Which bonds hold base pairs in DNA together?
Hydrogen bonds
Sulfhydryl bonds
Nonpolar covalent bonds
Peptide bonds
Polar covalent bonds
The Correct Answer is A
A. Hydrogen bonds: Hydrogen bonds hold complementary nitrogenous bases together between the two strands of the DNA double helix. Adenine pairs with thymine through two hydrogen bonds, while guanine pairs with cytosine through three hydrogen bonds. These relatively weak bonds allow the DNA strands to separate easily during replication and transcription while still maintaining overall structural stability of the molecule.
B. Sulfhydryl bonds: Sulfhydryl bonds (also known as disulfide bonds) occur between sulfur-containing amino acids such as cysteine in proteins. These bonds help stabilize the tertiary and quaternary structures of proteins. They are not involved in the bonding between DNA bases.
C. Nonpolar covalent bonds: Nonpolar covalent bonds involve equal sharing of electrons between atoms with similar electronegativities. While covalent bonds are important within molecules, the interactions between complementary DNA bases are not nonpolar covalent bonds. Instead, base pairing specifically relies on hydrogen bonding.
D. Peptide bonds: Peptide bonds are covalent bonds that link amino acids together to form polypeptide chains in proteins. They are fundamental to protein structure and are formed during protein synthesis on ribosomes. DNA molecules do not contain peptide bonds because they are composed of nucleotides rather than amino acids.
E. Polar covalent bonds: Polar covalent bonds occur when electrons are shared unequally between atoms with different electronegativities, creating partial charges within a molecule. While polar covalent bonds exist within individual DNA molecules, they do not hold the complementary base pairs together between the two strands.
Nursing Test Bank
Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is C
Explanation
A. facilitated diffusion: Facilitated diffusion is the movement of substances across a cell membrane via specific carrier proteins or channels, but it occurs along the concentration gradient (from high to low) and does not require cellular energy. It relies on the natural kinetic energy of molecules rather than ATP.
B. endocytosis: Endocytosis involves the engulfing of large particles or liquids by the cell membrane to bring them into the cell. Although it requires energy, it is a bulk transport mechanism rather than the specific movement of molecules against a concentration gradient using a carrier protein.
C. active transport: Active transport is the movement of molecules against their concentration gradient (from lower to higher concentration) using specific transport proteins and energy expenditure, typically in the form of ATP. Examples include the sodium-potassium pump and calcium pumps in cell membranes, which maintain essential ionic gradients for cell function.
D. osmosis: Osmosis is the passive movement of water molecules across a semipermeable membrane from an area of low solute concentration to high solute concentration. It does not require energy or a carrier protein and is specific to water movement.
E. diffusion: Diffusion is the passive movement of molecules from an area of higher concentration to lower concentration down their concentration gradient. It does not require energy or proteins and is driven by molecular motion, unlike active transport which moves substances against the gradient.
Correct Answer is A
Explanation
A. Endotoxin: Endotoxins are lipopolysaccharide (LPS) components of the outer membrane of Gram-negative bacteria. When these bacteria die, LPS is released into the host’s bloodstream, triggering a strong immune response. The lipid A portion of endotoxin activates macrophages to release cytokines such as interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-α), which act on the hypothalamus to induce fever.
B. Exotoxin: Exotoxins are proteins secreted by both Gram-positive and Gram-negative bacteria during their growth. They are highly potent and can cause tissue damage and specific clinical syndromes, but they are not primarily responsible for fever following bacterial death.
C. Capsule proteins: Capsule components contribute to bacterial virulence by preventing phagocytosis and promoting adherence. While they enhance bacterial survival, capsule proteins do not directly trigger the cytokine-mediated fever response that is associated with endotoxin release.
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