In some human populations, the proportion of individuals who are heterozygous for the sickle cell allele is much higher than would be expected by chance alone. Why?
A. Individuals with two normal alleles have an advantage over heterozygous individuals.
B. Individuals with two harmful alleles have an advantage over heterozygous individuals.
C. Individuals with two harmful alleles have an advantage over individuals with two normal alleles.
D. Heterozygous individuals have an advantage over individuals with two normal alleles.
D. Heterozygous individuals have an advantage over individuals with two normal alleles.
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The Southern Hemisphere has no equivalent of the arctic tundra because:a
it has no latitudes near the poles. b. it has insufficient altitudes. c. it has no land in the proper latitudes. d. it is too close to the sun during the winter. e. it has an inhospitable temperature.
Which of the following statements about Gram-negative bacteria and Gram-positive bacteria is true?
a. Only Gram-positive bacteria have a double layer of plasma membrane. b. Gram negative bacteria have much thicker cell walls than Gram-positive bacteria. c. Gram-positive bacteria have no plasma membrane, only a cell wall. d. Gram-negative bacteria are surrounded by a thin cell wall beneath an outer membrane.
In bacteria, the small, circular pieces of DNA that are located outside the chromosome are called
A) sex pili. B) plasmids. C) bacteriophages. D) flagella. E) endospores.
Which of these strategies does phage T7 use to ensure successful infection and replication within a host?
a. using host RNA polymerase to transcribe early proteins and then subsequently inhibiting host RNA polymerase activity b. rapidly synthesizing an anti-restriction protein before the entire T7 genome has entered the cell c. processing DNA concatemers using a head-full mechanism d. both a and b