What are you going to learn?
- What is the genetic code and how does it work?
- What are codons?
- What are the most important characteristics of the genetic code?
- What does it mean that the genetic code is nonoverlapping and degenerate?
- terms: stop codons, sense codons
Genetic code is the language, set of rules or instructions that the organism uses to translate information in a gene into a protein. It is the means by which it is possible to synthesize a protein based on the information stored in a gene. The code uses codons (sequences of three nucleotides), which specify the amino acid sequences of proteins.
To understand how the genetic code works, let's review the process of synthesizing a protein.
To synthesize a protein, the nucleotide sequence of the gene that codes for a protein is transcribed into mRNA (transcription). Then, the information is translated on the ribosomes into the actual amino acid sequence of a protein, with each group of three nucleotides, a codon, specifying one amino acid (translation). This set of rules by which we transcribe and translate the information in a gene is the genetic code.
Each nucleotide can have one of four possible bases - A, G, C, U (T in DNA). For that reason, there are 4 x 4 x 4 = 64 possible codons. 3 of these are stop codons (UAA, UAG, UGA) that give a signal to the ribosome to stop translation. The other 61 codons are called sense codons because they specify amino acids.
What is more, the genetic code has a few very important characteristics described below.
1) The genetic code is composed of codons.
A codon, a set of three nucleotides in mRNA, specifies one amino acid that is going to be present in the protein. For example, GCU codon codes for alanine.
2) The genetic code is nonoverlapping.
Each nucleotide is part of a single codon - in other words, one nucleotides belongs to just one codon. For example, a sequence of CUG GAC is going to code for 2 amino acids: leucine (CUG), aspartic acid (GAC). However, if it were overlapping, it could be read as follows: leucine (CUG), tryptophan (UGG), glycine (GGA), aspartic acid (GAC).
3) The genetic code is degenerate.
Amino acids can be specified by more than just one codon. For example, alanine can be specified by the following codons: 1) GCU, 2) GCC, 3) GCA, 4) GCG. This makes sense considering that we have 64 possible codons and only 20 different amino acids found in proteins. The codons that specify the same amino acid are called synonymous codons.
4) The genetic code is (almost) universal.
Each codon specifies the same amino acid in (almost) all organisms. Some exceptions, however, apply, for example in mitochondria.
1) composed of codons = one codon → one amino acid
2) nonoverlapping = one nucleotide belongs to one codon
3) degenerate = one amino acid is specified by more than one codon
4) universal = the same for (almost) all organisms
References:
Pierce, B. A. (2019). Genetics: A Conceptual Approach (Seventh ed.). W. H. Freeman.
Snustad, D. P., & Simmons, M. J. (2012). Principles of Genetics. Wiley.