What is the initial step in DNA replication?
A) unwinding of the DNA double helix B) formation of the mitotic spindle
C) breakdown of the nuclear envelope D) formation of the cleavage furrow
Answer: A
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When legumes with nitrogen-fixation nodules are given chemical fertilizers with high nitrogen concentrations, they abandon support of the nodules and end the symbiosis. Why?
A. The mineral nutrients inhibit nitrogenase activity. B. The plant can conserve valuable energy sources by terminating the symbiotic relationship. C. Nitrogen gas is no longer available in the soil. D. The dissolved chemical nutrients are toxic to Rhizobium bacteria. E. Dissolved nitrogen uncurls the root hairs, releasing the nodule.
“Double fertilization” produces nutrient tissue by the
union of a. two egg nuclei and one sperm nucleus. b. two sperm nuclei and one egg nucleus. c. two sperm nuclei and two egg nuclei. d. one sperm nucleus and endosperm mother cell nuclei. e. two sperm nuclei and one endosperm mother cell nucleus.
Protein filaments with a diameter of about 10 nm that are major components of the cytoskeleton are called ________.
A. microtubules B. microfilaments C. intermediate filaments D. mycelia
Bacterial and archaeal ribosomes are known as ________ ribosomes, based on their sedimentation coefficient.
A. 70S B. 80S C. 50S D. 30S