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The biochemical machinery in the cytoplasm of each cell has the
capacity to make every protein from just 20 amino acids
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The basic process is:
•

DNA provides the instructions in the form of a long sequence of bases
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Polypeptide synthesis:
transcription and splicing

Transcription: the process of making pre-mRNA using
part of the DNA as a template
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an enzyme acts on a specific region of the DNA
causing the two strands to separate and expose
the nucleotide bases in that region
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The nucleotide bases on one of the two DNA
strands, known as the template strand, pair with
their complementary nucleotides from the pool
which is present in the nucleus
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The process can be likened to a bakery:
The basic equipment and ovens (cell organelles) can manufacture
any variety of cake (protein) from relatively few basic ingredients
(amino acids)
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By
choosing different recipes at different times, rather than making
everything all the time, the baker can meet seasonal demands,
adapt to changing customer needs and avoid waste
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In this way, an exposed guanine base on the DNA
binds with the cytosine base of a free nucleotide
and so on
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As the RNA polymerase adds the nucleotides one
at a time to build a strand of pre-mRNA, the DNA
strands rejoin behind it
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5
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Splicing of pre-mRNA:
The DNA of a gene in eukaryotic cells is
made up of sections called exons that code
for proteins and sections called introns that
do not
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The base sequences corresponding to the introns are removed and
the functional exons are joined together
during splicing
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The mRNA molecules are too large to
diffuse out of the nucleus and so once they
have been spliced, they leave via a nuclear
pore
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