Bio 180 Ch 16 (CSI) – Flashcards
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What does the one-gene, one-enzyme hypothesis state? a. Genes are composed of stretches of DNA.
b. Genes are made of protein.
c. Genes code for ribozymes.
d. A single gene codes for a single protein.
answer
d
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Which of the following is an important exception to the central dogma of molecular biology?
a. Many genes code for RNAs that function directly in the cell. b. DNA is the repository of genetic information in all cells.
c. Messenger RNA is a short-lived "information carrier."
d. Proteins are responsible for most aspects of the phenotype
answer
a
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DNA's primary structure is made up of just four different bases, and its secondary structure is regular and highly stable. How can a molecule with these characteristics hold all the information required to build and maintain a cell?
a. The information is first transcribed, then translated.
b. The messenger RNA produced from DNA has much more
complex secondary structures, allowing mRNA to hold much more information.
c. A protein coded for in DNA has much more complex primary and secondary structures, allowing it to hold much more information.
d. The information in DNA is in a code form that is based on the sequence of bases.
answer
d
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Why did researchers suspect that DNA does not code for proteins directly?
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Because there is no chemical complementarity between nucleo- tides and amino acids; and because in eukaryotes, DNA is in the nucleus but translation occurs in the cytoplasm.
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Which of the following describes an important experimental strategy in deciphering the genetic code?
a. comparing the amino acid sequences of proteins with the base
sequence of their genes
b. analyzing the sequence of RNAs produced from known DNA
sequences
c. analyzing mutants that changed the code
d. examining the polypeptides produced when RNAs of known
sequence were translated
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d
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What is a stop codon?
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A codon that signals the end of translation.
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Explain why Morse code is an appropriate analogy for the genetic code.
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Because the Morse code and genetic code both use simple elements (dots and dashes; 4 different bases) in different orders to encode complex informa- tion (words; amino acid sequences).
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How did experiments with Neurospora crassa mutants support the one-gene, one-enzyme hypothesis?
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They supported an important prediction of the hypothesis: Losing a gene (via mutation) resulted in loss of an enzyme
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Why does a single-base deletion mutation within a protein-coding sequence usually have a more severe effect than a deletion of three adjacent bases?
a. because single-base deletions prevent the ribosome from binding
to mRNA
b. because single-base deletions stabilize mRNA
c. because single-base deletions change the reading frame
d. because single-base deletions alter the meaning of individual
codons
answer
c
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When researchers discovered that a combination of three deletion mutations or three addition mutations would restore the function of a gene, most biologists were convinced that the genetic code was read in triplets. Explain the logic behind this conclusion.
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In a triplet code, addition or deletion of 1-2 bases disrupts the reading frame "downstream" of the mutation site(s), resulting in a dysfunctional protein. But addition or deletion of 3 bases restores the reading frame—the normal sequence is disrupted only between the first and third mutation. The resulting protein is altered but may still be able to function normally. Only a triplet code would show these patterns.
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Explain why all point mutations change the genotype, but why only some point mutations change the phenotype
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A point mutation changes the nucleotide sequence of an existing allele, creating a new one, so it always changes the genotype. But because the genetic code is redundant, and because point mutations can occur in DNA sequences that do not code for amino acids, these point mutations do not change the protein product and therefore do not change the phenotype.
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What problems would arise if the genetic code contained only 22 codons—one for each amino acid, a start signal, and a stop signal?
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Every copying error would result in a mutation that would change the amino acid sequence of the protein and would likely affect its function.
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Scientists say that a phenomenon is a "black box" if they can describe it and study its effects but don't know the underlying mechanism that causes it. In what sense was genetics—meaning the transmission of heritable traits—a black box before the central dogma of molecular biology was understood?
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Before the central dogma was under- stood, DNA was known to be the hereditary material, but no one knew how particular sequences of bases resulted in the production of RNA and protein products. The central dogma clarified how genotypes produce phenotypes.
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QUANTITATIVE One of the possibilities that researchers interested in the genetic code considered was that the code was overlapping, meaning that a single base could be part of up to three codons. How many amino acids would be encoded in the sequence
5′ AUGUUACGGAAU 3′ by a non-overlapping and maximally overlapping code?
a. 4 (non-overlapping) and 16 (overlapping)
b. 4 and 12
c. 4 and 10
d. 12 and 4
answer
c
