It was the sound of young America tearing off its post-Kennedy shroud, a moment when a generation coalesced in one pent-up shriek to embrace the trans-Atlantic crossing of Beatlemania.

And the song that sparked the unforgettable fire in millions of kids watching the Ed Sullivan Show on February 9, 1964 - boys and girls - was She Loves You, a tune hammered out by John Lennon and Paul McCartney in a Newcastle hotel only nine months earlier.

The tune had actually flopped when released on an obscure label in the US in September 1963, eight weeks before JFK's callow presidency was ended by Lee Harvey Oswald's rifle.

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But something in that song, with its unforgettable hook, Yeah Yeah Yeah, shimmering harmonies and rolling drum intro, fused a collective charge in a pop culture that had been comatose since Elvis cut his hair and joined the army.

Today, half century down the line, the energy from that first performance of She Loves You is still a crackling, vibrant touchstone in the collective consciousness of those millions of baby-boomers who first tuned in and later dropped out to the songs of John, Paul, George and Ringo.

It was the third song they performed that night for the avuncular Sullivan, with All My Loving and Till There Was You providing the building blocks before the ear-splitting crescendo that reshaped American youth forever.

Within weeks, five Beatles singles occupied the top five slots in the Billboard Hot 100. She Loves You was at number 2 but eventually nudged I Want To Hold Your Hand to sit imperiously at the summit, beginning a 15-week stay in the charts. Today, it remains the fifth most-played Beatles song on American radio behind the likes of Hey Jude, Yesterday and I Want To Hold Your Hand, and was last year placed at 64 in Rolling Stone's 500 greatest songs of all time.

It sold 3.8 million copies in the country that gave the world rock 'n' roll but which now bowed in homage to four young guys from Liverpool.

Steve Marinucci, who runs the Beatles Examiner website in the States, has no doubt that the high-voltage energy released by She Loves You provided the initial impetus for the Beatles to endure in America.

He said: "Sinatra and Elvis had both attracted screamers but nothing compared to the Beatles. She Loves You was the one song where any talk of resistance to the band finally broke. It was a seminal moment in American pop culture and the Beatles have amazingly remained at its centre ever since."

There is one ironic postscript to She Loves You that would surely bring a smile to the face of Uncle Sam himself. When McCartney first played the song to his father, Jim, he was taken aback by the older man's withering putdown of the song's most famous element.

The older McCartney said: "Son, there are enough Americanisms around. Couldn't you sing, 'Yes, yes, yes,' just for once?"

McCartney replied: "You don't understand, Dad. It wouldn't work."

Ken McNab is the author of The Beatles in Scotland