A letter by Francis Crick, the co-discoverer of DNA, outlining the Nobel Prize-winning achievement to his young son is expected to fetch as much as $2 million when it is sold at auction in April, Christie's said on Tuesday.

Crick and James Watson unraveled the double-helix structure and function of deoxyribonucleic acid (DNA) while working together in Cambridge, England, in 1953. They received the Nobel Prize in medicine in 1962 for their ground-breaking work.

In the seven-page, handwritten letter, Crick, who was 33 years old at the time, described the discovery to his 12-year-old son Michael, who was away at a British boarding school.

"When you come home we will show you the model," he wrote in the letter.

Crick went on to say he believed DNA is a code and that the order of the bases (the letters) makes one gene different from another gene.

"In other words we think we have found the basic copying mechanism by which life comes from life. You can understand that we are very excited," Crick added, before signing the letter, "Lots of love, Daddy."

In his later years Crick was a professor at the Salk Institute for Biological Studies in La Jolla, California. He died in 2004.

The letter, which is being sold by Crick's son, will be part of the books and manuscripts sale on April 10.

A letter dated August 2, 1939, by physicist Albert Einstein to President Franklin Delano Rooseveletwarning him of the potential danger of "the construction of extremely powerful bombs" through nuclear fission sold for more than $2 million at auction in 2002.