2.6.5 Skills: Interpreting Sequences

• 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

• 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

• Use a table of the genetic code to deduce which codon(s) corresponds to which amino acid
• The 20 amino acids are all coded for in the Table of mRNA Codons
• Some amino acids are only coded for by one codon eg. methionine (Met), coded by AUG
• Other amino acids have several codons that code for them eg. arginine (Arg), coded by CGU, CGC, CGA, CGG, AGA and AGG

Step 1: Look up His in the table

Codon CAU (no others)

Step 2: Look up Trp in the table

Codon UGG (no others)

Step 3: Look up Gly in the table

Codons GGU, GGC, GGA and GGG

Step 4: Look up Leu in the table

Codons CUU, CUC, CUA, CUG, UUA, UUG

• 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 a known base sequence

Step 1: split the mRNA sequence into triplets

AUG ACU GGG CCU CCC CAA UAU UAG

Step 2: look up the first triplet in the mRNA Codons and Amino Acids Table:

AUG = Met

Step 3: repeat for the remaining triplets

ACU = Thr

GGG = Gly

CCU = Pro

CCC = Pro

CAA = Gln

UAU = Tyr

UAG = Stop

Step 4 : link the amino acids together

Met-Thr-Gly-Pro-Pro-Gln-Tyr