In 1954, at the age of 37, Crick completed his PhD thesis: "X-Ray Diffraction: Polypeptides and Proteins" and received his degree.
Crick then worked in the laboratory of David Harker at Brooklyn Polytechnic Institute, where he continued to develop his skills in the analysis of X-ray diffraction data for proteins, working primarily on ribonuclease and the mechanisms of protein synthesis.
David Harker, the American X-ray crystallographer, was described as "the John Wayne of crystallography" by Vittorio Luzzati, a crystallographer at the Centre for Molecular Genetics in Gif-sur-Yvette near Paris, who had worked with Rosalind Franklin.

After the discovery of the double helix model of DNA, Crick's interests quickly turned to the biological implications of the structure.
In 1953, Watson and Crick published another article in Nature which stated: "it therefore seems likely that the precise sequence of the bases is the code that carries the genetical information".

In 1956, Crick and Watson speculated on the structure of small viruses.
They suggested that spherical viruses such as Tomato bushy stunt virus had icosahedral symmetry and were made from 60 identical subunits.

After his short time in New York, Crick returned to Cambridge where he worked until 1976, at which time he moved to California.
Crick engaged in several X-ray diffraction collaborations such as one with Alexander Rich on the structure of collagen.
However, Crick was quickly drifting away from continued work related to his expertise in the interpretation of X-ray diffraction patterns of proteins.

George Gamow established a group of scientists interested in the role of RNA as an intermediary between DNA as the genetic storage molecule in the nucleus of cells and the synthesis of proteins in the cytoplasm (the RNA Tie Club).
It was clear to Crick that there had to be a code by which a short sequence of nucleotides would specify a particular amino acid in a newly synthesised protein.
In 1956, Crick wrote an informal paper about the genetic coding problem for the small group of scientists in Gamow's RNA group.
In this article, Crick reviewed the evidence supporting the idea that there was a common set of about 20 amino acids used to synthesize proteins.
Crick proposed that there was a corresponding set of small "adaptor molecules" that would hydrogen bond to short sequences of a nucleic acid, and also link to one of the amino acids.
He also explored the many theoretical possibilities by which short nucleic acid sequences might code for the 20 amino acids.

During the mid-to-late 1950s Crick was very much intellectually engaged in sorting out the mystery of how proteins are synthesised.
By 1958, Crick's thinking had matured and he could list in an orderly way all of the key features of the protein synthesis process:
genetic information stored in the sequence of DNA molecules
a "messenger" RNA molecule to carry the instructions for making one protein to the cytoplasm
adaptor molecules ("they might contain nucleotides") to match short sequences of nucleotides in the RNA messenger molecules to specific amino acids
ribonucleic-protein complexes that catalyse the assembly of amino acids into proteins according to the messenger RNA
The adaptor molecules were eventually shown to be tRNAs and the catalytic "ribonucleic-protein complexes" became known as ribosomes.
An important step was later realisation (in 1960) that the messenger RNA was not the same as the ribosomal RNA.
None of this, however, answered the fundamental theoretical question of the exact nature of the genetic code.
In his 1958 article, Crick speculated, as had others, that a triplet of nucleotides could code for an amino acid.
Such a code might be "degenerate", with 4×4×4=64 possible triplets of the four nucleotide subunits while there were only 20 amino acids.
Some amino acids might have multiple triplet codes.
Crick also explored other codes in which, for various reasons, only some of the triplets were used, "magically" producing just the 20 needed combinations.
Experimental results were needed; theory alone
In 1954, at the age of 37, Crick completed his PhD thesis: "X-Ray Diffraction: Polypeptides and Proteins" and received his degree. Crick then worked in the laboratory of David Harker at Brooklyn Polytechnic Institute, where he continued to develop his skills in the analysis of X-ray diffraction data for proteins, working primarily on ribonuclease and the mechanisms of protein synthesis. David Harker, the American X-ray crystallographer, was described as "the John Wayne of crystallography" by Vittorio Luzzati, a crystallographer at the Centre for Molecular Genetics in Gif-sur-Yvette near Paris, who had worked with Rosalind Franklin.
After the discovery of the double helix model of DNA, Crick's interests quickly turned to the biological implications of the structure. In 1953, Watson and Crick published another article in Nature which stated: "it therefore seems likely that the precise sequence of the bases is the code that carries the genetical information".
In 1956, Crick and Watson speculated on the structure of small viruses. They suggested that spherical viruses such as Tomato bushy stunt virus had icosahedral symmetry and were made from 60 identical subunits.
After his short time in New York, Crick returned to Cambridge where he worked until 1976, at which time he moved to California. Crick engaged in several X-ray diffraction collaborations such as one with Alexander Rich on the structure of collagen. However, Crick was quickly drifting away from continued work related to his expertise in the interpretation of X-ray diffraction patterns of proteins.
George Gamow established a group of scientists interested in the role of RNA as an intermediary between DNA as the genetic storage molecule in the nucleus of cells and the synthesis of proteins in the cytoplasm (the RNA Tie Club). It was clear to Crick that there had to be a code by which a short sequence of nucleotides would specify a particular amino acid in a newly synthesised protein. In 1956, Crick wrote an informal paper about the genetic coding problem for the small group of scientists in Gamow's RNA group. In this article, Crick reviewed the evidence supporting the idea that there was a common set of about 20 amino acids used to synthesize proteins. Crick proposed that there was a corresponding set of small "adaptor molecules" that would hydrogen bond to short sequences of a nucleic acid, and also link to one of the amino acids. He also explored the many theoretical possibilities by which short nucleic acid sequences might code for the 20 amino acids.
During the mid-to-late 1950s Crick was very much intellectually engaged in sorting out the mystery of how proteins are synthesised. By 1958, Crick's thinking had matured and he could list in an orderly way all of the key features of the protein synthesis process:
genetic information stored in the sequence of DNA molecules
a "messenger" RNA molecule to carry the instructions for making one protein to the cytoplasm
adaptor molecules ("they might contain nucleotides") to match short sequences of nucleotides in the RNA messenger molecules to specific amino acids
ribonucleic-protein complexes that catalyse the assembly of amino acids into proteins according to the messenger RNA
The adaptor molecules were eventually shown to be tRNAs and the catalytic "ribonucleic-protein complexes" became known as ribosomes. An important step was later realisation (in 1960) that the messenger RNA was not the same as the ribosomal RNA. None of this, however, answered the fundamental theoretical question of the exact nature of the genetic code. In his 1958 article, Crick speculated, as had others, that a triplet of nucleotides could code for an amino acid. Such a code might be "degenerate", with 4×4×4=64 possible triplets of the four nucleotide subunits while there were only 20 amino acids. Some amino acids might have multiple triplet codes. Crick also explored other codes in which, for various reasons, only some of the triplets were used, "magically" producing just the 20 needed combinations. Experimental results were needed; theory alone 
Francis Crick