ill mark brainliest

explain how muscles “know” they are supposed to move?
explain why it is possible for you to extend your arm all the way, and then curl it right back up toward your body, if muscles can only pull?
about how many muscles do you have in your body?

hi! I can only help with one part of the answer. Muscles use a part of the brain I think called the cerebellum. Something called that which is cordination of muscles. It allows muscles to retract and extend. So its possible for you arm to extend and rool back.  

Explanation:

600 muscles in your body, here you go! :)

Explanation:

the sugar-phosphate backbone forms the structural framework of nucleic acids, including dna and rna. this backbone is composed of alternating sugar and phosphate groups, and defines directionality of the molecule.