DNA directs the cell’s activities by telling it what proteins to make and when. These proteins form structural elements in the cell and regulate the production of other cell products. By controlling protein synthesis, DNA is hugely important in directing life.
Protein synthesis is a two-step process. DNA resides in the nucleus, but proteins are made in the cytoplasm. The cell copies the held in DNA onto RNA molecules in a process called transcription. Proteins are synthesized at the ribosomes from the codes in RNA in a process called translation.
Before getting into the way that the on DNA can be transcribed and then translated into protein, we have to spend some time studying the major players in the process: DNA and RNA.
DNA and the Genetic Code
The sequence of nucleotides in DNA makes up a code that controls the functions of the cell by telling it what proteins to produce. Cells need to be able to produce
20 different amino acids in order to produce all the proteins necessary to function. DNA, however, has only four nitrogen bases. How can these four bases code for the
20 amino acids? If adenine, thymine, guanine, and cytosine each coded for one particular amino acid, DNA would only be able to code for four amino acids. If two bases were used to specify an amino acid, there would only be room to code for
16 (
) different amino acids.
In order to be able to code for 20 amino acids, it is necessary to use three bases (which offer a total of 64 coding combinations) to code for each amino acid. These triplets of nucleotides that make up a single coding group are called codons or genes. Two examples of codons are CAG, which codes for the amino acid glutamine, and CGA, which codes for arginine.
Codons are always read in a non-overlapping sequence. This means that any one nucleotide can only be a part of one codon. Given the code AUGCA, AUG could be a codon for the amino acid methionine, with CA starting a new codon. Alternatively, GCA could be a codon specifying alanine, while the initial AU was the last two letters of a previous codon. But AUG and GCA cannot both be codons at the same time.
Degeneracy of the Genetic Code
There are 64 codons but only 20 amino acids. What happens to the other 44 coding possibilities? It happens that some of the different codons call for the same amino acid. The genetic code is said to be degenerate because of its redundancy.
Experiments have shown that there are also three stop codons, which signal when a protein is fully formed, and one start codon, which signals the beginning of an amino acid sequence.
Mutations of the Genetic Code
Since the sequence of nucleotides in DNA determines the order of amino acids in proteins, a change or error in the DNA sequence can affect a protein’s function. These errors or changes in the DNA sequence are called mutations.
There are two basic types of mutations: substitution mutations and frameshift mutations.
Substitution Mutation
