Receptors detect changes in the environment. For a stimulus to elicit a response, certain cells must have an appropriate receptor. Stimulation of the receptor initiates a specific signal pathway.
Answer:
<u>A chemical bond between the phosphate group of one nucleotide and the sugar of a neighboring nucleotide holds the backbone together. Chemical bonds (hydrogen bonds) between the bases that are across from one another hold the two strands of the double helix together.</u>
Explanation:
secondary statement makes a seem correct
<span>It helps scientists understand how populations and species react to changes in their environment, and this knowledge can be used to help these populations and species survive.</span>
The youngest would be the ones at the top and the oldest would be at the bottom. the oldest is B and the ones around it, as they get bigger get younger. hope this helps :)
