Question

RECALL Why is selenocysteine called the 21 st amino acid when there are many more amino acids found than the 20 basic ones coded for by the genetic code?

Short answer

Selenocysteine is called the 21st amino acid due to its unique codon (UGA) and specific incorporation mechanism, distinguishing it from other non-standard amino acids.

Step-by-step solution

Understand Basic Amino Acids

The genetic code typically includes 20 common amino acids that are directly encoded by the universal genetic code.

Define Selenocysteine

Selenocysteine is an amino acid that contains selenium, which replaces sulfur in the typical cysteine molecule. It is often referred to as the 21st amino acid.

Incorporation into Proteins

Selenocysteine is incorporated into proteins by unique mechanisms involving a specific tRNA and a special sequence in the mRNA known as the SECIS element.

Comparison with Other Amino Acids

Unlike other non-standard amino acids, selenocysteine has a dedicated codon (UGA) and a specific incorporation mechanism, placing it distinctively as the 21st amino acid.

Key concepts

Amino Acids

Amino acids are the building blocks of proteins. They are organic compounds that play crucial roles in various biological processes. Each amino acid consists of an amino group \((NH_2)\), a carboxyl group \( (COOH) \), and a unique side chain. All together, there are 20 common amino acids encoded by the genetic code. However, more than these 20 exist in nature.
In proteins, amino acids are linked together by peptide bonds, forming long chains that fold into specific shapes. These shapes determine the function of the protein.

Genetic Code

The genetic code is a set of rules used by living cells to translate information encoded within genetic material (DNA or mRNA) into proteins. The code specifies which amino acids will be used to build a protein. There are 64 possible codons (triplets of nucleotides), but only 20 amino acids are directly coded.
Each codon corresponds to a specific amino acid or a stop signal, which tells the ribosome to stop protein synthesis. The genetic code is nearly universal, meaning it is used by almost all organisms to build proteins.

Protein Synthesis

Protein synthesis is the process through which cells build proteins. It involves two main stages: transcription and translation.
1. Transcription: DNA is transcribed into messenger RNA (mRNA) in the nucleus. The mRNA carries the genetic information from the DNA to the ribosome.
2. Translation: The ribosome reads the mRNA sequence and translates it into an amino acid sequence, forming a protein. Transfer RNA (tRNA) molecules bring the appropriate amino acids to the ribosome, based on the codons in the mRNA sequence.
The polypeptide chain then folds into its functional form to perform its specific role in the cell.
Selenocysteine is incorporated into proteins during this process using a special mechanism involving a unique tRNA and the SECIS element.

tRNA

Transfer RNA (tRNA) is crucial in decoding the mRNA sequence into a protein. Each tRNA molecule carries a specific amino acid to the ribosome based on its anticodon, which is complementary to the mRNA codon.
The ribosome matches the tRNA anticodon with the mRNA codon, allowing the correct amino acid to be added to the growing polypeptide chain. For selenocysteine, a special tRNA (tRNA^Sec) is used to incorporate it correctly into the protein.
This mechanism sets selenocysteine apart from other non-standard amino acids.

SECIS Element

The SECIS element (Selenocysteine Insertion Sequence) is a specific nucleotide sequence in the mRNA that enables the incorporation of selenocysteine into proteins.
Located in the 3' untranslated region (UTR) of the mRNA, the SECIS element forms a unique secondary structure recognized by specific protein factors and the tRNA^Sec.
It ensures that the UGA codon, normally a stop signal, is re-coded to insert selenocysteine instead. This rare mechanism highlights the sophistication of protein synthesis and the special role of selenocysteine as the 21st amino acid.