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As the name suggests,
the template DNA strand serves
as a template for the pairing
of free ribonucleotides from the nucleoplasm
via complementary base pairing.
One thing to take note here is
the differences in the nitrogenous bases
of deoxyribonucleotide and that of ribonucleotides.
The bases found in DNA
are Adenine Guanine Cytosine and Thymine,
while the bases found in RNA
are Adenine Guanine Cytosine and Uracil.
Note that Thymine is only found in DNA,
while Uracil is only found in RNA.
Thus, in complementary base pairing,
A on DNA template strand will be
paired with U,
while T on the DNA template strand
will be paired with A.
RNA polymerase will then catalyse
the joining of adjacent ribonucleotides
through the formation of covalent phosphodiester bonds.
The mRNA is sythesised and elongated in the 5' to 3' direction.
As RNA polymerase moves in the 3' to 5' direction
along the DNA template strand,
it continues to separate the two DNA strands
and catalyse the assembly of ribonucleotides
via complementary base pairing.
Parts of the DNA that has just been transcribed
reanneals to reform a double helix.