How do bacteria protect themselves against viruses?

the immune systems include a vast array of restriction enzymes, which help cut up the viral DNA and render it ineffective.

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

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layer 3 is younger than layer 6.