Essence of the biological code
For polypeptide chain synthesis to occur in a cell, a clear set of instructions is required. The genetic (biological code) serves as such an instruction. Essentially, it is a mechanism for translating the "language" of nucleic acids (nucleotide sequences in DNA or RNA) into the "language" of proteins (amino acid sequences). Without this mechanism, the precise synthesis of structural and functional molecules in living systems would be impossible.
Main properties of the genetic code
Biochemists distinguish seven fundamental properties that describe the rules for reading genetic information:
- Triplet nature. The code number is three. This means the smallest unit of the code is a triplet (or codon), which consists of exactly three nucleotide residues. A single such triplet encodes one amino acid.
- Specificity. The rule of uniqueness: each specific triplet is capable of encoding only one amino acid, without exception.
- Degeneracy (redundancy). Conversely, the reverse rule also applies: the same amino acid can be encoded by several different triplets. Typically, their number ranges from 2 to 6. This provides an important reserve against errors.
- Universality. The biological code is identical (the same) across almost all existing species of organisms. These rules work identically in both bacteria and human cells.
Rules of reading and translation termination
The realization of genetic information follows strict rules regarding direction and termination:
- Presence of terminating codons. There are specific triplets that signal the cessation of protein synthesis. These are the so-called stop codons: UAA, UAG, UGA. Their main feature is that they do not encode any amino acids at all, but simply halt the process.
- Unidirectionality. Information reading (codon sequence) in mature messenger RNA (mRNA) occurs strictly in one direction: from the 5' end to the 3' end.
- Collinearity. This is the property of linear correspondence. It means that the sequence of codons in a mature mRNA molecule corresponds absolutely and precisely to the amino acid sequence in the final synthesized protein.
Briefly: answers to topical questions
Which amino acids are encoded by only a single codon (exceptions to the degeneracy property)?
Only methionine and tryptophan are encoded by a single unique codon, which is an exception to the degeneracy (redundancy) property of the genetic code.
Degeneracy means that more than one triplet can correspond to a single amino acid. Most amino acids are encoded by multiple codons:
- Leucine — encoded by 6 codons.
- Serine — encoded by 6 codons.
- Glycine — encoded by 4 codons.
- Alanine — encoded by 4 codons.
If an amino acid is encoded by multiple codons, in most cases they differ only in the third letter.
What exceptions exist to the universality property of the genetic code?
Exceptions to the universality of the genetic code include mitochondrial DNA and a few other instances.
The property of universality means that the genetic code is nearly identical across all species of organisms. Minor differences are noted in mitochondrial DNA.
What is the essence of the non-overlapping property of the genetic code?
The essence of the non-overlapping property is that codons in mRNA are read sequentially one after another.
A single nucleotide cannot simultaneously belong to two adjacent codons (triplets). Reading proceeds triplet by triplet; each nucleotide is part of only one triplet.
What is a start (initiation) codon and which amino acid does it encode?
A start (initiation) codon is the AUG triplet, which encodes the amino acid methionine and is located at the beginning of a gene, initiating the translation process.
This codon enters the peptidyl site (P-site) of the ribosome, where initiator tRNA delivers methionine. If AUG triplets are found within the gene (in its middle) or elsewhere, they simply code for methionine and do not act as start codons. In prokaryotes, the starting amino acid is N-formylmethionine.
How many possible triplets (codons) exist in total, and how many of them are sense codons?
There are 64 possible combinations of three nucleotides (codons) in total.
Of these, 61 codons are sense (coding) codons, meaning they are capable of encoding the 20 proteinogenic amino acids. The remaining 3 codons (UAA, UAG, UGA in mRNA or ATT, ACT, ATC in DNA) are non-coding nonsense triplets. These are terminating codons (stop codons) that do not code for amino acids, but instead act as punctuation marks signaling the termination of protein synthesis.
The properties of the genetic code can be remembered by their first letters: T (Triplet nature), S (Specificity), D (Degeneracy), U (Universality), U (Unidirectionality), C (Collinearity).
Class and exam questions
What is the difference between code specificity and degeneracy?
Specificity means that one triplet encodes strictly one amino acid. Degeneracy, on the other hand, means that a single amino acid can be encoded by several different triplets at once (from 2 to 6).
What are stop codons and what types exist?
These are terminating triplets indicating the end of protein synthesis. They do not encode amino acids. They include UAA, UAG, and UGA.
In which direction is the genetic code read?
Information reading (codons) in mature mRNA occurs in only one direction — from the 5' end to the 3' end.
What does the term "collinearity" mean?
Collinearity is the rule whereby the sequence of triplets in a gene (and mature mRNA) fully corresponds to the amino acid sequence in the protein.
What else to study on this topic
- Mechanism of transcription
- Polypeptide chain synthesis
- DNA replication mechanism
- Synonymous codons
- Anticodons and their functions
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