Transcript The Synthesis of Proteins

The Synthesis of Proteins
The sequence of bases in DNA ultimately determines the sequence
of amino acids in proteins.
At a time of protein synthesis some
of hydrogen bonds break and the
DNA molecule partly unravels to
expose a sequence of bases.
The essential molecular structures concerned with protein
synthesis are:
•
DNA - The molecule that contains all the information needed to transfer
hereditary traits. The information is comprised in the sequence of bases in the
polymer chains.
•
Messenger RNA (m-RNA) - A single-stranded RNA molecule that carries the
information from the DNA molecule to structures (ribosomes) outside the cell
nucleus where protein synthesis takes place. m-RNA determines the particular
protein to be synthesised.
•
Transfer RNA (t-RNA) - A much smaller single-stranded RNA molecule
consisting of three bases that selects specific amino acids and “escorts” them
to the growing protein chain so that they join at just the proper position.
•
Ribosome - A structure outside the cell nucleus where protein synthesis
occurs.
•
ATP (adenosine triphosphate) - A molecule that “stores” energy and can
release its energy under specific conditions.
•
Amino acids - Of general formula H2NCRHCOOH.
•
Activated amino acids - Amino acid is complexed with ATP.
How can the base language of DNA be translated to the amino acid
language of proteins?
• There are four kinds of bases (A, C, G, T) in DNA and 20 amino acids.
If each base coded for one particular amino acid then only 4 of the 20 amino
acids could be used in protein synthesis.
If a pair of bases coded for an amino acid then 16 different amino acids could
be used. This is still not enough.
AA
AC
AG
AT
CC
CG
CT
GG
GT
TT
CA
GA
TA
These are the
possible base pairs.
GC
TC
TG
• In fact, a group of 3 bases codes for an amino acid. 64 (43) different
amino acids could be accounted for in this way.
The steps in protein synthesis may be represented as follows:
• m-RNA is synthesised in the cell nucleus by pairing with an untwisted
portion of a DNA chain
DNA molecule partly unravels.
m-RNA molecule is synthesised by base pairing.
• The m-RNA then migrates outside of the nucleus and becomes
complexed with a ribosome.
• An amino acid (a1, a2, a3, etc.) interacts with an ATP molecule and
becomes activated so that it can complex with a molecule of t-RNA.
a1
+
ATP

a1*
• The activated amino acid complexes with a molecule of t-RNA that has
the proper structure to accept this particular amino acid. The t-RNA
amino acid complex then undergoes base pairing with an appropriate
section of the m-RNA.
Such a section comprises three
nucleotides, and the set of three
bases included in this section of
m-RNA is called a codon. The
complementary codon of the t-RNA
is called an anti-codon. One
codon corresponds to a particular
amino acid.
• Another t-RNA amino acid complex undergoes a similar process. The
particular amino acid that is selected is determined by the next codon
in the m-RNA sequence. This amino acid then forms a peptide bond
with its neighbour.
• This process continues and the polypeptide chain grows to form a
protein. As the protein chain breaks loose, the t-RNA molecules are
released and recycled.