What is Central Dogma – An Inheritance Mechanism of Translation

You have landed on the right page if you are keen to understand the basic concepts of central dogma in a simplified way. This central dogma process helps in the synthesis of proteins. Proteins are building blocks of our body as they strengthen our body. Not only strength, but they also act as hormones and serve as enzymes that catalyse the hundreds of chemical reactions necessary for life. 

The present article aims to make you understand all the complex processes of central dogma in a simplified way. We will first understand what central dogma means. Once we understand the outline, we will study the steps involved in central dogma- transcription and translation. In the end, we will study some questions based on concepts.

What is central dogma?

Central dogma states that genetic information flows from DNA to RNA to Protein. F.H.C. Crick postulated it in 1958. According to the “central dogma”, DNA is transcribed to RNA; mRNA is translated to proteins. It is a two-step process by which gene information flows into proteins. 

Steps of Central Dogma:

The steps involved in central dogma are:

1. DNA replication
2. Transcription- Genetic information from one strand of the DNA is copied into RNA.
3. Translation-synthesis of the polypeptide chain

DNA replication:

DNA replication refers to forming an exact copy of itself during cell division. Watson and  Crick  (1953)  suggested that  DNA  replication is  semi­-conservative. According to the semi-­conservative method, the two strands of the DNA separate, and the complementary strand is synthesised from the medium. The replication of DNA requires many enzymes and protein factors. The newly synthesised DNA  possesses one strand contributed by parent  DNA  and another newly synthesised. 

Transcription

The transcription unit comprises three parts- 

1. Structural gene: Structural gene is a gene that codes for RNA and proteins. So, the structural gene has a 5’-3’ end (coding strand) and a 3’- 5’ end (template strand). DNA-dependent RNA polymerase catalyses polymerisation in the 5’→3’ direction. In the template strand, 5’-3’ end, the sequence of bases is the same as in RNA (Uracil replaces thymine). Nothing is coded for by the coding strand. 
2. A promoter: It is located towards the 5′ end of the structural gene and provides a binding site for enzyme RNA polymerase. RNA polymerase binds to a specific promoter. It defines the template and coding strands.
3. A Terminator: It is located at the 3′ end of the coding strand and defines the end of the transcription process.

Mechanism of Transcription:

RNA polymerase first binds to the promoter, and after binding, it moves along the DNA and causes the local unwinding of DNA duplex into two chains. Out of two chains, one strand acts as a template (antisense strand), and the other strand is coding (complementary strand or sense strand). When RNA polymerase reaches the terminator signal on the DNA, it leaves DNA, and fully formed mRNA (primary transcript) is released. As the mRNA grows, the transcribed region of the DNA molecule becomes spirally coiled and regains double-helical form. 

Translation:

The translation mechanism involves polymerising amino acids to form a polypeptide. These amino acids are transported from an intracellular pool to the ribosomes, forming a polypeptide chain. These polypeptide chains are assembled into proteins in the cytoplasm. 

Mechanism of translation:

The steps involved in translation are:

(a) Activation of amino acids: In translation, amino acids are joined by peptide bonds. The formation of a peptide bond requires energy in the form of ATP. So, in this step, amino acid reacts with ATP to form amino acid AMP complex and pyrophosphate by an activating enzyme called aminoacyl-tRNA synthetase in the presence of Mg2+. Now, this amino acid AMP enzyme complex is called an activated amino acid. The pyrophosphate gets hydrolysed to two inorganic phosphates (2pi).

(b) Charging of tRNA: During this step, the amino acid AMP-enzyme complex (formed in step an above) joins with the amino acid binding site of tRNA. At that site, the COOH group of the amino acid bonds with the OH group of the terminal base triplet CCA nucleotide base. Aminoacyl tRNA synthetase, a similar enzyme, catalyses the process. A charged tRNA is the term used to describe the ensuing tRNA-amino acid combination. During this process, the enzyme and AMP are eventually liberated. The enzyme that has been released can be activated and used to link one tRNA molecule to another. Finally, reaching the ribosomes, the tRNA amino acid complex.

(c) Activation of the ribosome: Ribosome comprises around 80 distinct proteins and structural RNAs. Ribosome exists as two subunits; a large subunit (the 70S) and a small subunit (30S). First, a small subunit encounters an mRNA that joins the larger subunit. The peptide bonds between amino acids occur between two sites in the large subunit. Some additional sequences in mRNA are not translated and are thus known as untranslated regions (UTR). They are present at both 5′ -end (before start codon) and at 3′ -end (after stop codon). For initiation, the ribosome binds at the start codon (AUG), and the chain elongation begins. In the same way, amino acids are added one by one and form polypeptide sequences. Translation is stopped when a release factor binds to the stop codon at the end, and the entire polypeptide is released from the ribosome.

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