Why does the thermosphere have the highest temperature of all the atmosphere layers?

Thermosphere is the part of the Earth's atmosphere that lies above the surface of the earth. It extends to the outer space. It lies above the mesosphere the middle layer of the atmosphere and directly below the exosphere that is top layer of the atmosphere. It exhibit highest temperature of all the atmospheric layers because it absorbs X-rays and ultraviolet rays coming from the sun. These rays increases the temperature of the thermosphere.

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.