Video Transcript
The first experiment that cracked
the first “word” of the genetic code is called the poly(U) experiment and was
carried out in the ’60s by two scientists, M. Nirenberg and H. Matthaei. They created synthetic RNA chains
only made of uracil bases, poly(U) RNA, for example, five prime UUU UUU UUU UU three
prime. They added these poly(U) chains to
20 tubes filled with the cytoplasm of bacteria, containing the translation
machinery. Each tube also contained one type
of the 20 amino acids. In which tube did they observe a
synthesis of protein? Use the codon wheel to find the
answer. (A) In the phenylalanine tube, (B)
in the glutamic acid tube, or (C) in the cysteine tube.
The poly(U) experiment is the key
experiment that allowed scientists to start deciphering the genetic code. The genetic code is the set of
rules that all living cells use to translate information encoded in RNA molecules
into chains of amino acids, which form proteins.
In order to decipher this genetic
code, Marshall Nirenberg and Heinrich Matthaei first needed the machinery that would
be able to build proteins with the amino acids that they would provide in a test
tube. Later, other scientists discovered
that this machinery is made of ribosomes and tRNA that are able to assign an amino
acid to a specific codon of messenger RNA.
Nirenberg and Matthaei managed to
get this machinery by breaking bacteria cells and collecting their cytoplasm. Then, they distributed this into 20
tubes as shown on the diagram. In addition, each tube contained
only one type of amino acid that was radioactively tagged. This way, if this amino acid was
incorporated in proteins during synthesis, they could detect radioactive
proteins. Then, they added poly(U) RNA
sequences to each tube.
After one hour, Matthaei found that
only the tube containing phenylalanine had a high level of concentrated
radioactivity. This meant protein synthesis had
happened in this tube, which was the phenylalanine tube. This was due to the fact that the
poly(U) RNA could only give codons made of three uracil bases. Nowadays, you just need to look at
the codon wheel to find out that a UUU codon corresponds to a phenylalanine amino
acid.
The answer to this question is thus
answer (A), in the phenylalanine tube.
