Which mutations cause a frameshift?
Nonsense and deletion
Missense and insertion
Missense and nonsense
Deletion and insertion
The Correct Answer is D
A. Nonsense and deletion: A nonsense mutation occurs when a codon that normally codes for an amino acid is changed into a stop codon, causing premature termination of protein synthesis. Although deletions can cause frameshifts, nonsense mutations themselves do not alter the reading frame. Therefore, this combination does not consistently produce frameshift mutations.
B. Missense and insertion: A missense mutation results from a single nucleotide substitution that changes one amino acid in the protein sequence. This alters protein structure but does not shift the reading frame. While insertions can cause frameshifts, the combination listed does not accurately represent the mechanisms responsible for frameshift mutations.
C. Missense and nonsense: Both missense and nonsense mutations are types of point mutations caused by single base substitutions. These mutations affect the identity of a codon or create a premature stop codon but do not alter the grouping of codons into triplets. As a result, they do not shift the reading frame of the genetic code.
D. Deletion and insertion: Frameshift mutations occur when nucleotides are inserted into or deleted from the DNA sequence in numbers not divisible by three. Because the genetic code is read in triplets (codons), such changes shift the reading frame downstream of the mutation. This alters all subsequent codons and often results in a drastically altered or nonfunctional protein.
Nursing Test Bank
Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is A
Explanation
A. Chitin: Fungal cell walls are primarily composed of chitin, a long-chain polymer of N-acetylglucosamine. Chitin provides structural strength and rigidity to the fungal cell wall, allowing the organism to maintain shape and resist osmotic pressure. Its presence distinguishes fungi from bacteria and makes it a target for antifungal therapies.
B. Peptidoglycan: Peptidoglycan is a key structural component of bacterial cell walls, particularly in Gram-positive and Gram-negative bacteria. It provides rigidity and protects against osmotic lysis but is absent in fungal cells, so it is not responsible for their structural integrity.
C. Lipopolysaccharide: Lipopolysaccharides (LPS) are molecules found in the outer membrane of Gram-negative bacteria. They contribute to bacterial virulence and can trigger immune responses in humans. LPS is not found in fungi and does not play a role in fungal infections.
Correct Answer is B
Explanation
A. Yeast: Yeast are single-celled fungi with relatively low resistance to physical and chemical agents. They are susceptible to heat, disinfectants, and antiseptics because they lack specialized protective structures like endospores, making them easier to control compared to more resistant microbial forms.
B. Bacterial endospores: Bacterial endospores are highly resistant, dormant structures formed by certain bacteria such as Bacillus and Clostridium species. They have a tough protective coat, low water content, and metabolic inactivity, which make them impervious to heat, radiation, desiccation, and many chemical disinfectants. Endospores can survive extreme environmental conditions for extended periods, making them the most resistant microbial form.
C. Fungal spores: Fungal spores provide some resistance to environmental stress and disinfectants, but they are significantly less resistant than bacterial endospores. Most fungal spores can be inactivated by standard sterilization techniques such as autoclaving.
D. Protozoan cysts: Protozoan cysts are protective forms that allow protozoa to survive harsh environments, including changes in pH and desiccation. While they are moderately resistant, they are not as impervious to sterilization methods as bacterial endospores.
E. Naked viruses: Naked (non-enveloped) viruses are more resistant than enveloped viruses to detergents and some disinfectants due to the absence of a lipid envelope. However, they are still more susceptible to heat, radiation, and chemical agents than bacterial endospores.
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