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Let us discuss about the mechanism of gene expression and central dogma of molecular biology.
Mechanism of Gene Expression:
A gene contains the blue print or code for a particular polypeptide in the form of a specific sequence of its base pairs. It transfers its code to mRNA transcribed from it.
The process is called transcription. Other genes transcribe rRNA and tRNAs. The mRNA binds to ribosomes and with the help of suitable tRNAs selects the required amino acids and links them in a proper sequence received from the gene to form a polypeptide. This process occurs on the ribosomes and is called translation.
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The polypeptide chain so formed may act as a structural protein and form some organelle, or form proteinaceous biochemical such as hemoglobin, insulin, etc., or serve as an enzyme and catalyze some chemical reaction, or have a motile role like that of myosin, or act as a membrane receptor. In other words, the polypeptide may contribute to a morphological or a functional trait (phenotype) of the cell and organism. Thus, the molecular structure of a gene is expressed in a phenotype.
This process may be briefly stated as given below:
Central Dogma of Molecular Biology:
The flow of information within a cell generally follows a pathway, with genes coded in DNA are transcribed into three types of RNA (rRNA, mRNA and tRNA), which then combine to read the genetic code and translate the information into proteins (or enzymes).
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These specific proteins, in the form of specific enzymes control specific function, characteristic of that gene. This concept of one way flow of genetic information from DNA to a specific protein is known as Central Dogma of molecular biology-a term coined by Francis Crick (Fig. 7.1).
There are three main steps involved in this one way flow of information from DNA to specific protein:
(i) Replication of DNA by a template mechanism;
(ii) Transcription involving transfer of genetic information from DNA to rRNA, mRNA and tuna;
(iii) Translation: conversion of specific information of mRNA into a specific protein. In this process rRNA and tRNA play important role.
Protein and enzyme coding is specified by a “genetic code”, which contains the language of protein synthesis.
Reverse Transcription (Teminism):
An exception to this one way flow of information was reported in 1970. H. Temin and D. Baltimore independently discovered reverse transcription in some viruses. This discovery was important in understanding cancer and, hence, these two scientists were awarded Nobel Prize along with R. Dulbecco in 1975. During reverse transcription RNA molecule is used as a template to make a single stranded copy of DNA (Figs. 7.1 & 7.2)
