JUMPING GENES
In the 1940s, Barbara McClintock, an American geneticist made an important discovery, while
working on inheritance in corn plants (Fig. 31.102 A). She found that certain genetic elements can
move from one location to another in a chromosome or even from one chromosome to another. These
mobile elements or “Jumping genes” may occupy the position in the middle of other genes and disrupt
them.
McClintock had discovered the jumping genes before other geneticists began to realize their
importance, indeed, the few workers understood her findings until the 1970s, after the mobile genes
had discovered in E. coli.
Strong evidences, now, suggest that all cells carry segments of DNA, called transposable elements or
transposons or jumping genes, that are able to move from one site to another (Fig. 31.102 B). She found
that a transposon can be inserted into the middle of a gene and make the gene no functional.
These mobile genetic elements can function as natural mutagens that develop genetic differences. Thus,
transposons would be powerful forces in evolution. In 1983, at the age of 81, she received the Nobel
Prize for her significant work.
Contribution of Khorana
(a) Artificial synthesis of a gene for yeast alanine t-RNA: The yeast alanine t-RNA molecule is the
first t-RNA molecule whose molecular structure was worked out by R.W. Holley and his
coworkers in 1965. Khorana and his associates (1969) used this information for the deduction of
the structure of the gene for yeast alanine t-RNA. They calculated that the gene for yeast
alanine t-RNA has 77 base pairs. For the artificial synthesis of this gene they had before them
the work of assembling the 77 pairs of nucleotides in exact order. Their technique involved first
the chemical synthesis of short deoxynucleotide sequence, which then were joined by hydrogen
bonds to form complementary strands. The second enzymatic synthesis was to join the double
stranded pieces. For this purpose the DNA repair polynucleotide ligase was employed. enzyme,
the (b) Artificial synthesis of a gene for bacterial tyrosine t-RNA: In 1975 Khorana and his
associates have completed the synthesis of a gene for E. coli tyrosine t-RNA. Tyr. t-RNA has 126
pairs of nucleotides.
