18. Transcription
• Transcription occurs in the cell nucleus where DNA is housed. In
transcription DNA is unzipped and the enzyme RNA polymerase runs along
the template strand of the DNA. The template strand of DNA can be
identified by finding the nucleotide sequence T A C at the 3’ end. This
identifies that strand as the template and the other strand, the
information strand, will not be used in this transcription. As the RNA
polymerase runs along the DNA template strand it will add the
complementary RNA nucleotides to the DNA nucleotides. This means that
G will be paired with C and A will be paired with U and T paired with
A. When the single helix mRNA strand is complete it will separate from
the DNA and the DNA will re-zip into the double helix. In eukaryotes, once
the mRNA is transcribed it will then be processed. A cap and tail will be
added to the ends of the mRNA strand. Then the strand will be spliced.
The introns will be removed and the remaining exons will be spliced
together. The completed mRNA strand has groups of three nucleotides
known as codons. These groups of three will code for a particular amino
acid in translation. AUG will code for the start amino acid.

19. Translation
• Translation occurs when the mRNA strand moves out of the nucleus
and into the cytoplasm. At this point mRNA, rRNA and tRNA all
come together. On the large ribosomal unit are two sites- the A site
and the P site. These will be the sites of polypeptide synthesis and
elongation. The tRNA molecules have an amino acid (the monomer
of proteins) attachment site and it also carries an anticodon. The
anticodon is the complementary nucleotide sequence to a given
codon. The tRNA will pick up the appropriate amino acid in the
cytoplasm that is coded for by the mRNA codon that its anticodon
matches. In translation the mRNA will run through the rRNA from
the 5’ end (with A U G) to the terminating codon at the 3’ end. The
first codon, A U G, will start in the A site. There, the tRNA with the
appropriate anticodon, U A C, will meet up with the start codon
bringing with it the appropriate amino acid, methionine.