CAIE A Level Biology 9700 · Nucleic acids and protein synthesis

Nucleic acids and protein synthesis: describe question

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A small section of a messenger RNA (mRNA) molecule from a fungus has the base sequence:

5'-AUG GGU UAC UGA-3'

The table below shows some mRNA codons and the amino acids they code for (using the universal genetic code):

mRNA codon	Amino acid
AUG	methionine (start)
GGU	glycine
UAC	tyrosine
UGA	stop
UAG	stop

(a) Describe how this mRNA sequence is translated into a polypeptide chain. Include in your description the roles of mRNA, codons, tRNA, anticodons, and ribosomes.

(b) In a mutant of this fungus, a substitution mutation changes the third codon from UAC to UAG. Describe how this mutation would affect the polypeptide produced.

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5 marks, so the mark scheme is looking for about 5 separate creditworthy points. Making the same point twice in different words earns it once.

Describe means: Say what happens or what something is like, in order and with detail. You are not being asked why. If there is data, quote it: name the trend, then give figures with units at the points where it changes.

Nucleic acids and protein synthesis: describe question

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