How many amino acids are common to all living systems?
A) 20
B) 10
C) 25
D) 15
E)30
The correct answer and explanation is :
The correct answer is:
A) 20
Explanation:
All living organisms on Earth use a set of 20 standard amino acids to build proteins. These amino acids are universal to nearly all life forms, from bacteria to humans, making them fundamental building blocks of life.
Why 20 amino acids?
Proteins are polymers made up of amino acids linked by peptide bonds. The sequence and combination of these amino acids determine the structure and function of proteins. The genetic code, which is nearly universal, encodes these 20 amino acids using codons in DNA and RNA.
The set of 20 amino acids includes:
- Nonpolar amino acids: alanine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, proline
- Polar uncharged amino acids: serine, threonine, cysteine, tyrosine, asparagine, glutamine
- Positively charged amino acids: lysine, arginine, histidine
- Negatively charged amino acids: aspartic acid, glutamic acid
- Glycine, the smallest amino acid, is often considered separately because of its unique properties.
Universality and Importance
This universality suggests a common evolutionary origin of life. Even though some organisms may have variations or use a few rare amino acids (like selenocysteine or pyrrolysine), these are considered special cases, not part of the universal set encoded by the genetic code.
The reason life settled on these 20 amino acids relates to their chemical diversity, which allows proteins to fold into complex 3D shapes essential for biological functions such as catalysis (enzymes), structural support, signaling, and transport.
Other amino acids
While many other amino acids exist in nature (some synthesized artificially or found in non-protein forms), only these 20 are directly encoded by the universal genetic code and incorporated into proteins during translation.
Summary:
- The standard genetic code encodes 20 amino acids.
- These amino acids are universal in all known living organisms.
- They provide chemical diversity required for protein structure and function.
- Some rare amino acids exist but are exceptions and not part of the universal set.
Thus, the answer is 20 amino acids common to all living systems.