First DNA transcription takes place in the nucleus of the cell. RNA polymerase
binds to the start of a gene on the double helix and it unwinds the DNA into single
strands. As it goes along it uses one of the single strands as a template and builds a
complementary mRNA strand. RNA polymerase uses RNA instead of DNA.
Therefore, thymine is replaced by uracil. The mRNA breaks off from the template
strand and moves from the nucleus into the cytoplasm through a nuclear pore,
translation begins and DNA returns to its original double-helix formation. Then
mRNA comes into contact with the ribosomes which consist of two parts; a large
subunit and a small subunit. Ribosomes read the mRNA sequence and convert it
into an amino acid sequence. The ribosomes begin translation at the start codon
AUG and in this case, stop at the stop codon UGA. tRNA molecules transfer the
amino acid to the ribosome. tRNA contains an anticodon on one end and an amino
acid on the other. The tRNA with a complementary anticodon temporarily binds
onto the codon, then the ribosome reads the next codon and another tRNA with a
complementary anticodon binds. A peptide bond joins the two amino acids, and as
the ribosome continues down the mRNA strand the first tRNA falls off, leaving its
amino acid. The ribosome continues down the strand until the stop codon is
reached, where the last tRNA falls off, leaving a chain of amino acids, which will
then fold upon itself to give it its function.
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