chapter 14
Which of these is currently considered the best definition of a gene?
A gene codes for either a polypeptide or an RNA molecule As our understanding of protein synthesis has expanded, we have seen that one gene may code for several polypeptides. We have also discovered many RNA molecules with diverse but important functions in the cell.
The genetic code is essentially the same for all organisms. Based on this information, one can logically assume which of the following statements to be correct?
A gene from an organism can theoretically be expressed by any other organism.
Which of the following statements about mutations is false?
A knock-out mutation results in a total absence of the mutated protein. A knock-out mutation refers to the loss of a protein's function but not necessarily to its complete absence.
The tRNA anticodon, GAC, is complementary to the mRNA codon with the sequence _____.
CUG
Which of the following is NOT true of a codon?
It extends from one end of a tRNA molecule.
What must happen to a newly made polypeptide before it can be secreted from a cell?
Its signal sequence must target it to the ER, after which it goes to the Golgi.
The initiator tRNA attaches at the ribosome's _____ site.
P The initiator tRNA attaches to the ribosome's P site.
A mutant bacterial cell has a defective aminoacyl-tRNA synthetase that attaches a lysine to tRNAs with the anticodon AAA instead of the normal phenylalanine. What would be a potential consequence for a cell in which this happens? Remember that phenylalanine may be encoded by multiple codons
Proteins in the cell will include lysine instead of phenylalanine at amino acid positions specified by the codon UUU.
Which of the following statements is true about protein synthesis in prokaryotes?
Translation can begin while transcription is still in progress.
If the sequence ATGCATGTCAATTGA were mutated such that a base were inserted after the first G and the third T were deleted, how many amino acids would be changed in the mutant protein?
Two The second and third codons in the new sequence are different from the original codons.
the figure represents tRNA that recognizes and binds a particular amino acid (in this instance, phenylalanine). Which codon on the mRNA strand codes for this amino acid?
UUC
Of the following, which is the most current and complete description of a gene?
a DNA sequence that is expressed to form a functional product: either RNA or polypeptide
Which of the following DNA mutations is likely to have the most negative effect on the protein it specifies?
a base-pair deletion
Which of the following types of mutation, resulting in an error in the mRNA just after the 3 AUG start codon, is likely to have the most serious effect on the polypeptide product?
a deletion of two nucleotides
During RNA processing a(n) _____ is added to the 3' end of the RNA
a long string of adenine nucleotides A poly-A tail is added to the 3' end of the RNA.
Which component is NOT directly involved in translation?
a single nucleotide insertion downstream of, and close to, the start of the coding sequence
What enzyme catalyzes the attachment of an amino acid to tRNA?
aminoacyl-tRNA synthetase This enzyme matches a particular tRNA with a particular amino acid.
When the ribosome reaches a stop codon on the mRNA, no corresponding tRNA enters the A site. If the translation reaction were to be experimentally stopped at this point, which of the following sets of structures would you be able to isolate from the cell?
an assembled ribosome with a polypeptide attached to the tRNA in the P site
The anticodon of a particular tRNA molecule is __________.
complementary to the corresponding mRNA codon
Translation occurs in the _____
cytoplasm Ribosomes, the sites of translation, are found in the cytoplasm.
Which of the following types of mutations could result in a frameshift mutation?
either an insertion or a deletion of a base
The RNA segments joined to one another by spliceosomes are _____.
exons Exons are expressed regions
True or false? A codon is a group of three bases that can specify more than one amino acid.
false A codon is a group of three bases that can specify only one amino acid.
The flow of information in a cell proceeds in what sequence?
from DNA to RNA to protein In a cell, inherited information flows from DNA to RNA to protein
There are 61 mRNA codons that specify an amino acid, but only 45 tRNAs. This is best explained by the fact that
the rules for base pairing between the third base of a codon and tRNA are flexible
A codon consists of __________ bases and specifies which __________ will be inserted into the polypeptide chain.
three ... amino acid Three nucleotide bases make up a codon and specify which amino acid comes next in the sequence.
The anticodon loop of the first tRNA that will complement this mRNA is
3' GGC 5'.
A particular triplet of bases in the template strand of DNA is 5' AGT 3'. The corresponding codon for the mRNA transcribed is
3' UCA 5'.
in which direction does the RNA polymerase move?
3' → 5' along the template strand
The direction of synthesis of an RNA transcript is _____.
5' —> 3' Nucleotides are added to the 3' end of RNA.
Which of the following does not occur in prokaryotic gene expression, but does occur in eukaryotic gene expression?
A poly-A tail is added to the 3' end of an mRNA and a cap is added to the 5' end.
Which of the following mutations would likely be most dangerous to a cell?
Deletion of one nucleotide Deletion of one nucleotide would shift the reading frame, altering the coding for all subsequent amino acids. In most cases, this would destroy the function of the protein produced from this gene. Read about the genetic code and point mutations.
If a DNA sequence is altered from TAGCTGA to TAGTGA, what kind of mutation has occurred?
Deletion. The original sequence has lost the base C.
Which one of the following is true of tRNAs?
Each tRNA binds a particular amino acid. Aminoacyl-tRNA synthetase joins a specific amino acid to a tRNA, a single strand of RNA about 80 nucleotides long.
Which of the following is NOT true of RNA processing?
Exons are cut out before mRNA leaves the nucleus
Which of the following is a function of a poly-A tail in mRNA?
It helps protect the mRNA from degradation by hydrolytic enzymes.
What is the effect of a nucleotide-pair substitution that results in a nonsense mutation in a gene?
It introduces a premature stop codon into the mRNA.
Which of the following best describes the significance of the TATA box in eukaryotic promoters?
It is the recognition site for a specific transcription factor.
Which mutation(s) would not change the remainder of the reading frame of a gene sequence that follows the mutation(s)?
One addition and one deletion mutation. This combination results in no net change in the number of bases, so the reading frame would eventually be restored.
After an RNA molecule is transcribed from a eukaryotic gene, what is removed and what is spliced together to produce an mRNA molecule with a continuous coding sequence?
introns ... exons Introns, intervening sequences, are removed, and the exons, expressed sequences, are spliced together
Which of the following experimental procedures is most likely to speed up mRNA degradation in a eukaryotic cell?
removal of the 5' cap
In eukaryotic cells, transcription CANNOT begin until __________.
several transcription factors have bound to the promoter
Spliceosomes are composed of _____.
small RNAs and proteins Spliceosomes are complexes composed of small RNAs and proteins.
Examine the simple metabolic pathway in the figure. According to Beadle and Tatum's hypothesis, how many genes are necessary for this pathway A------------->B--------------->C (enzyme one) (enzyme two)
two
During RNA processing a(n) _____ is added to the 5' end of the RNA.
modified guanine nucleotide The 5' cap consists of a modified guanine nucleotide.
Where does RNA polymerase begin transcribing a gene into mRNA?
It starts after a certain nucleotide sequence called a promoter In both eukaryotes and prokaryotes, RNA polymerase binds to the gene's promoter and begins transcription at a nucleotide known as the start point, although in eukaryotes the binding of RNA polymerase to the promoter requires transcription factors.
What is the function of RNA polymerase?
It unwinds the double helix and adds nucleotides to a growing strand of RNA. RNA polymerase has several functions in transcription, including unwinding the DNA double helix and adding RNA nucleotides
If a mutated DNA sequence produces a protein that differs in one central amino acid from the normal protein, which of the following kinds of mutations could have occurred?
An addition mutation and a deletion mutation. If the mutations occur within the same codon, only that codon (amino acid) will be altered.