chapter 13 – transcription – Flashcards
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The genetic code is said to be triplet, meaning that there ________.
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are three bases in mRNA that code for an amino acid
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The code is nonoverlapping, meaning that, assuming "standard translation," a given base participates in the specification of one and only one amino acid.
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True
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Both the codons UUU and UUC specify the amino acid phenylalanine. This is possible because the genetic code is __________.
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degenerate
Degeneracy of the code means that a given amino acid can be specified by more than one triplet codon.
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The genetic code is degenerate, meaning that a codon can specify more than one amino acid.
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False.
The degeneracy of the genetic code means that an amino acid may be coded for by more than one codon. However, a single codon can only ever specify one amino acid.
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A DNA sequence produces a mutant protein in which several amino acids in the middle of the protein differ from the normal protein. What kind of mutation could have occurred?
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An addition and a deletion mutation
A single addition or deletion would change the reading frame of the protein, but if another mutation occurred to cancel the effects of the first mutation, only those amino acids between the mutations would change.
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Which of the following statements about eukaryotic transcription is false?
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A polycistronic mRNA may be transcribed if the gene products are used in the same pathway or needed at the same time.
This statement is false. Polycistronic mRNAs are produced only in prokaryotes. In eukaryotes, a single gene is transcribed at a time.
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What is the initiator triplet in both prokaryotes and eukaryotes? What amino acid is recruited by this triplet?
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AUG; methionine
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When examining the genetic code, it is apparent that ________.
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there can be more than one codon for a particular amino acid
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The relationship between a gene and a messenger RNA is that ________.
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mRNAs are made from genes
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RNA synthesis from a DNA template is called __________.
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transcription
Transcription is initiated when the cell signals for the expression of a particular gene and involves the synthesis of RNA from a DNA template.
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When considering the initiation of transcription, one often finds consensus sequences located in the region of the DNA where RNA polymerase(s) binds. Which of the following is a common consensus sequence?
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TATA
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A 3′ poly-A tail and a 5′-cap are common components of prokaryotic RNAs.
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False.
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A 5′-cap describes the addition of a base, usually thymine, to the 5′ end of a completed peptide.
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False.
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Which subunit of RNA polymerase establishes template binding to a promoter in prokaryotes?
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Sigma.
The sigma subunit recognizes the promoter sequence. Different sigma subunits can be employed to regulate the expression of genes at the transcriptional level.
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Which of the following contains the three posttranscriptional modifications often seen in the maturation of mRNA in eukaryotes?
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heteroduplex formation, base modification, capping
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Transcription factors function to help move ribosomes along the mRNA.
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False.
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It has been recently determined that the gene for Duchenne muscular dystrophy (DMD) is more than 2000 kb (kilobases) in length; however, the mRNA produced by this gene is only about 14 kb long. What is a likely cause of this discrepancy?
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The introns have been spliced out during mRNA processing.
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If one compares the base sequences of related genes from different species, one is likely to find that corresponding ________ are usually conserved, but the sequences of ________ are much less well conserved.
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exons; introns
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Which of the following is a characteristic of RNA splicing in Eukaryotes?
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Exon/intron boundaries are typically characterized by a 5' GU splice junction and a 3' AG splice junction.
Correct. These splice junctions are recognized by the spliceosome so that accurate removal of introns is possible.
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A snRNP is best described as _______.
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small RNAs associated with protein complexes in the nucleus
Correct. snRNPs recognize the 5' and 3' splice junctions and the branch point sequence, excise the intron, and splice together the exons.
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Which of the following is most likely attributable to a base substitution at a 5' splice junction?
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A longer than usual final transcript
Correct. Such a mutation could block intron removal, resulting in a longer than usual transcript.
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Identify the the following elements on a diagram of a transcription bubble.
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Top strand:
5', DNA coding strand (non-template), 3'
Bottom strand:
3', DNA template strand, 5'
Ribbon:
5', RNA, 3'
The RNA polymerase reads the template strand from the 3' to 5' direction, and synthesizes RNA from 5' to 3'. The template strand is the DNA strand that is based paired to RNA as transcription proceeds. In this example, the RNA is paired with the bottom strand. The bottom strand is oriented with 3' end to the left and 5' end to the right. The coding strand (non-template) is complementary and anti-parallel to the template strand. Therefore the coding strand is oriented with the 5' end to the left and the 3' end to the right.
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What is the direction of transcription in this diagram?
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from left to right
The RNA polymerase reads the template strand from the 3' to 5' direction, and synthesizes RNA from 5' to 3'. The template strand is the DNA strand that is based paired to RNA as transcription proceeds. In this example, the RNA is paired with the bottom strand. The bottom strand is oriented with 3' end to the left and 5' end to the right. This means transcription is proceeding from left (3') to right (5').
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Where would you expect to find the promoter for this gene?
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left side
The promoter is located in the DNA that is adjacent to the start site of transcription. The start site corresponds with the 5' end of the RNA (or the 3' end of the DNA template strand).
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Identify the components involved in the initiation of transcription.
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Prokaryotic Initiation of Transcription:
Pribnow Box, -35 consensus, sigma subunit
Eukaryotic Initiation of Transcription:
.30 TATA box, enhancers, RNA Polymerase III
Neither;
DNA polymerase, rho
While the basics of transcriptional initiation in prokaryotes and eukaryotes are similar, they differ in complexity.
Prokaryotic RNA polymerase, with the addition of the sigma subunit, recognizes the Pribnow box and the -35 consensus.
Eukaryotic promoters and consensus sequences are considerably more diverse, and the three different RNA polymerases recognize different promoters, transcribe different genes, and produce different RNAs. Most eukaryotic promoters have a -30 TATA box. In addition to the core promoter, enhancer sequences around the eukaryotic gene bind activator proteins that interact with initiation complex to dramatically increase the efficiency of transcription. Many protein-protein interactions are necessary to initiate transcription in eukaryotes.
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In prokaryotes, which component must disassociate to allow for elongation of the transcriptional complex?
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sigma subunit
The sigma subunit helps RNA polymerase bind the promoter. This interaction is so strong that the RNA polymerase cannot leave the promoter. When the sigma subunit disassociates, this interaction is weakened, which allows the RNA polymerase to leave the promoter for elongation to begin.
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Which statements are true?
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-Since prokaryotes lack a nucleus, translation can begin before the transcript is fully transcribed.
-Three different RNA polymerases in eukaryotes transcribe different classes of genes.
-Eukaryotic RNA polymerase II requires general transcription factors for initiation of transcription.
One of the fundamental differences between prokaryotes and eukaryotes is that prokaryotes lack a nucleus that separates transcription from translation. Additionally, prokaryotes have only one RNA polymerase that transcribes all of the genes, whereas eukaryotes have three different RNA polymerases that recognize different promoters, transcribe different genes, and produce different RNAs.
In prokaryotes, RNA polymerase/sigma factor can initiate transcription alone, but in eukaryotes, RNA polymerases require many protein-protein interactions with several general transcription factors for initiation of transcription to occur.
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Where does the process of transcription fit into the central dogma of molecular genetics?
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Molecule:
DNA --> RNA --> protein
Process:
transcription --> translation
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Messenger RNA molecules are very difficult to isolate in prokaryotes because they are rather quickly degraded in the cell.
Can you suggest a reason why this occurs?
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mRNA molecules generally exist as single-stranded structures, so they are quite labile.
Prokaryotic cells exist in a more unstable environment than many cells of multicellular organisms, and labile mRNA is needed to accomplish such rapid responses.
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Eukaryotic mRNAs are more stable and exist longer in the cell than do prokaryotic mRNAs. Is this an advantage or a disadvantage for a pancreatic cell making large quantities of insulin?
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advantage
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One form of posttranscriptional modification of most eukaryotic RNA transcripts is the addition of a poly-A sequence at the 3 end. The absence of a poly-A sequence leads to rapid degradation of the transcript. Poly-A sequences of various lengths are also added to many prokaryotic RNA transcripts where, instead of promoting stability, they enhance degradation. In both cases, RNA secondary structures, stabilizing proteins, or degrading enzymes interact with poly-A sequences.
Considering the activities of RNAs, what might be the general functions of 3-polyadenylation?
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-regulation of mRNA longevity
-regulation of mRNA transport to the cytoplasm
-regulation of mRNA association with cellular components, like proteins, lipids, or nucleic acids