Page 4 - Science Focus (Issue 018)

Jumping Genes:

Barbara McClintock and Transposable DNA Elements

༪ᚔٙਿΪjBarbara McClintockၾDNAᔷЗɿ

By Chantelle Sullivan 蘇盈安

Have your friends ever shared with you a McClintock also found another element that
seemingly impossible scientific notion, and had you controls the Ds transposon, which she named Ac
thinking “There’s no way that’s possible?” This was for activator. Ac contains all the tools and enzymes
similar to what 1983 Nobel laureate in Physiology and needed for Ds to move around. She found that if Ac
Medicine Barbara McClintock faced when she came helps Ds move out of the color gene in cases where
to her conclusion that some genes can move around Ds was already lying in the C gene, the anthocyanin
the genome, and reinsert itself at other loci (positions pigment production can be resumed. If this happens
of the DNA). McClintock had made her discovery in one of the cells during kernel development where
even before the structure of DNA was elucidated. cell division continuously occurs, the resulting purple
However, like Gregor Mendel, the first geneticist cell will divide and make clones of itself, and forms
who is famous for his pea plant experiments, spots or streaks. As a result, the kernel will be speckled.
McClintock’s discovery had not been accepted by When McClintock shared her findings in the
her contemporaries but was rediscovered later.
1951 Annual Symposium at the Cold Spring Harbor
In the 1940s, McClintock was working in the Cold Laboratory and in a publication in the same year,
Spring Harbor Laboratory and conducted studies she received “puzzlement, even hostility” [1] since
focusing on the color patterns of Zea mays (maize). the concept was deemed conceptually difficult
At the time, many geneticists were puzzled by the and seemingly unlikely to occur. In addition to
variation of kernel colors on the same corn cob, which transposable elements, she also presented her findings
can be white, purple or speckled. People assumed that genes can be silenced (i.e. gene expression can
that the speckled kernels to be mutations of the white be halted) by enzymes, which also received ridicule
color gene and thought nothing more of it, as the and negative criticism. She subsequently stopped
assumption was too hard to prove true. McClintock publishing more data regarding transposable
took the problem into her own hands, and conducted elements, as she thought she “must await the right
experiments on maize, which entailed the endless time for conceptual change.” [2] McClintock and her
breeding of these plants. research were ahead of her time.
From her experiments, she obtained evidence It was only in the 1970s when her research
that pointed towards the existence of transposable resurfaced and was given a second look by other
DNA elements which inserted themselves into the biologists in the community. This was due to the
1
color (C) gene to alter the abundance of the purple emergence of data from others who have also
pigment in corn called anthocyanin. She called those identified transposable elements in bacteria and
transposable DNA elements Ds, for disassociation. viruses. Afterwards, Barbara McClintock gained
When Ds jumped into C, the gene can be disrupted the approval and acceptance of the scientific
and the anthocyanin production is therefore halted. community, and was awarded the Nobel Prize in
In this case, the kernel loses its purple pigmentation Physiology and Medicine in 1983. With her seminal
and becomes white. discovery of transposable elements, McClintock was
the pioneer of cytogenetics .
2

1 Transposable element: A DNA sequence that can change its
position in the genome
2 Cytogenetics: The study of the structure and function of
chromosomes