To read/write to the DS1307 chip you use the Wire library see Wire - Arduino Reference The DS1307 memory is not mapped to the MCU memory space so you cannot just read a MCU memory location to get a value from the DS1307. Using the Wire library you tell the DS1307 what memory location you want to read/write on that device (not MCU memory) and how many bytes to read/write and then read/write them.
A poor analogy would be a office filing cabinet. Your sitting at home and and want to access some file from the cabinet in the office at work. You cannot directly access the cabinet but you can phone someone at the office and tell them what file you want to alter and then they read the file to you or you dictate the changes to them.