Researchers have found that there a numerous transposons or “jumping genes” within the genetic code that is responsible for development of the embryo and its growth. These were earlier thought to be junk or useless parts of the genome.
The jumping genes were first found in corn DNA by biologist Barbara McClintock who won the Nobel in 1983. Since then this phenomenon has been studied in detail by numerous scientists.
The team of researchers have found that these DNA sequences or transposons are called jumping genes because they can introduce copies of themselves or change their location in the genetic code. Till now these were thought to be genetic parasites that replicated without purpose and served no use. Half of the genetic code was found to be transposons.
Now a new study has shown that these sequences are actually very important for growth, development and also evolution of the species. Around a quarter of them for example are responsible for working on the genetic switches that turn off and on and can alter protein synthesis and gene action.
These regulatory sequences are known to contain bits of transposons which have now become part of the genome. These transposons are commonly found to insert themselves where they did not belong and this may lead to mutations. While mutations are necessary for evolution, they are also responsible for genetic diseases and cancers.
The team of researchers led by Developmental biologist Miguel Ramalho-Santos from University of California, San Francisco looked at a transposon that forms 17 percent of the human genome – transposon LINE1.
They wanted to study if this gene sequence just existed or contributed to embryonic development. They found that in order to replicate LINE1 makes a RNA copy of itself. Thereafter an enzyme converts this RNA into DNA.
This DNA sequence gets incorporated into the genome. Ramalho-Santos says that this movement of the gene could be disastrous for the growing embryo. But it seemed that the embryos have a high degree of transposon activity and movement. The purpose of the transposon movement within the embryos was the puzzle the team was trying to solve. The study results were published in the latest issue of the journal Cell this week.
To understand what LINE1 did, they devised a method to reduce the LINE1 RNA content by 80 to 90 percent. Then they tested the effect this restriction had on the mouse embryos and their stem cells. The results showed that removal or reduction of the LINE1 RNA led to a reduce capacity of the embryonic cells to replenish themselves. As LINE1 action was stopped, the team noted that the embryo stopped at the two cell stage. This means that the embryo then cannot implant itself or get embedded in the wall of the uterus for further development. This translates into no developing fetus and no baby.
The team found that the RNA from the transposon works with two proteins that allow the embryo to develop from its two cell stage.
Ramalho-Santos says that transposons are not “selfish parasites” as believed earlier but are helpful parts of the genome that help on embryonic development. Nearly 500,000 copies of LINE1 exist in the human genome say experts. All of their roles in humans are still unclear.