The following conclusions were reached in the experiments to unravel the genetic code by H.G. Khorana:
DNA specifies the sequence of amino acids in proteins and the information
is realyed through a RNA molecule. This enabled direct correlation
between DNA and a protein to be accomplished.
An amino acid is translated from a 3-letter, non-overlapping nucleotide
sequence as opposed to a 2- or 1-
letter code from
messenger RNA.
- the proof of this is present in the nature of the genetic code
- the above statement is valid, however, it does not rule out the fact
that a single mRNA (messenger RNA)
may have multiple initiation points for translation. Therefore there will
be different reading frames within
the same mRNA, and hence specification for more than one polypeptide chain
(overlapping genes).
Four cases of overlapping genes discovered, for example in viruses. (Fig
1).
The genetic code table proposed by Khorana differs in some aspects to the
table which is present today
(Fig
2).
Codon assignments(which code stands for which amino acid) for Escherichia
coli:
a) "the code as shown for E.coli, will probably hold essential for
other organisms as well" (Khorana, 1965)
- we know today that the genetic code is nearly universal
- in other words the genetic code varies in certain aspects for mitochondrial
DNA (mtDNA), for
example UGA in the normal code
is a stop codon and in mtDNA in humans and yeast it codes for
trptophan.
b) "two meanings for a few of the codons, for example
AUG and GUC stand for insertion of
amino acids
and for chain initiation signals" (Khorana, 1965)
- valid statement to present knowledge, but only AUG
is initiator code.
c) the code is highly degenerate, and there are enteries for all of the
64 trinucleotides
- today we know through Crick's work in 1966 that degeneracy is due to
the wobble position
d) there are two chain termination codons, UAA and
UAG but not known whether UGA is termination
code or nonsense
-today it is established that there aree three stop codes UAA,UGA
and UAG.
Two
binding sites on a 70s ribosmal particle
- 30s and 50s ribosmal involved in protein synthesis
- 30s particles bind fmet-tRNA in presence of poly-AUG
- noninitiator tRNA's are bound only after the addition of 50s particle
to 30s particle
- possible to carry out the dipeptide, formylmethionylmethionine, synthesis
by stepwise formation of
appropriate complex containing all necessary components