Central Dogma and The Genetic Code

You may have seen a sci-fi movie giving codes/names to agents so that they can communicate internal matters without being detected during secret mission. Similarly, the genetic code (discovered by Francis Crick and his team) is the set of codons that correspond to specific amino acids. Codons are three letters denoting the nucleotides.

We know that the flow of any information occurs systemically. Like any systemic manner, central dogma is a concept that explains the genetic flow of information. 

Central Dogma

The central dogma is a fundamental concept that helps to understand the flow of genetic information within a biological system, typically from DNA to RNA to protein. It was first proposed by Francis Crick in 1957 and has since become a cornerstone of our understanding of how genetic information is stored, replicated, and expressed in living organisms.

The central dogma consists of three main processes:

1. DNA Replication: This is the process by which DNA molecules make exact copies of themselves. It occurs during cell division and ensures that each daughter cell receives identical genetic information.
2. Transcription: During transcription, a specific segment of DNA works as a template to produce a complementary RNA molecule. This RNA molecule, messenger RNA (mRNA), carries the genetic code from DNA to the ribosomes in the cytoplasm.
3. Translation: Translation is the process by which the genetic code carried by the mRNA is used to build a protein. This step occurs in the ribosomes, where transfer RNA (tRNA) molecules introduce amino acids to the ribosome in the correct sequence specified by the mRNA. Based on the mRNA’s instructions, the ribosome then assembles these amino acids into a protein.

In summary, the central dogma describes the unidirectional flow of genetic information. It starts with DNA replication, followed by transcription to produce mRNA, and finally translation to synthesize proteins. It’s important to note that while this model is generally applicable, there are exceptions and additional complexities in gene regulation. Like post-transcriptional modifications and the role of non-coding RNAs.

Genetic Code

It is a set of rules that specifies how the information in DNA and RNA translates into the sequence of amino acids to form proteins. It’s the language the cell uses to read the instructions in the genetic material and produce the proteins that carry out various bodily functions. The genetic code is universal and standard to almost all living organisms, from bacteria to humans.

Understanding the genetic code is essential for deciphering the instructions encoded in DNA and RNA and predicting the amino acid sequence of proteins based on the genetic information. This knowledge has been instrumental in advancing our understanding of genetics and molecular biology and has practical applications in fields like genetic engineering and biotechnology.

So, do you remember the nucleotides? They are A-adenosine, T-thymine, C-cytosine, and G-guanine. The thymine (T) changes into Uracil (U) in RNA. So, the codons are formed by the four nucleotides in RNA (A, U, C, and G). 

There are 20 standard amino acids and four types of nucleotides arranged in three (4^3), giving rise to 64 codons. Since three codons (UAA, UAG, and UGA) are the stop codons, the remaining 61 are sense codons and code the 20 amino acids. Likewise, the codon AUG codes the amino acid-methionine (met), the start codon.

Key features of the Genetic Code

1. Codons: The genetic code is read in groups of three nucleotides (triplets) on the mRNA molecule. Each triplet of nucleotides is called a codon. There are 64 possible codons (4 nucleotide options for each of the three positions in a codon), and each codon relates to a specific amino acid to start or stop protein synthesis.
2. Start Codon: The codon AUG serves as the start codon, indicating the beginning of protein synthesis. It also codes for methionine amino acids.
3. Stop Codons: There are three stop codons (UAA, UAG, and UGA), which signal the termination of protein synthesis. The ribosome releases the newly synthesized protein when it interacts with a stop codon.
4. Amino Acid Assignments: Each of the 64 codons is associated with a specific amino acid or a signal for translation initiation or termination. For example, the codon UUU codes for the amino acid phenylalanine, while UGA is a stop codon.
5. Redundancy: The genetic code is degenerate or redundant, meaning that more than one codon specifies most amino acids. This redundancy provides some robustness to the system, as errors or mutations in the DNA sequence may not always result in a change in the amino acid sequence of the encoded protein.
6. Universality: The genetic code is nearly universal across all known life forms, with only minor variations. This universal code suggests a common evolutionary origin for all living organisms.
7. Non-Coding Codons: Some codons do not code for amino acids but instead serve as regulatory elements within the mRNA, controlling aspects of translation. 

Relation between Central Dogma and Genetic Code

Central dogma and genetic code are closely related concepts of molecular biology. This close relation is due to both concepts describing the flow of genetic information within living organisms.

The following points describe the relation between genetic code and central dogma in brief:

Genetic Code and Transcription:

• Transcription is a part of the processes in central dogma. It involves the conversion of genetic information from DNA to RNA. Here, the genetic code plays a crucial role. 
• A specific segment of DNA is a template for synthesizing a complementary RNA molecule called messenger RNA (mRNA).
• The genetic code decides how the information in the DNA transcribes into the sequence of nucleotides in the mRNA, following the rules of the genetic code. 

Genetic Code and Translation

• Another critical process of central dogma, translation, synthesizes protein using the information encoded in mRNA. 
• The genetic code is vital in translation as it specifies the relationship between codons and the amino acids they represent. 
• With their specific anticodons, transfer RNA (tRNA) molecules are recognized and paired with mRNA codons according to the genetic code. Each tRNA carries the corresponding amino acid, allowing the ribosome to assemble the amino acids correctly to produce a protein.  

In summary, the genetic code governs how genetic information is transcribed from DNA to mRNA and then translated into proteins, the functional molecules in the cell. It is an integral part of the central dogma, which outlines the flow of genetic information from DNA replication to transcription and translation. Together, these concepts provide a comprehensive framework for understanding how genetic information is stored, processed, and expressed in living organisms.

References

1. Mercadante AA, Dimri M, Mohiuddin SS. Biochemistry, Replication and Transcription. [Updated 2023 Aug 14]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK540152/
2. Crick F. (1970). Central dogma of molecular biology. Nature, 227(5258), 561–563. https://doi.org/10.1038/227561a0 
3. Ille, A. M., Lamont, H., & Mathews, M. B. (2022). The Central Dogma revisited: Insights from protein synthesis, CRISPR, and beyond. Wiley interdisciplinary reviews. RNA, 13(5), e1718. https://doi.org/10.1002/wrna.1718