Transposon - U2 Lesson 2
A transposon (or transposable element) is a small piece of
DNA that inserts itself into another place in the genome.
Geneticist Barbara McClintock discovered these genetic
elements while studying corn in the 1940s. McClintock
noticed that spontaneous breaks in the plant’s chromosomes
correlated with the color patterns of the corn. She later
revealed that certain bits of DNA — she called them “jumping
genes” — have the ability to move within and between
chromosomes, causing the breaks she saw.
How these jumping genes get around can vary. For example,
some make copies of themselves first, and then these copies
insert into another position of the genome. Others move
directly to another position by a “cut and paste” mechanism.
Transposons have since been identified in a variety of organisms, ranging from bacteria to humans. In the
process of inserting into the genome, transposons can interrupt the normal spelling of DNA, creating gene
mutations with a variety of effects. They may turn nearby genes off, preventing their ability to create
protein, or they may turn them on, increasing the amount of protein made.
There is evidence that transposons aren’t just “selfish genes” intent on replicating themselves or genomic
“junk” that provides no benefit to the host. They may play a creative role in building new functional parts of
the genome Recent research has shown that transposons may help plants respond and adapt to
environmental stress by regulating other genes. In bacteria, transposons often carry genes that impart
resistance to antibiotic substances, helping the bacteria survive.
Questions
1. What is a transposon?
2. What types of effects can transposons have on the genome?
First marsupial genome decoded
By Nicole Davis, Communications, May 9th, 2007
Photo courtesy of Phil Myers, Museum of Zoology, University of
Michigan
The human genome is littered with so-called junk DNA, relics
of “jumping genes” that hopped about chromosomes for more
than a billion years. Although these jumping genes have been
widely regarded as parasites, concerned only with selfpropagation, a new study suggests they in fact played a
creative role in evolution — spreading key genetic innovations
across the genome.
This insight emerges from the work of an international research team led by scientists at the Broad
Institute of MIT and Harvard, which announced today the completion of a high-quality genome sequence of
the opossum Monodelphis domestica — the first marsupial to have its DNA decoded. Appearing in the May
10 issue of Nature, the findings provide a fresh look at the evolutionary origins of the human genome. They
also shed light on the genetic differences between placental mammals (including humans, mouse and dogs)
and marsupial mammals, such as opossums and kangaroos.
“Marsupials are the closest living relatives of placental mammals, which include humans,” said senior
author Kerstin Lindblad-Toh, the co-director of the Broad Institute’s Genome Sequencing and Analysis
Program. “Because of this relationship, the opossum genome offers a unique lens through which to view the
evolution of our own genome.”
In the last few years, the functionally important elements of the human genome have been identified
through genomic comparisons with other placental mammals. These genetic “working parts” are shared
universally across all placental mammals, and therefore must have been present when the creatures arose,
about 100 million years ago.
But how did these critical features evolve in the first place?
The scientists knew important clues could be found if they could search the recent past, rather than far-off
times in evolutionary history. For this, marsupials held the key. Marsupials are closely related to placental
mammals, but the two groups diverged 180 million years ago — well before placental mammals appeared.
So, by comparing the opossum and human genomes, the scientists were able to pinpoint the genetic
elements that are present in placental mammals but missing from marsupials — that is, the ones that
appeared just before the divergence of placental mammals.
Interestingly, about one-fifth of the key functional elements in the human genome arose during this recent
evolutionary period. By focusing on these “newer” innovations, the scientists made two remarkable
findings:

The vast majority (~95%) of recent genetic innovation lies not in protein-coding genes, but rather
the regulatory elements that influence genes’ activity. This result implies that mammals have
evolved not so much by inventing new kinds of proteins, as by tweaking the molecular controls that
dictate when and where proteins are made.

Most surprisingly, many of the new DNA instructions are derived from the jumping genes, or
“transposons”, which make up our so-called junk DNA. The percentage is at least 16% — and is
likely much higher, as many transposon-derived sequences have mutated beyond the point of
recognition.
“Transposons have a restless lifestyle, often shuttling themselves from one chromosome to another,” said
first author Tarjei Mikkelsen, a Broad Institute researcher and a Harvard-MIT Health Sciences and
Technology graduate student. “It is now clear that in their travels, they are disseminating crucial genetic
innovations around the genome.”
“Biology depends upon the precise coordination of large sets of genes that are switched on and off
together,” said Eric Lander, director of the Broad Institute of MIT and Harvard and an author of the Nature
paper. “One of the great mysteries in evolution is how this synchrony arises. These findings suggest a
simple answer — genetic controls can evolve in one location in the genome and then be distributed
elsewhere by transposons.”
Other important findings to emerge from the analysis of the opossum genome include:

the discovery that the opossum has many genes involved in immunity, challenging the notion that
marsupials possess only ‘primitive’ immune systems;

insights into the evolutionary origins of the random inactivation of one of the two X chromosomes
in females, a process unique to placental mammals; and

the unusual structure of the opossum genome, which has fewer chromosomes than the human
genome (9 pairs versus 23 pairs, respectively) but a longer total length (3.4 billion versus 3 billion
bases, respectively). Opossum chromosomes also exhibit distinct genetic compositions from human
chromosomes, providing strong support for a recent theory of chromosome evolution.
The sequencing of the M. domestica genome was made possible by a close collaboration with the marsupial
research community. In addition to comparative genomics studies, the genome sequence provides a
fundamental resource for scientists who study opossums as model organisms, and will help shed light on
the unique aspects of marsupial biology. For instance, newborn opossums can repair damage to their spinal
cord and thus are the focus of research in regenerative medicine. Opossums are also the only known
mammals, other than humans, in which ultraviolet (UV) radiation behaves as a complete carcinogen.
Genetic tools may enable deeper insights into the molecular mechanisms of melanoma and other UVinduced cancers, both in opossums and in humans.
Senior author Kerstin Lindblad-Toh led the opossum genome project, working together with first author
Tarjei Mikkelsen as well as several other Broad Institute scientists, including Jean Chang, Michele Clamp,
April Cook, James Cuff, Manuel Garber, Manfred Grabherr, Michael Kamal, Michael Kleber, Eric Lander,
Evan Mauceli, Ted Sharpe, Claire Wade, Xiaohui Xie, Michael Zody, and members of the Genome Sequencing
Platform and Whole Genome Assembly team.
Question
How did transposons contribute to the evolution of mammals?