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MOLECULAR GENETICS
THE CHARACTERISTICS OF THE GENETIC CODE
The gene
is a part of a chromosome which functions as a unit to
produce a polypeptide.
Two steps
are required
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Transcription.
The
synthesis of mRNA use the gene on the DNA molecule as
a template. This happens in the nucleus of
eukaryotes.
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Translation.
The
synthesis of a polypeptide chain using the genetic
code on the mRNA molecule as its guide.
Transcription: The synthesis of a
strand of mRNA
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Uses an
enzyme RNA polymerase.
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Proceeds
in the same direction as replication (5
to 3)
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Forms a
complementary
strand of mRNA
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It
begins at a promotor
site which signals the beginning of gene
is not much further down the molecule (about 20 to 30
nucleotides).
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After
the end of the gene is reached there is a
terminator sequence
that tells RNA polymerase to stop transcribing.
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Editing the mRNA
In
prokaryotes the transcribed mRNA goes straight to
the ribosomes in the cytoplasm.
In eukaryotes the freshly transcribed mRNA in the
nucleus is about 5000 nucleotides long
When the same mRNA is used for translation at the
ribosome it is only 1000 nucleotides long.
The
mRNA has been edited. The parts which are kept
for gene expression are called
EXONS.
The parts which are edited out (by snRNP
molecules) are called
INTRONS. |
Translation
Location:
The ribosomes in the cytoplasm that provide the
environment for translation.
The
genetic code is brought by the mRNA molecule.
What is the genetic code?
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The
genetic code consists of the sequence of bases found
along the mRNA molecule.
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There
are only four
letters to this code (A, G, C and U).
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The code
needs to be complex enough to represent
20 different
amino acids
used to build proteins.
How many
combinations of these four bases are needed to code for
20 different amino acids?
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If one
base represented one amino acid this would only be
able to produce 4 different combinations. (A, C, G
and U)
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If pairs
of bases represented each amino acid this would only
be able to produce 4 x 4 = 16 combinations.
(AA, AC, AG, AU, CA,
CC, CG, CU etc).
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If
triplets of bases represented each amino acid, this
would be able to produce 4 x 4 x 4 = 64 combinations.
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This is
enough combinations to code for the 20 amino acids
but is the code actually made of triplets?
Nature is logical!
Over 10
years biochemists synthesised bits of mRNA with
different combinations. Then they used them to
synthesise polypeptides. The results proved the logical
answer was correct. The genetic code is made of
triplets of bases called
codons.
tRNA
molecules do the translating. At one end the have a
specific amino
acid attached. At the other end they have an
anticodon which
is complementary to the mRNA codons.
The tRNA
molecules with the correct anticodons are lined up with
their bases
complementary to the mRNA codons.
Two tRNA
moleules at a time can fit on the ribosome. A
peptide bond
forms between their amino acids, the first tRNA leaves
the ribosome and mRNA move along to accept a new tRNA.
The
process of translation proceeds in the same direction
as replication and transcription (5
to 3).
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© Paul Billiet 2008 |
The Open Door Web Site