In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: A time course was established for the change in tissue mass , RNA and DNA content , as well as hydroxyproline content .
### Output Text:
A time course was established for the change in tissue mass , RNA and DNA content , as well as <span class="chemical">hydroxyproline</span> content . 