Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Triplet Code
Name: Angelica
Status: Student
Grade:  9-12
Location: VA
Country: United States
Date: September 2005

There are 64 amino acids overall but why does only 20 of them exist in humans?

There are actually many more than 64 amino acids, because the class of substances "amino acids" only requires one carboxyl group and one amine group. The presence of other groups is not excluded, and some of the group of 20 do contain sulfur. I don't think anyone knows why the "essential amino acids" are 20 in number. Even given the number, no one knows why these amino acids are "left handed", that is plane polarized light is rotated in only one direction. There are many unanswered questions in molecular biology.

Vince Calder

There are only 20 amino acids in nature, not 64. There are 64 different triplet codons in DNA and RNA (4 x 4 x4). A doublet code could only code for 4 x 4 = 16 codons. 64 is the minimum number of codons required to code for 20 amino acids.

Ron Baker, Ph.D.

Click here to return to the Molecular Biology Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory