The genetic code, in simple terms, refers to the relationship between the bases and amino acids. The genetic code is consistent across all organisms (whose genetic codes have been studied), except in mitochondria and protozoan nuclear DNA.
DNA contains four nucleotide bases: adenine, guanine, cytosine, and thymine, while RNA has four nucleotide bases: adenine, guanine, cytosine, and uracil. This “alphabet” of four letters is responsible for carrying the code that results in the synthesis of various protein molecules.
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Understanding the Genetic Code and Its Role in Protein Synthesis
The process of protein synthesis is known as Translation because it translates the language of the nucleotide sequence in mRNA into the language of an amino acid sequence.
The translation process requires the genetic code, which expresses the information contained in the nucleic acid sequence to produce a specific amino acid sequence. In other words, the genetic code is embedded in the sequence of nucleotides in mRNA, which is transcribed from the DNA.
A codon is a triplet of bases along the mRNA that codes for a specific amino acid. The flow of information is unidirectional: information moves from nucleotide sequences to the amino acid sequence of proteins, but it never transfers back from proteins into the DNA sequence, a concept referred to as the central dogma of molecular biology by Francis Crick.
Deciphering the Genetic Code
Marshall Nirenberg and Heinrich Matthaei at the National Institutes of Health conducted a series of precise and logical experiments to decode the genetic language.
They were among the first to characterize specific coding sequences. This achievement was made possible by advancements that allowed for protein synthesis in vitro and the synthesis of RNA strands in vitro.
Amino Acids: The Building Blocks of Proteins
Amino acids are the subunits of proteins and share the same basic structure: a central carbon atom (the α-carbon), an amino group (NH2) attached to the α-carbon, a carboxyl group (COOH), and a side group (R).
There are 20 naturally occurring amino acids, which differ in their side group. All amino acids, except glycine, have an asymmetrical α-carbon atom, giving rise to D or L stereoisomer forms. However, only the L form is found in humans. The dynamic structure of hemoglobin (Hb) enables it to transport oxygen (O2) within mammalian blood.
A single amino acid change can cause Hb to form fibers, altering its ability to transport oxygen. The specific sequence of amino acids results in a unique three-dimensional structure for that protein, and these structures are related to their function.
For instance, proteins can bind to other proteins and molecules, sometimes acting as enzymes to facilitate chemical reactions within the bound molecules without altering the protein’s structure itself.
An example of the importance of amino acid sequences is sickle-cell anemia, a genetic disease caused by a single base change in the coding region for the β-globin section of Hb.
This mutation results in a single amino acid change, which alters Hb’s physical properties, causing the sickle-shaped red blood cells to clump together and block blood vessels.
The Genetic Code: Amino Acids and Codons
Almost all amino acids are specified by two, three, or four different codons. For example, three amino acids arginine, serine, and leucine are specified by six codons. Methionine and tryptophan are encoded by single codons.
Many synonymous codons specifying the same amino acid differ in the third base (such as Proline), and this flexibility is known as the Wobble hypothesis. The third base is usually less specific than the first two, allowing mutations to have a lesser impact on protein function.
The code also includes signals for the start and stop of translation. The codon AUG signals the start, while UGA, UAA, and UAG serve as stop codons.
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Six Key Features of the Genetic Code
The genetic code has six distinct characteristics:
1. Triplet Code: Each codon is made up of three nucleotides, imparting specificity to each amino acid. There are three termination codons (UAA, UAG, and UGA) that do not encode for any amino acid.
2. Specificity: The genetic code is unambiguous and specific. A given codon always codes for the same amino acid. For instance, UUU always codes for phenylalanine.
3. Degeneracy: The genetic code is degenerate, meaning that some amino acids are specified by multiple codons. For example, Valine can be coded by GUU, GUC, GUA, or GUG. The third base in a codon is often less specific, which reduces the impact of mutations.
4. Universality: The genetic code is nearly universal, with the exception of mitochondria and protozoan nuclear DNA. This consistency across organisms supports the idea of a shared evolutionary history.
5. Non-overlapping: The genetic code is read in groups of three nucleotides, with no overlap between codons. Each codon is read independently, and after reading one triplet, the reading frame shifts by three nucleotides.
6. Unpunctuated or Commaless: Once translation begins, codons are read continuously with no breaks between them until a stop codon is encountered.
By the end of this article, it is hoped that the concept of the central dogma of molecular genetics and the features of the genetic code have been clearly understood.
The genetic code plays a crucial role in the synthesis of proteins and the function of living organisms, including those in agricultural genetics. Understanding these principles is essential for advancing breeding programs and improving livestock and crop productivity.
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