ADL 02
Addition of the new amino acids involves three steps.
1. Binding of the charged tRNA to the A site. This step requires correct base-pairing between the codon on the mRNA and the anticodon on the tRNA. 2. Formation of the new peptide bond. In the process, the polypeptide chain is transferred from the tRNA in the P site to the amino acid on the tRNA in the A site. 3. Movement of the mRNA through the ribosome. In this step, the discharged tRNA shifts to the E site (where it is released) and the tRNA carrying the growing polypeptide shifts to the P site.
Given the following mRNA sequence, which of the following mutations would affect the protein sequence? 5 - AUG CAG UUA GCG UGC UAG - 3 5 - AUG CAG UUG GCG UGC UAG - 3 5 - AUG CAG UUA GCA UGC UAG - 3 5 - AUG CAA UUA GCG UGC UAG - 3 5 - AUG CAC UUA GCA UGC UAG - 3 5 - AUG CAA UUA GCG UGU UAG - 3
5 - AUG CAC UUA GCA UGC UAG - 3
Which single-base mutation would insert a premature stop codon into the following protein sequence? NMet-Gln-Leu-Arg-CysC 5 - AUG CAG UUA GCG UGC AAG - 3 5 - AUG AAG UUA GCG UGC UAG - 3 5 - AUG CAG UUA UUG UGC UAG - 3 5 - AUG CAG AUA GCG UGC UAG - 3 5 - AUG CAG UAA GCG UGC UAG - 3
5 - AUG CAG UAA GCG UGC UAG - 3
What normally binds to an open A-site on the ribosome? The 3' UTR (untranslated region) of the mRNA A tRNA that carries the next amino acid to be added to the polypeptide chain An empty tRNA after its amino acid has been added to the polypeptide chain The initiator tRNA that carries the first amino acid in the polypeptide chain
A tRNA that carries the next amino acid to be added to the polypeptide chain
Khorana used synthetic mRNAs to determine genetic code possibilities. To do so, he translated synthetic mRNA in vitro in the presence of individual 14C-labeled amino acids. Using the repeating dinucleotide Poly-UG, he identified a polypeptide with which two amino acids alternating?
Cys Val
tRNA interactions with mRNA and the ribosome
During translation, new amino acids are added one at a time to the growing polypeptide chain. The addition of each new amino acid involves three steps: Binding of the charged tRNA to the A site. This step requires correct base-pairing between the codon on the mRNA and the anticodon on the tRNA. Formation of the new peptide bond. In the process, the polypeptide chain is transferred from the tRNA in the P site to the amino acid on the tRNA in the A site. Movement of the mRNA through the ribosome. In this step, the discharged tRNA shifts to the E site (where it is released) and the tRNA carrying the growing polypeptide shifts to the P site.
_________ of translation always happens at the start codon of the mRNA.
Initiation
Which of the following statements concerning peptide bond formation is correct? It is catalyzed by peptidyl transferase. It is catalyzed by an enzymatic protein. It uses water. It requires GTP.
It is catalyzed by peptidyl transferase.
What events occur when one of these codons appears at the A site of the ribosome? Check all that apply. The ester bond linking the polypeptide to the tRNA in the P site is hydrolyzed. 50S and 30S ribosomal subunits assemble into 70S ribosome. GTP is hydrolyzed, causing release of the RF from the A site. Release factor RF1 or RF2 binds to the A site. The 70S ribosome disassembles into 50S and 30S ribosomal subunits
The ester bond linking the polypeptide to the tRNA in the P site is hydrolyzed. GTP is hydrolyzed, causing release of the RF from the A site. Release factor RF1 or RF2 binds to the A site. The 70S ribosome disassembles into 50S and 30S ribosomal subunits
In the early 1960s a number of investigators conducted a series of elegant experiments that helped to decipher the genetic code. Which of the following describes a bona fide property of the genetic code? The genetic code is a doublet code composed of two nucleotides per codon. Spacers or markers separate codons from one another. The genetic code consists of nonoverlapping codons. The genetic code consists of overlapping codons.
The genetic code consists of nonoverlapping codons. Experiments by Hans Fraenkel-Conrat in 1960 demonstrated that the genetic code consists of nonoverlapping codons.
How does the eukaryotic initiation complex locate the true start codon? The true start codon is the formyl-ATG, which will encode for fMet in the protein. The true start codon is the first ATG encountered downstream of the Kozak sequence. The true start codon is the first ATG encountered downstream of the Shine-Dalgarno sequence. The initiation complex moves the small ribosomal subunit through the 5 UTR, scanning for the start AUG. The preinitiation complex moves the ribosome through the 3 UTR, scanning for the Kozak sequence.
The initiation complex moves the ribosome thru the 3 UTR, scanning for the Kozak sequence.
Which of the following best describes the first step in the formation of the translation initiation complex? The small ribosomal subunit binds to an mRNA sequence near the 5' end of the transcript The small ribosomal subunit binds to an mRNA sequence near the 3' end of the transcript. The large ribosomal subunit binds to the small ribosomal subunit. The large ribosomal subunit binds to an mRNA sequence near the 5' end of the transcript
The small ribosomal subunit binds to an mRNA sequence near the 5' end of the transcript.
What is the role of codons UAA, UGA, and UAG in translation? They indicate the start of the coding sequence and trigger translation. They indicate the duplication of previous amino acid. They indicate the start of new gene. They indicate the end of the coding sequence and trigger translation termination.
They indicate the end of the coding sequence and trigger translation termination.
Part A - The molecular components of translation Although the process of translation is similar in bacteria and eukaryotes, there are some important differences.
Translation in bacteria: Shine-Dalgarno sequence tRNA^(fMet) Translation in eukaryotes: Kozak sequence Translation in both: mRNA, GTP hydrolysis, tRNA^(Ser), release factor, UAA codon (stop). Not part of translation: promoter, RNA polymerase. The general aspects of translation are similar in all organisms. However, there are some differences between bacterial and eukaryotic translation, especially during the initiation phase. In bacteria, the start codon lies just downstream of a recognition sequence called the Shine-Dalgarno sequence. In eukaryotes, the start codon is located within a different recognition sequence, the Kozak sequence. In bacteria, the amino acid on the initiator tRNA is a modified methionine called N-formylmethionine (fMet). In eukaryotes, the amino acid on the initiator tRNA is methionine.
Bacterial translation initiation
Translation initiation in bacteria involves the assembly of a 70S initiation complex from two ribosomal subunits, mRNA, and the initiator tRNA. Three initiation factors (IF1, IF2, and IF3) and GTP are also required. The assembly of these components occurs in a particular order, as follows: IF3 binds to the 30S subunit and prevents its association with the 50S subunit, keeping the 30S subunit available for the assembly of the remaining components. The 16S rRNA of the 30S subunit base-pairs with the Shine-Dalgarno sequence, which lies just upstream of the start codon in the mRNA. This base pairing positions the mRNA correctly on the 30S subunit. The initiator tRNA (tRNAfMet), with IF2 and GTP bound, base-pairs with the start codon in the mRNA. IF1 binds to the A site of the 30S initiation complex. The 50S subunit joins the 30S initiation complex. The initiation factors are released. The resulting 70S initiation complex is now ready for translation elongation.
In prokaryotes, the methionine that initiates the formation of a polypeptide chain differs from subsequently added methionines in that _______. its tRNA anticodon is not complementary to the AUG codon a formyl group is attached to the initiating methionine the initiating methionine is not an amino acid incorporation of the initial methionine does not require a tRNA
a formyl group is attached to the initiating methionine This modification is not present on methionine residues added during elongation.
Locations of the processes involved in protein synthesis a. Nucleolus b. Cytoplasm c. Ribosomes d. Nucleus e. Rough ER
a. formation of ribosomal subunits b. attachment of an amino acid to tRNA c. translation of cytoplasmic proteins d. transcription and RNA processing e. translation of secreted proteins
When a peptide bond is formed between two amino acids, one is attached to the tRNA occupying the P site and the other _______. is attached to the tRNA occupying the E site is attached to the tRNA occupying the A site is attached through hydrogen bonds to the mRNA is free in the cytoplasm
is attached to the tRNA occupying the A site
The RNA that has an amino acid attached to it, and that binds to the codon on the mRNA, is called a ___
tRNA
In eukaryotes, the initiation factor proteins eIF1A and eIF3 join with a ________ carrying the amino acid ______ to form the preinitiation complex.
tRNA, methionine
_________ of translation happens when the ribosome hits a stop codon on the mRNA
termination The process of translation, or protein synthesis, is a crucial part of the maintenance of living organisms. Proteins are constantly in use and will break down eventually, so new ones must always be available. If protein synthesis breaks down or stops, then the organism dies.
The process, performed by the ribosome, of reading mRNA and synthesizing a protein is called _______.
translation
What is the Shine-Dalgarno sequence? A short conserved nucleotide sequence downstream of the stop codon that signals termination of translation A short conserved nucleotide sequence upstream of the AUG start codon that binds initiator tRNA A conserved nucleotide sequence that identifies a transcriptional start site A short conserved nucleotide sequence upstream of the AUG start codon that serves to align the mRNA on the bacterial small ribosomal subunit
A short conserved nucleotide sequence upstream of the AUG start codon that serves to align the mRNA on the bacterial small ribosomal subunit. The Shine-Dalgarno sequence is a purine-rich sequence of six nucleotides localized three to nine nucleotides upstream of the AUG start codon.
During elongation, the charged tRNA is recruited to which site on the ribosome?
A site
Translation is directly dependent on all of the following associations except _______. complementary base pairing between mRNA and DNA complementary base pairing between mRNA and tRNA association of the 30S and the 50S ribosomal subunits complementary base pairing between mRNA and rRNA
complementary base pairing between mRNA and DNA Transcription, not translation, is dependent on this association.
A mutagen has introduced a frame-shift mutation by adding one nucleotide base. Which of the following would be classified as a reversion mutation for this particular mutant? deleting 1 base or adding 1 base deleting 1 base or adding 3 bases adding 1 base deleting 2 bases deleting 1 base or adding 2 bases
deleting 1 base or adding 2 bases
Binding of what protein initiates translation-termination events that result in polypeptide release and dissociation of ribosomal subunits?
release factor
Aminoacyl-tRNA synthetases are the enzymes responsible for attaching the correct amino acid to the corresponding tRNA molecule. How many different tRNA synthetase enzymes are typically required to accomplish this task in the cell? 61 20 64 1
20 . There is one tRNA synthetase enzyme for each amino acid. Each enzyme is responsible for charging all iso-accepting tRNAs with the specific corresponding amino acid.
Life as we know it depends on the genetic code: a set of codons, each made up of three bases in a DNA sequence and corresponding mRNA sequence, that specifies which of the 20 amino acids will be added to the protein during translation. Imagine that a prokaryote-like organism has been discovered in the polar ice on Mars. Interestingly, these Martian organisms use the same DNA → RNA → protein system as life on Earth, except that there are only 2 bases (A and T) in the Martian DNA, and there are only 17 amino acids found in Martian proteins. Based on this information, what is the minimum size of a codon for these hypothetical Martian life-forms? 2 bases 3 bases 4 bases 5 bases 6 bases The answer cannot be determined from the information provided.
5 bases. In the most general case of x bases and y bases per codon, the total number of possible codons is equal to xy . In the case of the hypothetical Martian life-forms, is the minimum codon length needed to specify 17 amino acids is 5 (25 = 32), with some redundancy (meaning that more than one codon could code for the same amino acid). For life on Earth, x = 4 and y = 3; thus the number of codons is 43, or 64. Because there are only 20 amino acids, there is a lot of redundancy in the code (there are several codons for each amino acid).
Determine which of the following sequences and structures represent part of mature eukaryotic mRNA. Check all that apply. termination sequence poly-A tail intron start codon 3'-UTR stop codon promoter 5'-UTR AAUAAA 5'-cap
5' cap 5' UTR start codon stop codon AAUAAAA poly A tail 3' UTR = stop codon, AAUAAA, poly-A tail
Using mathematical reasoning, a triplet genetic code gives a possible ________ different codons, while a doublet genetic code would yield ________ different codons. Thus, the triplet code accounts for 20 known amino acids and indicates redundancy in the genetic code.
64, 16
Once elongation is underway, tRNAs involved in the process occupy a series of sites on the complexed ribosome. The occupation of sites occurs in the following order A Site, P Site, E Site P Site, E Site, A Site A Site, S Site, E Site A Site, P Site, E Site, S Site
A P E
protein targetting pathway PFK insulin
PFK : cytoplasm only insulin : ER - GOLGI - OUTSIDE There are two general targeting pathways for nuclear-encoded proteins in eukaryotic cells. Proteins that will ultimately function in the cytoplasm (PFK, for example) are translated on free cytoplasmic ribosomes and released directly into the cytoplasm. Proteins that are destined for the membranes or compartments of the endomembrane system, as well as proteins that will be secreted from the cell (insulin, for example), are translated on ribosomes that are bound to the rough ER. For proteins translated on rough ER, the proteins are found in one of two places at the end of translation. If a protein is targeted to a membrane of the endomembrane system, it will be in the ER membrane. If a protein is targeted to the interior of an organelle in the endomembrane system or to the exterior of the cell, it will be in the lumen of the rough ER. From the rough ER (membrane or lumen), these non-cytoplasmic proteins move to the Golgi apparatus for processing and sorting before being sent to their final destinations.
RNA is a part of many cellular processes, particularly those associated with protein synthesis: transcription, RNA processing, and translation. Drag the labels to the appropriate bins to identify the step in protein synthesis where each type of RNA is utilized. a. transcription/RNA processing b. translation c. not used in protein synthesis
a. pre-mRNA, snRNA b. tRNA, rRNA, mature mRNA c. RNA primers In eukaryotes, pre-mRNA is produced by the direct transcription of the DNA sequence of a gene into a sequence of RNA nucleotides. Before this RNA transcript can be used as a template for protein synthesis, it is processed by modification of both the 5' and 3' ends. In addition, introns are removed from the pre-mRNA by a splicing process that is catalyzed by snRNAs (small nuclear RNAs) complexed with proteins. The product of RNA processing, mRNA (messenger RNA), exits the nucleus. Outside the nucleus, the mRNA serves as a template for protein synthesis on the ribosomes, which consist of catalytic rRNA (ribosomal RNA) molecules bound to ribosomal proteins. During translation, tRNA (transfer RNA) molecules match a sequence of three nucleotides in the mRNA to a specific amino acid, which is added to the growing polypeptide chain. RNA primers are not used in protein synthesis. RNA primers are only needed to initiate a new strand of DNA during DNA replication.
Which step in translation initiation is unique to eukaryotes? ribosome assembly translocation of the ribosome in the 3 direction formation of the initiation complex formation of the preinitiation complex binding of ribosomes to the 5' cap of the mRNA
binding of ribosomes to the 5' cap of the mRNA
In the ER, misfolded proteins are identified and bound by what molecules?
chaperone
Elongation factors translocate the ribosome in the 3 direction by a distance of two codons. three codons. two nucleotides. one nucleotide. one codon.
one codon
