The genetic code and mRNAAQA A-Level Biology: Revision notes

The sequence of bases in a gene is a code for the order of amino acids in a polypeptide. A sequence of three DNA bases, called a triplet, codes for a specific amino acid. On mRNA the equivalent three bases are a codon.

There are four bases, so a doublet code would give only 4² = 16 combinations, too few for 20 amino acids. A triplet code gives 4³ = 64 combinations, which is enough. A polypeptide of n amino acids needs at least 3n bases in the coding region (plus a stop codon).

The genetic code is:

• Universal: the same triplet codes for the same amino acid in all organisms. This is why a human gene can be expressed in a bacterium.
• Non-overlapping: each base is part of only one triplet, and the bases are read in order, three at a time.
• Degenerate: most amino acids are coded for by more than one triplet.

Some triplets are stop codons, which code for no amino acid and end translation. A start codon (AUG) begins translation.

The genome is the complete set of genes in a cell. The proteome is the full range of proteins that a cell is able to produce.

The genome is the information; the proteome is the set of products made from it.

Messenger RNA (mRNA) carries a copy of a gene from the DNA to the ribosomes. It is:

• a single-stranded, linear polynucleotide (not folded, with no base pairing);
• made of nucleotides containing ribose, a phosphate group and a base: adenine, uracil, guanine or cytosine;
• arranged in codons, each of three bases.

mRNA is much shorter than the DNA molecule and is made in the nucleus (in eukaryotes) before moving to the cytoplasm.

Transfer RNA (tRNA) brings amino acids to the ribosome. It is a single polynucleotide strand folded into a clover-leaf shape, held by hydrogen bonds between paired bases.

Each tRNA has:

• an anticodon, a sequence of three bases that pairs with a complementary codon on mRNA;
• an amino acid binding site at one end, to which one specific amino acid attaches.

There are many types of tRNA, one for each amino acid (or group of triplets).

To work out an amino acid sequence from DNA or mRNA, use the data given (you do not need to recall codons).

Worked example: DNA template strand TAC CGA AGG.

1. Write the complementary mRNA: A pairs with U, T with A, C with G, G with C, so the mRNA is AUG GCU UCC.
2. Read in groups of three and use the data (for example AUG = methionine, GCU = alanine, UCC = serine).
3. The polypeptide is methionine, alanine, serine.

To find an anticodon, write the base pairs of the codon: the codon UUC has the anticodon AAG.