Which of the following accurately describes the way in which a muscle moves? a. when actin filaments are pulled toward the center of the sarcomere, the fiber shortens. b. interaction and attachment between myosin and actin cause a muscle fiber to relax. c. no atp is needed is the contraction of a muscle. d. during contraction, cross- bridges pull the myosin to the center of the sarcomere.

Keeping in mind the life cycle of bacteriophages, consider the following problem: during the reproductive cycle of a temperate bacteriophage, the viral dna inserts into the bacterial chromosome where the resultant prophage behaves much like a trojan horse. it can remain quiescent, or it can become lytic and initiate a burst of progeny viruses. several operons maintain the prophage state by interacting with a repressor that keeps the lytic cycle in check. insults (ultraviolet light, for example) to the bacterial cell lead to a partial breakdown of the repressor, which in turn causes the production of enzymes involved in the lytic cycle. as stated in this simple form, would you consider this system of regulation to be operating under positive or negative control?

Which example describes a mutualistic relationship between organisms? young wasps prey on caterpillars. crabs eat the remains of dead fish. tapeworms feed on food in the intestines of cats ants protect a tree on which they feed.

What are the subsystem of the earth