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Sophia Kossida (sophia.kossida@merton.oxford.ac.uk)
Thu, 11 Jan 1996 09:54:11 +0000
Dear Gail,
I'm sending you the "protein_synthesis" file translated into HTML.
Please, serve it up from where you think it's more appropriate.
Thank you in advance.
Take care
Sophia
Protein Synthesis
Protein synthesis, a one-dimensional procedure - the information in a
linear sequence of nucleotides is used to specify a linear chain of amino
acids - depends on the collaboration of several classes of RNA molecules
and requires a series of preparatory steps:
- A messenger RNA (mRNA) which encodes the protein to be
synthesised is generated by the DNA in nucleus in a process called DNA
transcription. Transcription generates not only the mRNAs that carry
the information for protein synthesis but transfer, ribosomal and other RNA
molecules that have structural and catalytic functions. All these RNA
molecules are synthesised by RNA polymerase enzymes which bind very
tightly when they collide with a specific DNA sequence: the
promoter. The promoter sequence is the one which defines which DNA
strand is to be transcribed by defining the direction of RNA polymerase
movement. All the above is based upon the fact that the DNA strand serving
as template must be traversed from its 3' end to its 5' end.
Discussion question: Plausible explanations for the complicated molecules
of both bacterial and eucaryotic RNA polymerases in contrast with the
single-chain and with one fifth of the mass of the later; RNA polymerases
of some bacterial viruses depicting the same efficiency though.
- Each of the 20 amino acids from which the protein is to be built must
be attached to its specific transfer RNA (tRNA) molecule. Actually,
only the tRNA molecule, and not its attached amino acid, determines where
the amino acid is added during protein synthesis. Specific enzymes called
aminoacyl-tRNA synthetases couple each amino acid to its appropriate
tRNA molecule. Then the tRNA molecule binds to the appropriate nucleotide
sequence on the mRNA. Amino acids are added to the carboxyl-terminal end of
a growing polypeptide chain. Most of the amino acids are represented by
more than one codon and the genetic code is said to be
degenerate.
Discussion question: Existence of tRNAs. Is it necessary? In which sense?
Could the tRNAs be replaced by other mechanisms?
- The events in protein synthesis are catalysed on the ribosomes
which are large complexes of RNA and protein molecules. The ribosome
moves stepwise along the mRNA chain. The elongation phase of protein
synthesis on a ribosome requires a three-step cycle which is repeated over
and over during the synthesis of a protein chain. An aminoacyl-tRNA
molecule binds to the A-site on the ribosome in step 1; a new peptide
bond is formed in step 2; and the ribosome moves a distance of three
nucleotides along the mRNA chain in step 3, ejecting an old tRNA molecule
and "resetting" the ribosome so that the next aminoacyl-tRNA
molecule can bind. A protein chain is released from the ribosome when one
of three of different stop codons is reached. In eucaryotes only one
species of polypeptide chain is usually synthesised from each mRNA
molecules. The binding of many ribosomes to an individual mRNA molecule
generates polyribosomes. The overall rate of protein synthesis in
eucaryotes is controlled by initiation factors.
Discussion question: Why should rRNA molecules exist at all, and how
did they come to play an important role in the structure and function of
the ribosome?