Skill:
• Deducing the DNA base sequence for the mRNA strand
mRNA → DNA
mRNA is a complementary copy of a DNA segment (gene) and consequently can be used to deduce the gene sequence
For converting a sequence from mRNA to the original DNA code, apply the rules of complementary base pairing:
- Cytosine (C) is replaced with Guanine (G) – and vice versa
- Uracil (U) is replaced by Adenine (A)
- Adenine (A) is replaced by Thymine (T)
Example: (mRNA) AUG CCA GUG ACU UCA GGG ACG AAU GAC UUA
Answer: (DNA) TAC GGT CAC TGA AGT CCC TGC TTA CTG AAT
Skill:
• Use a table of mRNA codons and their corresponding amino acids to deduce the sequence of amino acids
coded by a short mRNA strand of known base sequence
mRNA → Polypeptide
In order to translate an mRNA sequence into a polypeptide chain, it is important to establish the correct reading frame
The mRNA transcript is organised into triplets of bases called codons, and as such three different reading frames exists
- An open reading frame starts with AUG and will continue in triplets to a termination codon
- A blocked reading frame may be frequently interrupted by termination codons
Once the start codon (AUG) has been located and reading frame established, the corresponding amino acid sequence can be deduced using the genetic code
Example: (mRNA) GUAUGCACGUGACUUUCCUCAUGAGCUGAU
Answer: (codons) GU AUG CAC GUG ACU UUC CUC AUG AGC UGA U
Answer: (amino acid) Met His Val Thr Phe Leu Met Ser STOP
The Genetic Code (Grid)
