Making of bone....

...complex but beautifully logical and finely tuned!
First, cartilage does not "turn into" bone. Chondrocytes die through programmed cell death (apoptosis) and are then REPLACED by bone forming cells (osteocytes -> osteoblasts). These cells secrete a protein called osteoid, which is then hardened (utilizing other protiens, enzymes, and Vitamin D) with calcium and phosphate. Osteoclasts then model the final bone structure based on DNA instructions. Actually, though the skeleton provides critical infrastructure, the real critical function of bone is to provide a nanosecond-by-nanosecond supply of calcium, which is the major electrolyte involved in cell depolarization, neuronal signalling, and muscle contraction. It all works so beautifully! This is what millions of years of natural selection has accomplished...a symphony of bone!