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5. Definere begrebet 'reading frame' for mRNA. Angive at mRNA altid læses fra 5' imod 3'
Stryer, s.135; s.827
Devlin, s.236, s.245

Reading frame – groups of three non-overlapping nucleotides on the mRNA. The reading frame is established after the start codon AUG is located.
 

mRNA is always read in the 5’-3’ direction. This has important consequences.
The transcription also has the direction 5’-3’. If the direction of translation would be opposite that of transcription, only fully synthesized mRNAs could be translated. In contrast, because the directions are the same, mRNA can be translated while it is being synthesized. This actually occurs in prokaryotes, where almost no time is lost between transcription and translation; they are closely coupled in space and time.
On the other hand, in eukaryotes, the newly synthesized mRNA must first exit the nucleus and go to the free ribosomes or to the rER before it can be translated.

As already said, mRNA is always written, transcribed and read in the 5’-3’ direction.
Amino acids are both written and synthesized from the amino-terminal residue to the carboxy-terminus. Comparisons of mRNA sequences with sequences of the proteins they encode reveal a perfect, collinear, non-overlapping and gap-free correspondence. Deduced sequences can however differ from the mRNA sequences due to posttranscriptional modifications.

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