C. Incorrect! Threonine is an amino acid, not a nucleotide base.

Transcription

1 MCAT Biology - Problem Drill 05: RNA and Protein Biosynthesis Question No. 1 of Which of the following bases are only found in RNA? Question #01 (A) Ribose. (B) Uracil. (C) Threonine. (D) Adenine. Ribose is the five-carbon sugar in RNA. It has a 2 OH, whereas deoxyribose, found in DNA, has a 2 H. B. Correct! Uracil is a pyrimidine base found only in RNA, not in DNA. Threonine is an amino acid, not a nucleotide base. Adenine is found in both RNA and DNA. We are looking for a base that is RNAspecific. RNA is a polynucleotide containing four different nitrogenous bases: the purines (adenine and guanine) and the pyrimidines (cytosine and uracil) and a sugarphosphate backbone (ribose). The correct answer is (B).

2 Question No. 2 of 10 Instructions: (1) Read the problem statement and answer choices carefully; (2) Work the problems on paper as needed; (3) Pick the correct answer; and (4) Go back to review the core concept tutorial as 2. Which of the following are components of ribonucleic acid? Question #02 (A) Amino acids. (B) Deoxyribose. (C) Thymine. (D) Ribose. Amino acids are components of proteins, not RNA. Deoxyribose is found in DNA, not RNA. Thymine is found in DNA, not RNA. D. Correct! Ribose is the sugar component of RNA nucleotides. RNA is a polynucleotide containing four different nitrogenous bases: the purines (adenine and guanine) and the pyrimidines (cytosine and uracil) and a sugarphosphate backbone (ribose). The correct answer is (D).

3 Question No. 3 of What is the difference between RNA and DNA? Question #03 (A) RNA and DNA use different nitrogenous bases. (B) RNA contains a different sugar. (C) RNA uses different patterns of hydrogen bonding. (D) RNA does not have the same 2-D or 3-D structure. Three of the four bases included in the RNA and DNA are the same (A, G, C). Only one base is different (T in DNA, U in RNA). B. Correct! RNA contains ribose, while DNA contains deoxyribose. Hydrogen bonding between the purines involves 2 H bonds and 3 H bonds in pyrimidines in both DNA and RNA. Double-stranded RNA is capable of forming helical structures just like doublestranded DNA. DNA and RNA are both nucleic acids (unbranched polynucleotides), but they do have some basic differences. DNA contains the sugar deoxyribose while RNA contains ribose. DNA contains the bases adenine, thymine, cytosine, and guanine, while RNA contains adenine, uracil, cytosine, and guanine. DNA is a double helix while RNA is often single-stranded, though it may form double-stranded or looped structures. When RNA is double stranded, it adopts a helical structure like DNA. The correct answer is (B).

4 Question No. 4 of Put the following events of transcription in the correct order. Question #04 I. The Polymerase catalyzes addition of ribonucleoside triphosphates. II. The DNA double helix is unwound. III. The Polymerase encounters a termination signal. IV. The RNA molecule is processed and leaves the nucleus. (A) II, I, III, IV (B) I, III, IV, II (C) I, II, III, IV (D) II, I, IV, III A. Correct! The transcription starts with a DNA template and ends with RNA leaving the nucleus. The transcription starts with a DNA template and ends with RNA leaving the nucleus. The transcription starts with a DNA template and ends with RNA leaving the nucleus. The transcription starts with a DNA template and ends with RNA leaving the nucleus. DNA Template Transcription factors binding to gene promoter RNA polymerase unwinds DNA next to the gene and initiates synthesis Elongation in the 5 to 3 direction Termination at stop signal Processing/Splicing (remove introns and add poly-a tail) RNA ready for transport out of the nucleus The correct answer is (A).

5 Question No. 5 of What is the major function of RNA in the cell? Question #05 (A) It functions in storage of genetic information. (B) It produces the amino acids that comprise proteins. (C) It is the catalytic molecule of the cell. (D) It functions as a mediator between DNA and protein. DNA, not RNA, functions in the storage of genetic information. trna molecules transport amino acids to ribosomes during protein synthesis, but they do not create amino acids. While there are some RNA molecules that have catalytic functions, the major catalytic molecule in the cell is proteins. D. Correct! RNA is the intermediate phase of information flow in the cell. The information stored in DNA is transcribed into RNA, which is subsequently translated into protein. Several species of RNA exist in the cell. Three major species are important for the MCAT, and they all function in protein biosynthesis: mrna (the interface between the stored information in DNA and the functional/structural version of that information as protein), trna (carries amino acids to the ribosomes), and rrna (a component of the ribosomes). The correct answer is (D).

6 Question No. 6 of What is the function of mrna in protein biosynthesis? Question #06 (A) It is the site of protein synthesis. (B) It guarantees that the correct amino acid is added. (C) It transports amino acids to the ribosome. (D) It carries the sequence information for proteins. Ribosomes are the sites of protein synthesis, and they contain rrna instead of mrna. mrna does not have a proofreading function. Removal of a misincorporated amino acid is mediated by trna synthetases. trna transports amino acids to the ribosome, not mrna. D. Correct! mrna is the intermediate step in the flow of information in the cell. mrna carries the sequence for proteins as a series of three-base codons with each codon specifying an amino acid. DNA Template Transcription mrna Translation Peptide/Protein The correct answer is (D).

7 Question No. 7 of Which of the following mrna molecules will code for a peptide, if read from the first base? Question #07 (A) 5 -ATGGTGCAGTTATAG-3 (B) 5 -UGGGCGAAUCCCUAG-3 (C) 5 -UAGCCCAUUCACUUA-3 (D) 5 -AUGCCCUUUGAUUAA-3 This molecule contains thymine, so it is a DNA molecule, not RNA. This molecule does not have the appropriate start signal, so protein biosynthesis will not be initiated. This molecule has a stop codon in the first position, so initiation of protein synthesis will not occur. D. Correct! The first codon in this molecule is a start codon, and if we read across 3 bases at a time, we find that the last codon is one of the stop codons. The start signal in mrna for protein biosynthesis is AUG, which codes for methionine. This codon will interact with a trna bearing the anti-codon UAC. Three stop codons are possible: UAA, UAG, and UGA. They do not code for amino acids. The correct answer is (D).

8 Question No. 8 of Which of the following correctly describes ribosomes? Question #08 (A) They are composed only of a nucleic acid and rrna. (B) The mrna is bound to the A site. (C) The ribosome will read the mrna in the 5 3 direction. (D) The ribosome is a nuclear structure. rrna is a nucleic acid, and it is a component of ribosomes, but several ribosomal proteins are also included in the structures called ribosomes. An aminoacyl-trna molecule (i.e., a loaded trna), not mrna, occupies the A site of ribosomes. C. Correct! Ribosomes read and physically move along mrna molecules in the 5 3 direction. Ribosomes are located in the cytoplasm. Often, a single mrna molecule will be read by several ribosomes at once in the cytoplasm. In this case, the mrna molecule is like a string and the ribosomes run along its length like beads. Such a structure is termed a polyribosome, or a polysome, and it increases the efficiency of translation. The correct answer is (C).

9 Question No. 9 of What prevents the coupling of transcription and translation in eukaryotes? Question #09 (A) The existence of the nuclear membrane. (B) Steric hindrance of RNA polymerase and ribosomes. (C) Regulatory proteins ensure that one process finishes before the other starts. (D) Transcription and translation are coupled in eukaryotes. A. Correct! The existence of the nuclear membrane physically separates the flow of information within the cell into two halves. mrna is produced in the nucleus and must be transported out through nuclear pores to be translated in the cytoplasm. In eukaryotic cells, RNA Polymerase and ribosomes do not function in the same compartment (i.e., one is nuclear, the other is cytoplasmic). The reason for sequential transcription and translation is not regulatory proteins; it is the existence of the nuclear membrane. Transcription and translation are never coupled in eukaryotes. Transcription and translation may be coupled in prokaryotes, because they do not have nuclear membranes. The correct answer is (A).

10 Question No. 10 of Which of the following is not true regarding protein biosynthesis? Question #10 (A) There may be more than one trna for each amino acid. (B) Mutations that change the reading frame can result in shortened proteins. (C) Degeneracy refers to the fact that 20 codons exist for the 64 amino acids. (D) The nascent polypeptide chain is held at the P site of the ribosome. In many cases, an amino acid may be linked to more than one trna, because of the degeneracy of the genetic code; however, each trna is specific for one and only one amino acid. A mutation that shifts the reading frame can cause an mrna molecule to be read incorrectly so that premature stop codons appear, causing protein biosynthesis to be terminated early. C. Correct! This answer has the facts completely backward. There are 20 amino acids, but 64 possible codons. The growing polypeptide is held by a trna molecule located in the P site, while the trna molecule bringing the next amino acid to the chain enters at the A site. Degeneracy, or redundancy, in the genetic code refers to the fact that there are 64 possible codons but only 20 amino acids. This means that many amino acids can be specified by more than one codon, and consequently by more than one trna. This is due to what is known as the Wobble Hypothesis. The correct answer is (C).