Why are there semi lunar valves in arteries

Recombinant dna (rdna) creates offspring which are genetically identical to the parent is the process of breeding only organisms with desirable traits involves the removal of the nucleus of a cell combines genes from organisms of different species in a lab

Is there any species that went extinct in recent years due to natural causes (not caused by human interaction). if so, what caused it?

You see a white marker with an orange circle and black lettering. what does this marker tell you