Early work on gene structure was carried out largely through the examination of mutations in bacteria and viruses. This research led Francis Crick to propose in 1958 that genes and proteins are colinear, that there is a direct correspondence between the nucleotide sequence of DNA and the amino acid sequence of a protein. The concept of colinearity suggests that the number of nucleotides in a gene should be proportional to the number of amino acids in the protein encoded by that gene. In a general sense, this concept is true for genes found in bacterial cells and in many viruses, although these genes are slightly longer than would be expected if colinearity were strictly applied because the mRNAs encoded by the genes contain sequences at their ends that do not specify amino acids. At first, eukaryotic genes and proteins were also assumed to be colinear, but there were hints that eukaryotic gene structure is fundamentally different. Eukaryotic cells were found to contain far more DNA than is required to encode proteins. Furthermore, many large RNA molecules observed in the nucleus were absent from the cytoplasm, suggesting that nuclear RNAs undergo some type of change before they are exported to the cytoplasm.
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