Starfish has a endoskeleton
a coral has a exoskeleton
sponge has neither
While both will equate changes in a genetic chain, deletion would have more severe consequences. A substitution mutation usually only causes minor disorders and diseases, like sickle cell anemia. Whereas deletion causes much more serious diseases, like cystic fibrosis, and turner syndrome which is where a female is born with only one X chromosome.
Think of it as a zipper. The top zipper is the leading strand, the bottom is the lagging, and the actual zipper thing is the helicase unwinding the 2 parts. You need the top and bottom strands to have a replication fork
