode alanine. Therefore, the substitution is synonymous.
96. If the codon UAU changes to UAA, what is the likely result?
A. Tyrosine remains unchanged
B. Tyrosine is replaced by phenylalanine
C. A premature stop signal is introduced
D. The reading frame shifts
Correct Answer: C
Explanation:
UAU encodes tyrosine, whereas UAA is a stop codon. Therefore, the mutation introduces a premature termination signal.
97. If one nucleotide is deleted from the middle of a coding sequence, what is the most likely consequence?
A. Synonymous mutation only
B. Frameshift mutation
C. No effect on reading frame
D. Conversion of RNA to DNA
Correct Answer: B
Explanation:
Deletion of a single nucleotide changes the grouping of downstream nucleotides into triplets, causing a frameshift.
98. A researcher wants to express a human protein efficiently in E. coli. Which strategy may improve expression while preserving the protein sequence?
A. Randomly change amino acids
B. Use codon optimization
C. Remove all codons
D. Convert all RNA to protein directly
Correct Answer: B
Explanation:
Codon optimization can redesign synonymous codons to better match the codon usage and translational machinery of E. coli, potentially improving heterologous protein expression.
99. Why can two different DNA sequences encode exactly the same protein?
A. The genetic code is ambiguous
B. The genetic code is degenerate
C. DNA cannot be translated
D. Proteins contain nucleotides
Correct Answer: B
Explanation:
Because multiple codons can encode the same amino acid, different nucleotide sequences can produce the same amino acid sequence.
100. Which statement best summarizes the biological significance of the genetic code?
A. It stores ATP in cells
B. It provides the rules for converting nucleotide information into protein information
C. It catalyzes DNA replication directly
D. It prevents all mutations
Correct Answer: B
Explanation:
The genetic code is the fundamental molecular system that connects nucleotide sequences in mRNA with amino acid sequences in proteins. It is therefore central to gene expression and virtually all aspects of molecular biology.
0 Comments