Which element is found in nucleic acids but not in proteins?

What is thought to have caused the mass extinction at the end of the cretaceous period?

Which is an advantage of having memory cells when a pathogen is encountered for a second time. a) the memory cells are what proliferate into clones of cells in response to the binding of an antigen. b) memory cells are vital to the primary immune response in that they immediately recognize pathogens even at the first encounter. c) memory cells are essentially effector cells that are short lived and attack antigens even without having a receptor specific to that antigen. d) it ensures that more lymphocytes with a receptor specific to a particular antigen will be present than in a host that had never encountered that pathogen.

Indicate the coat color and the proportion of offspring with that color for each of the following crosses of rabbits. assume all are homozygous. agouti x albino a) 1/2 albino, 1/2 agouti b) all agouti c) 3/4 agouti, 1/4 albino