Properties of Genetic Code — Definition
Definition
Imagine a secret language that all living things use to build themselves. That's essentially what the genetic code is! It's a set of instructions, written in the language of DNA and RNA, that tells your cells exactly how to make proteins. Proteins are the workhorses of your body – they do almost everything, from building tissues and muscles to carrying oxygen and fighting infections. So, getting the instructions right is super important.
This 'secret language' isn't like English with letters and words. Instead, it uses a sequence of four different 'letters' called nucleotides: Adenine (A), Guanine (G), Cytosine (C), and Uracil (U) in RNA (or Thymine (T) in DNA).
The key rule of this language is that every 'word' is exactly three letters long. These three-letter words are called 'codons'. For example, UUU is a codon, and it tells the cell to add a specific amino acid called Phenylalanine.
Similarly, GGG tells the cell to add Glycine.
There are 64 possible combinations of these three-letter codons (since there are 4 bases, and each codon has 3 positions, ). However, there are only 20 common amino acids that make up proteins.
This means that most amino acids are specified by more than one codon. This property is called 'degeneracy' or 'redundancy' of the genetic code. It's like having several different ways to say the same thing, which can be a good thing if there's a small mistake in the DNA sequence – it might still lead to the correct amino acid being added.
Not all codons specify amino acids. Some special codons act like punctuation marks. One codon, AUG, is usually the 'start' signal, telling the cell where to begin making a protein, and it also codes for the amino acid Methionine. Then there are three 'stop' codons (UAA, UAG, UGA) that signal the end of protein synthesis. When the cell encounters a stop codon, it knows the protein is complete.
Crucially, this code is read in a very precise way: it's non-overlapping (each nucleotide is part of only one codon) and comma-less (there are no gaps or skipped nucleotides between codons). And perhaps most astonishingly, this genetic code is almost universal, meaning that a codon like UUU specifies Phenylalanine in humans, bacteria, plants, and almost all other living organisms. This universality is a powerful piece of evidence for the common ancestry of all life on Earth.