Short answers, pulled from the story.
Tim Paterson created the original operating system known as 86-DOS during the summer of 1980 while working for Seattle Computer Products. Paterson delivered the code in August 1980 to help the company sell its 8086-based boards.
Microsoft acquired 86-DOS from Seattle Computer Products in July 1981 for 50,000 dollars. Bill Gates and Paul Allen licensed the existing product and renamed it MS-DOS without rewriting the code from scratch.
MS-DOS and PC DOS began to diverge significantly after twelve years and finally separated in 1993. The split occurred because the original licensing agreement allowed Microsoft to sell the operating system to other computer manufacturers while IBM wanted exclusive rights.
Microsoft declared all versions of MS-DOS 6.22 and older obsolete on the 31st of December 2001. Support for MS-DOS 7.0 ended on the same date, and support for MS-DOS 7.10 and MS-DOS 8.0 ended on the 11th of July 2006.
Microsoft released the source code for SCP MS-DOS 1.25 and a mixture of Altos MS-DOS 2.11 and TeleVideo PC DOS 2.11 on the 25th of March 2014. The company later re-licensed the code under the MIT License on the 28th of September 2018 and released the code for MS-DOS 4.00 on the 25th of April 2024.