London: An international team of scientists has for the first time mapped all the genes that are activated in the first few days of a fertilised human egg.

Led by Sweden’s Karolinska Institutet, the researchers showed that the newly identified genes can interact with the “junk DNA” — essential to the start of development.

There are approximately 23,000 human genes in total.

In the current study, scientists found that only 32 of these genes are switched on two days after fertilisation and by the day three, there are 129 activated genes.

Seven of the genes found and characterised had not been discovered previously.

“These genes are the ‘ignition key’ that is needed to turn on human embryonic development. It is like dropping a stone into water and then watching the waves spread across the surface”, said principal investigator Juha Kere, professor at the Karolinska Institutet.

The researchers had to develop a new way of analysing the results in order to find the new genes.

Most genes code for proteins but there are a number of repeated DNA sequences that are often considered to be so-called “junk DNA” but are, in fact, important in regulating gene expression.

“Our results provide novel insights into the regulation of early embryonic development in human,” added Outi Hovatta, professor at Karolinska Institutet’s department of clinical science, intervention and technology.

The team identified novel factors that might be used in reprogramming cells into so-called pluripotent stem cells for possible treatment of a range of diseases including infertility.

At the start of an individual’s life, there is a single fertilised egg cell.

One day after fertilisation there are two cells, after two days four, after three days eight and so on, until there are billions of cells at birth.

The order in which our genes are activated after fertilisation has remained one of the last uncharted territories of human development.

The study is forthcoming in the journal Nature Communications.