Question 1

Activation of amino acids

Accepted Answer

The answer of Activation of amino acids
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Question 2

The tRNAs may bind to the standard stop codons

Accepted Answer

The tRNAs may bind to the standard stop codons in cellular organelles which use a modified genetic code, to accommodate the tRNA for the unusual amino acid selenocysteine, or when the anticodon in a tRNA mutates to a sequence which can bind. All of these options are possible reasons for tRNAs binding to stop codons.

Question 3

The standard free energy change for the following reaction is about zero. amino acid + ATP + tRNA aminoacyl-tRNA + AMP + PP_i The driving force for this reaction in the cell is

Accepted Answer

The subsequent hydrolysis of pyrophosphate releases energy, which drives the reaction forward despite the near-zero standard free energy change. This energy is used to power the subsequent reaction of peptide bond formation.

Question 4

Each tRNA molecule has a separate activating enzyme.

Accepted Answer

The answer of Each tRNA molecule has a separate activating...

Question 5

Which of following best describes the two-step process for adding amino acids to tRNAs? AA = amino acid,P_i = phosphate,PP_i = pyrophosphate

Accepted Answer

The first step is the activation of the amino acid by attachment to AMP, and release of pyrophosphate (PPi). This reaction requires ATP. The second step is transfer of the activated amino acid to the 3'-end of the tRNA molecule, forming an aminoacyl-tRNA complex. This reaction requires the input of energy, which is provided by the attachment of the PPi to water, forming inorganic phosphate (Pi) and releasing energy. Therefore, option B is the correct choice as it describes the two-step process for adding amino acids to tRNAs.

Question 6

The process of amino acid activation

Accepted Answer

The process of amino acid activation involves the formation of an ester bond between the amino acid and tRNA through the activation of the amino acid carboxyl group by ATP. This ester bond links the amino acid and tRNA, preparing the amino acid for use in protein synthesis. Option A is incorrect because the formation of peptide bonds occurs during protein synthesis, not amino acid activation. Option B is incorrect because a mixed anhydride involves the reaction of two acid derivatives, which is not the case in amino acid activation. Option D is incorrect because at least one of the statements in A-C is true.

Question 7

Which of the following statements concerning tRNAs is false?

Accepted Answer

tRNAs are not transcribed by DNA polymerases. Instead, they are transcribed by RNA polymerases from DNA templates.

Question 8

Which linkage best describes the covalent bond between an amino acid (AA) and its tRNA?

Accepted Answer

The covalent bond between an amino acid (AA) and its tRNA is called an aminoacyl-tRNA bond. The carboxyl group of the AA is linked to the 3' -OH of the ribose sugar of the tRNA through an ester bond. This linkage is important for ensuring the correct amino acid is added to the growing polypeptide chain during translation. The other choices are not correct as they do not accurately describe the chemistry of the covalent bond between the AA and tRNA.

Question 9

The amino acids are initially attached to either the 2' or 3' hydroxyl group of the tRNA.

Accepted Answer

Amino acids are initially attached to the 2' or 3' hydroxyl group of the tRNA through a process called aminoacylation or charging. This forms an aminoacyl-tRNA molecule that carries the amino acid to the ribosome for protein synthesis.

Question 10

The initial step in the formation of an aminoacyl-tRNA is

Accepted Answer

The initial step in the formation of an aminoacyl-tRNA is the activation of the amino acid by reaction with ATP. This results in the formation of an aminoacyl-AMP intermediate, which is subsequently transferred to the appropriate tRNA to form the aminoacyl-tRNA. Esterification of the tRNA (choice A) and activation of the tRNA by reaction with ATP (choice C) both occur later in the process. Interaction of the mRNA with the tRNA (choice D) is not directly involved in aminoacyl-tRNA formation.

Question 11

The activating enzymes can actually remove wrong amino acids attached to a tRNA.

Accepted Answer

Activating enzymes have proofreading ability and can recognize if the wrong amino acid is attached to a tRNA. They can then remove it before it is used in protein synthesis.

Question 12

Activation of amino acids always takes place

Accepted Answer

Activation of amino acids always takes place in the cytoplasm, specifically on the tRNA molecules. The activation process involves attaching the appropriate amino acid to its corresponding tRNA molecule, which is catalyzed by aminoacyl-tRNA synthetases. This activated tRNA molecule can then bring the amino acid to the ribosome for protein synthesis. Activation of amino acids does not occur on the ribosomes, ribosomal subunits, or the rough endoplasmic reticulum.

Question 13

Using the genetic code,indicate which polypeptides would be synthesized if poly (UGG) were used as a synthetic mRNA?

Accepted Answer

The answer of Using the genetic code,indicate which polypeptides would...

Question 14

Specificity of aminoacyl-tRNA synthetases is always based on the anticodon.

Accepted Answer

The specificity of aminoacyl-tRNA synthetases is based on the amino acid that they recognize and attach to the proper tRNA. While the anticodon does play a role in this process, it is not the only factor determining specificity. Other factors, like the shape and size of the amino acid-binding pocket on the enzyme, also play a role.

Question 15

Amino acids linked to the 2'-end of a tRNA

Accepted Answer

Amino acids are initially attached to the 3'-end of tRNA molecules, not the 2'-end, before they are used in protein synthesis. This attachment is crucial for the correct positioning of amino acids during the translation process.

Question 16

The genetic code is said to be degenerate.This means that

Accepted Answer

Degenerate means that multiple codons can code for the same amino acid. For example, the codons GCU, GCC, GCA, and GCG all code for the amino acid alanine. Therefore, many amino acids are coded for by different codons. A and B are incorrect because each codon codes for a specific amino acid, and each anticodon can only interact with one specific triplet sequence in the mRNA. D is a true statement about the genetic code, but it is not related to the concept of degeneracy.

Question 17

Which of the following is not true?

Accepted Answer

Aminoacyl-tRNA synthetases require energy in the form of ATP to link amino acids to tRNA molecules.

Question 18

An aminoacyl-adenylate contains

Accepted Answer

An aminoacyl-adenylate contains an acid anhydride bond, which is formed between the carboxyl group of the amino acid and the phosphate group of ATP during the first step of aminoacyl-tRNA synthesis.

Question 19

A tRNA was determined to have the following anticodon sequence: 3'-GAI-5' (I represents the base hypoxanthine)
Which of the following codons can form base pairs with this anticodon?

Accepted Answer

The anticodon sequence 3'-GAI-5' can form base pairs with the codons 5'-CUA-3', 5'-CUC-3', and 5'-CUU-3' because they all have complementary bases to the hypoxanthine (I) in the tRNA anticodon. Specifically, the codons 5'-CUA-3' and 5'-CUC-3' each have adenine (A) opposite the hypoxanthine (I), and the codon 5'-CUU-3' has uracil (U) opposite the hypoxanthine (I). Therefore, all of these codons can potentially bind to the tRNA with the anticodon sequence 3'-GAI-5'.

Question 20

Which of the following is not a function of elongation factors in E.coli?

Accepted Answer

Elongation factors in E.coli do not have a function to keep the ribosomal subunits bound together. This function is carried out by other proteins, such as initiation factors and release factors. The other functions mentioned, such as guiding tRNA to its correct position, providing energy, and regenerating the factors by replacing GTP for GDP, are all functions of elongation factors.

Quiz 12: Protein Synthesis: Translation of the Genetic Message