In 1951 cells taken from an African American woman suffering from cancer were found to be unique because they carried on reproducing endlessly in the laboratory. Henrietta Lacks died of cervical cancer in 1951. Cultures from her cells have since been used to provide medical breakthroughs but as Farhana Haider reports, Henrietta Lacks was never asked if her cells could be used in medical research.

(Photo: Henrietta Lacks. Copyright: Lacks Family)

The epic mini-series about slavery in the USA hit TV screens in January 1977. Based on a novel by Alex Haley it imagined the lives of his ancestors who had been brought to the US from Africa on slave ships. It revolutionised perceptions about African-Americans and their history. Ashley Byrne has spoken to Leslie Uggams who played the character Kizzy in the series.

(Photo: Actors LeVar Burton, Todd Bridges and Robert Reed in Roots. Credit: Alamy)

In June 1980, the Guyanese opposition leader and academic, Dr Walter Rodney, was killed in a bomb explosion. He was one of the leaders of a movement trying to bridge the racial divide in Guyana’s politics. His supporters said he had been assassinated on the orders of the government. We hear from his widow, Patricia Rodney, and from Wazir Mohamed who was a young activist at the time.

(Photo: Walter Rodney. Credit: the Walter Rodney Family)

In 1931, nine black teenagers were convicted of raping two white girls in the southern US state of Alabama.

Eight were sentenced to death by an all-white jury; but after years of campaigning, all eventually went free.

We hear from the daughter of Clarence Norris, one of the accused.

Picture: Police escort two recently freed "Scottsboro Boys" New York, 1937, Credit: Associated Press

In 1925 a young black American dancer became an overnight sensation in Paris. Her overtly sexual act soon made her one of the most famous women in Europe. Her name was Josephine Baker - hear from her adopted son Jean-Claude Baker about her dancing, and her life.

(Photo: Josephine Baker in her heyday. Credit: Walery/Getty Images)

Birmingham in Alabama was one of the most segregated cities in the USA in 1963. In May that year thousands of black schoolchildren responded to a call from Martin Luther King to protest against segregation at the height of racial tensions. It became known as the Children's Crusade.

Gwendolyn Webb was 14 years old at the time and took part. Listen to her story.

(Photo: Firefighters turn their hoses on civil rights demonstrators in Birmingham, Alabama. Credit: AP Photo/Bill Hudson)

Andrew Goodman was one of the three civil rights workers killed by the Klu Klux Klan in Mississippi in 1964. He and the other two victims, James Chaney and Michael Schwerner, had been working on a project to register African-Americans to vote.

For Witness, Andrew's brother David recalls his brother's strong sense of justice and what his family lived through in the 44 days he was missing. He remembers how nationwide shock helped change America for good - and that it took the deaths of two white people to awake the conscience of middle America.

Picture: Andrew Goodman, Credit: Associated Press

In 1968, when Petula Clark touched Harry Belafonte's arm during a duet, it was the first time a white woman had touched a black man on US television.

The sponsor insisted it be cut from the programme, but the programme makers refused.

Louise Hidalgo speaks to the producer of the programme, Steve Binder.

(Photo: Harry Belafonte. Credit: Alan Meek/Express/Getty Images)

In 1974 American Vogue put a black model on its cover for the first time. We hear how Beverly Johnson made it to the front of the world's most famous fashion magazine.

In 1975, Lee Elder braved death threats to become the first African-American golfer to play at the prestigious US Masters in Augusta.

It was one of the last colour barriers in US sport and made him a hero to many black sportsmen - including Tiger Woods.

Lee Elder recalls the tournament for Witness.

PHOTO: Lee Elder playing golf later in life (Getty Images)

In September 1971 prisoners in a high security jail in the US rose up against their guards taking 42 people hostage. After 4 days of negotiations, armed police retook the jail. By the time the siege ended 39 people were dead.

Photo: Discussions inside the prison on 10th September 1971. Associated Press.

On August 28th 1963, the American civil rights leader, Martin Luther King, made his historic plea for an end to racial discrimination in the USA. Standing on the steps of the Lincoln Memorial, he addressed hundreds of thousands of activists who had marched to Washington to demonstrate for black rights.

Listen to John Lewis, the youngest speaker on the podium that day.

Photo: Associated Press.

In 1967, the world heavyweight champion, Muhammad Ali, refused to be indicted into the American military. His decision to follow his conscience and not serve in Vietnam galvanised radicals across the US.

Simon Watts speaks to Dr Nathan Hare about a visit by Muhammad Ali to Howard University at the height of the outcry over his refusal of the draft.

(Photo: Muhammad Ali in training. Credit: R McPhedran/Express/Getty Images)

The soul singer's April 1968 concert was held amid rioting and violence provoked by the assassination of Martin Luther King. But despite the fears of the city authorities, the streets of Boston were quiet the night James Brown and his band played. Listen to two people who were there.

(Photo: James Brown. Credit: AFP)

For nearly 40 years, the US government conducted an experiment on a group of African-American men without their knowledge - to see what would happen if their syphilis was left untreated.

Photo: US National Archive.