BIO CH 17
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Which of the following is a post-translational modification of a polypeptide?
Cleavage of a polypeptide into two or more chains
What is the proper order of the following events in the expression of a eukaryotic gene? 1. translation 2. RNA processing 3. transcription 4. modification of protein
3, 2, 1, 4
How many nucleotides are needed to code for a protein with 450 amino acids?
At least 1,350
Who formulated the one gene-one enzyme hypothesis?
Beadle and Tatum
Ribosomes provide the scaffolding on which tRNAs interact with mRNA during translation of an mRNA sequence to a chain of amino acids. A ribosome has three binding sites, each of which has a distinct function in the tRNA-mRNA interactions. Drag the appropriate tRNAs to the binding sites on the ribosome to show the configuration immediately before a new peptide bond forms. Note that one of the binding sites should be left empty.
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.
During translation, nucleotide base triplets (codons) in mRNA are read in sequence in the 5' → 3' direction along the mRNA. Amino acids are specified by the string of codons. What amino acid sequence does the following mRNA nucleotide sequence specify? 5′−AUGGCAAGAAAA−3′ Express the sequence of amino acids using the three-letter abbreviations, separated by hyphens (e.g., Met-Ser-Thr-Lys-Gly).
Met-Ala-Arg-Lys An amino acid sequence is determined by strings of three-letter codons on the mRNA, each of which codes for a specific amino acid or a stop signal. The mRNA is translated in a 5' → 3' direction.
Before a molecule of mRNA can be translated into a protein on the ribosome, the mRNA must first be transcribed from a sequence of DNA. What amino acid sequence does the following DNA nucleotide sequence specify? 3′−TACAGAACGGTA−5′
Met-Ser-Cys-His Before mRNA can be translated into an amino acid sequence, the mRNA must first be synthesized from DNA through transcription. Base pairing in mRNA synthesis follows slightly different rules than in DNA synthesis: uracil (U) replaces thymine (T) in pairing with adenine (A). The codons specified by the mRNA are then translated into a string of amino acids.
Which of the following catalyzes the linkage between ribonucleotides to form RNA during gene expression
RNA polymerase
__________ is the synthesis of RNA using information in the DNA.
Transcription
Suppose that a portion of double-stranded DNA in the middle of a large gene is being transcribed by an RNA polymerase. As the polymerase moves through the sequence of six bases shown in the diagram below, what is the corresponding sequence of bases in the RNA that is produced?
UGAGCC There are three principles to keep in mind when predicting the sequence of the mRNA produced by transcription of a particular DNA sequence. The RNA polymerase reads the sequence of DNA bases from only one of the two strands of DNA: the template strand. The RNA polymerase reads the code from the template strand in the 3' to 5' direction and thus produces the mRNA strand in the 5' to 3' direction. In RNA, the base uracil (U) replaces the DNA base thymine (T). Thus the base-pairing rules in transcription are A→U, T→A, C→G, and G→C, where the first base is the coding base in the template strand of the DNA and the second base is the base that is added to the growing mRNA strand.
Use the codon table to determine which mRNA triplets code for the amino acid cysteine, Cys. Select all that apply
UGC and UGU There are only two mRNA triplets that code for the amino acid Cys: UGU and UGC.
Place the events in the transcription of a gene in their proper order from left (first event) to right (last event)
first event RNA polymerase binds promoter RNA polymerase transcribes gene RNA polymerase reaches terminator RNA polymerase exits gene. RNA is released. last event Transcription begins when a molecule of RNA polymerase binds to a promoter. Transcription continues through the gene, producing the RNA. Once RNA polymerase reaches the terminator, the RNA is released, and RNA polymerase falls off the DNA.
Insertions and deletions are called __________ mutations.
frameshift
Polysomes may be defined as __________.
groups of ribosomes
The bonds that hold tRNA molecules in the correct three-dimensional shape are __________.
hydrogen bonds
Place the labels of Group 1 in their proper locations on this diagram showing the process of transcription. Then, use the labels of Group 2 to identify the corresponding RNA nucleotide that belongs in each pink target. Labels of Group 2 can be used once, more than once, or not at all.
look at diagram RNA polymerase reads the template strand of the DNA to produce the final RNA. In doing so, the enzyme follows the base pairing rules for RNA, with U complementary to A
A ___ mutation cause a wild-type amino acid to be replaced by a different amino acid
missense These are the three possible outcomes of small-scale substitutions in genes.
The sickle-cell β-globin mutation is an example of a __________.
missense mutation
A ___ mutation causes an early Stop codon to occur
nonsense
The type of point mutation that results in a premature stop codon is called a _________ mutation.
nonsense
By bombarding the fungus Neurospora crassa with X-rays, Beadle and Tatum were able to study __________ and characterize enzymes in a __________.
nutritional mutants; biochemical pathway
The structures called snRNPs are __________.
part of a spliceosome
One strand of a DNA molecule has the following sequence: 3-AGTACAAACTATCCACCGTC-5. In order for transcription to occur in that strand, there would have to be a specific recognition sequence, called a(n) __________, to the left of the DNA sequence indicated.
promoter
RNA molecules that function as enzymes are called __________.
ribozymes
In eukaryotic cells, a __________ by a __________ targets a growing peptide to the endoplasmic reticulum.
signal peptide; signal-recognition particle
A ___ mutation does not change the wild-type amino acid sequence.
silent
Use the table to sort the following ten codons into one of the three bins, according to whether they code for a start codon, an in-sequence amino acid, or a stop codon.
start/methionine: AUG stop codon: UAA, UAG, and UGA amino acid: CAC, GCA, UGC, AAA, ACU, and AUC Nearly every mRNA gene that codes for a protein begins with the start codon, AUG, and thus begins with a methionine. Nearly every protein-coding sequence ends with one of the three stop codons (UAA, UAG, and UGA), which do not code for amino acids but signal the end of translation
Which of the following terms associated with transcription describe regions of nucleic acid? terminator gene rna polymerase promoter
terminator, gene, and promoter The gene itself, the promoter, and the terminator are all DNA deoxyribonucleic acid) sequences. The RNA (ribonucleic acid) that is produced is another. The RNA polymerase enzyme that performs transcription is a protein and therefore not a nucleic acid.
An exception to the one gene-one enzyme hypothesis is __________.
that not all genes code for enzymes; some genes code for structural proteins such as keratin
Eukaryotic processing of the primary transcript includes __________.
the addition of a 5ꞌ cap, a 3ꞌ poly-A tail, and the splicing out of introns
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?
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).
After transcription begins, several steps must be completed before the fully processed mRNA is ready to be used as a template for protein synthesis on the ribosomes. Which three statements correctly describe the processing that takes place before a mature mRNA exits the nucleus?
A cap consisting of a modified guanine nucleotide is added to the 5' end of the pre-mRNA, noncoding sequences called introns are splice out by molecular complexes called spliceosomes, and a poly-A tail (50-250 adenine nucleotides) is added to the 3' end of the pre-mRNA Once RNA polymerase II is bound to the promoter region of a gene, transcription of the template strand begins. As transcription proceeds, three key steps occur on the RNA transcript: Early in transcription, when the growing transcript is about 20 to 40 nucleotides long, a modified guanine nucleotide is added to the 5' end of the transcript, creating a 5' cap. Introns are spliced out of the RNA transcript by spliceosomes, and the exons are joined together, producing a continuous coding region. A poly-A tail (between 50 and 250 adenine nucleotides) is added to the 3' end of the RNA transcript. Only after all these steps have taken place is the mRNA complete and capable of exiting the nucleus. Once in the cytoplasm, the mRNA can participate in translation.
Why is a frameshift missense mutation more likely to have a severe effect on phenotype than a nucleotide-pair substitution missense mutation in the same protein?
A substitution missense affects only one codon, but a frameshift missense affects all codons downstream of the frameshift. Many genetic diseases are caused by missense mutations.
What is a key difference in gene expression between eukaryotic and prokaryotic cells?
In prokaryotic cells, the mRNA transcript is immediately available as mRNA without processing.
Think about the DNA coding sequence of a gene. If an A were swapped for a T, what kind of mutation could it cause and why?
It could cause a silent, missense, or nonsense mutation because those are the types that can be caused by a nucleotide-pair substitution like this one. When the sequence of bases in a gene is changed by a single nucleotide-pair substitution, it can have different effects on the product depending on where it is and whether or not the mutated codon codes for the same amino acid as the original codon.
Genetic information of eukaryotic cells is transferred from the nucleus to the cytoplasm in the form of __________.
RNA
Which of the following best describes the arrangement of genetic information in a DNA molecule?
The three-nucleotide words of a gene are arranged in a nonoverlapping series on the DNA template strand.
Suppose that the triplet of nucleotides indicated in bold (AGC) spans two codons, that is, CTA and GCC. If the triplet AGC were deleted from this DNA coding sequence, what effect would it have on the resulting protein? 5'-ATGCTAGCCTATCGTAAC-3'
The two flanking codons would be altered, but the rest of the amino acid sequence would be the same because there would be no frameshift. When a deletion of a set of three nucleotides that is out of frame to the reading frame of codons occurs, it only affects the codons flanking it. Once the ribosome reads past that point, the rest of the codons are in frame because a full triplet was deleted at one time. Return to Assignment
__________ is the synthesis of a polypeptide using information in the mRNA.
Translation
When RNA is being made, the RNA base _________ always pairs with the base __________ in DNA.
U; A
Because the bacterial cell's DNA is not surrounded by a nuclear envelope, __________ occur(s).
coupled transcription and translation
The diagram below shows an mRNA molecule that encodes a protein with 202 amino acids. The start and stop codons are highlighted, and a portion of the nucleotide sequence in the early part of the molecule is shown in detail. At position 35, a single base-pair substitution in the DNA has changed what would have been a uracil (U) in the mRNA to an adenine (A). Based on the genetic code chart above, which of the following would be the result of this single base-pair substitution?
a nonsense mutation resulting in early termination of translation The effect of a single base substitution depends on the new codon formed by the substitution. To identify the new codon, it is first necessary to determine the reading frame for the amino acid sequence. The first codon starts with base 1, the second codon with base 4, the third with base 7, and so on.In this problem, the codon that contains the single base substitution begins with base 34. The original codon (UUA, which encodes the amino acid leucine) is converted by the single base substitution to UAA, which is a stop codon. This will cause premature termination of translation, also called a nonsense mutation.
The function of tRNA during protein synthesis is to __________.
deliver amino acids to their proper site during protein synthesis
Stop codons are unique because they __________.
do not code for amino acids that allow a releasing factor to bind to the A site of the ribosome
The TATA box is a __________ that allows for the binding of __________ and __________.
eukaryotic promoter; transcription factors; RNA polymerase II
During transcription in eukaryotes, a type of RNA polymerase called RNA polymerase II moves along the template strand of the DNA in the 3'→5' direction. However, for any given gene, either strand of the double-stranded DNA may function as the template strand. For any given gene, what ultimately determines which DNA strand serves as the template strand?
the base sequence of the gene's promoter In eukaryotes, binding of RNA polymerase II to DNA involves several other proteins known as transcription factors. Many of these transcription factors bind to the DNA in the promoter region (shown below in green), located at the 3' end of the sequence on the template strand. Although some transcription factors bind to both strands of the DNA, others bind specifically to only one of the strands. Transcription factors do not bind randomly to the DNA. Information about where each transcription factor binds originates in the base sequence to which each transcription factor binds. The positioning of the transcription factors in the promoter region determines how the RNA polymerase II binds to the DNA and in which direction transcription will occur.
Bacteria can transcribe and translate human genes to produce functional human proteins because __________.
the genetic code is nearly universal
Gene expression is __________.
the process by which DNA directs the synthesis of proteins
The "triplet code" refers to the fact that _________.
three nucleotides code for a single amino acid
RNA plays important roles in many cellular processes, particularly those associated with protein synthesis: transcription, RNA processing, and translation.
transcription/RNA processing: pre-mRNA, snRNA, and mRNA translation: rRNA and tRNA not used in protein synthesis: 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 of the following sequences shows a frameshift mutation compared to the wild-type mRNA sequence?
wild-type 5'-AUGCAUACAUUGGAGUGA-3' mutant 5'-AUGCAUACAUCUGGAGUGA-3' In order for an insertion or deletion to cause a frameshift mutation, it must cause the reading frame of each triplet of bases after it to be shifted by one or two places.
