Bonsoir, quelqu'un pourrait-il m'aider svp à répondre à ces 4 questions ? 

Researchers conducting tests in the harsh environment of Mono Lake in California
have discovered the first known micro-organism on Earth able to
thrive and reproduce using the toxic chemical arsenic. The
micro-organism substitutes arsenic for phosphorus in its cell
components.

"The definition of life has just expanded," said Ed Weiler, NASA's
associate administrator for the Science Mission Directorate at the
agency's Headquarters in Washington. "As we pursue our efforts
to seek signs of life in the solar system, we have to think more
broadly, more diversely and consider life as we do not know it."

Carbon,hydrogen, nitrogen, oxygen, phosphorus and sulfur are the six basic
building blocks of all known forms of life on Earth. Phosphorus is
part of the chemical backbone of DNA and RNA, the structures that
carry genetic instructions for life, and is considered an essential
element for all living cells.

Phosphorus is a central component of the energy-carrying molecule in all cells
(adenosine triphosphate) and also the phospholipids that form all
cell membranes. Arsenic, which is chemically similar to phosphorus,
is poisonous for most life on Earth. Arsenic disrupts metabolic
pathways because chemically it behaves similarly to phosphate.

"We know that some microbes can breathe arsenic, but what we've found is
a microbe doing something new -- building parts of itself out of
arsenic," said Felisa Wolfe-Simon, a NASA Astrobiology. "If
something here on Earth can do something so unexpected, what else can
life do that we haven't seen yet?"

The newly discovered microbe, strain GFAJ-1, is a member of a common
group of bacteria, the Gammaproteobacteria. In the laboratory, the
researchers successfully grew microbes from the lake on a diet that
was very lean on phosphorus, but included generous helpings of
arsenic. When researchers removed the phosphorus and replaced it with
arsenic the microbes continued to grow. Subsequent analyses indicated
that the arsenic was being used to produce the building blocks of new
GFAJ-1 cells. 

The key issue the researchers investigated was when the microbe was grown
on arsenic did the arsenic actually became incorporated into the
organisms' vital biochemical machinery, such as DNA, proteins and the
cell membranes.

The team chose to explore Mono Lake because of its unusual chemistry,
especially its high salinity, high alkalinity, and high levels of
arsenic. This chemistry is in part a result of Mono Lake's isolation
from its sources of fresh water for 50 years. 

"The idea of alternative biochemistries for life is common in science
fiction," said Carl Pilcher, director of the NASA Astrobiology
Institute at the agency's Ames Research Center in Moffett Field,
Calif. "Until now a life form using arsenic as a building block
was only theoretical, but now we know such life exists in Mono Lake."

1) Recall the main characteristics of the DNA molecule.
2) What are the main components of organic compounds on Earth ?
3) During the experiments, which element didn't scientists give to the
bacteria ? Explain why this element is important for bacteria.
4)What do exobiologists look for to discover extraterrestrial life today ?
Why is this discovery important ?