Determination of mRNA Base Sequence
- The rules of base pairing and the table of mRNA codons are needed to be able to convert between DNA sequences, mRNA base sequences and amino acid sequences
- A triplet is a sequence of three DNA bases that codes for a specific amino acid
- A codon is a sequence of three mRNA bases that codes for a specific amino acid
- A codon is transcribed from the triplet and is complementary to it
- When comparing the genetic code to amino acid sequences, mRNA codons are often used
- The four bases found in RNA molecules (adenine, uracil, cytosine and guanine) have the ability to form 64 different codons
- Multiple mRNA codons can encode the same amino acid
- This means that a change in the genetic code doesn’t necessarily result in a change in the amino acid sequence
- For example, UGU and UGC both code for the amino acid, cysteine
- This means that a change in the genetic code doesn’t necessarily result in a change in the amino acid sequence
- Some send important signals to the transcription machinery
- The START codon initiates the process of transcription and ensures it starts in the right location (this is always the amino acid methionine in eukaryotic cells, coded for by the codon AUG)
- STOP codons cause transcription to terminate and do not code for an amino acid e.g. UAA
- The genetic code is non-overlapping
- Each base is only read once in the codon it is part of
The rules of base pairing
- In DNA,
- Adenine (A) always pairs with Thymine (T)
- Cytosine (C) always pairs with Guanine (G)
- In RNA, Thymine (T) is replaced by Uracil (U)
- This means that the base Adenine (A) in DNA is transcribed to Uracil (U) in the mRNA strand
The mRNA Codons and Amino Acids table
- The first three bases of an mRNA strand form the first codon
- The first base of the codon is read from the first column of the table
- The second base of the codon is read from the top row of the table
- The third base of the codon is read from the final column of the table
mRNA Codons and Amino Acids Table