Short answers, pulled from the story.
The Missouri Compromise of 1820 was federal legislation passed on the 6th of March 1820 that admitted Maine as a free state and Missouri as a slave state while prohibiting slavery in the Louisiana Territory north of latitude 36°30'.
Representative James Tallmadge Jr. from New York submitted the Tallmadge Amendment to Missouri's request for statehood on the 13th of February 1819 to prohibit further migration of slaves into the territory and emancipate offspring at age 25.
President James Monroe signed the Missouri Compromise bill on the 6th of March 1820 after both houses agreed to the measures on the 5th of March 1820 following Senate votes on February 17 and 18, 1820.
Southern congressmen opposed initial restrictions because admitting Missouri without slavery would destroy the equal balance of power between eleven free states and eleven slave states in the Senate where each state received two senators regardless of population.
Stephen A. Douglas's Kansas Nebraska Act of 1854 effectively repealed the provisions of the Missouri Compromise that forbade slavery north of latitude 36°30' which caused outrage in the North and sparked political activity by Abraham Lincoln.