Gene Expression

Introduction

     The important characteristic of the gene is to store and express the genetic information that will contribute towards the phenotype, and will be passed on to successive generation.

Gene Metabolism Relationship

     The concept that genes have the information to produce enzymes which are proteinaceous in nature was proved by George Beade and Edward Talium in the early 1940s with their work on the fungus Neurospora crassa.

Central Dogma of Molecular Biology

     The RNA intermediate uses this information to direct the synthesis of proteins during translation. This unidirectional flow of information was described by F. H. C. Crick in 1958 as the Central Dogma of Molecular Biology.

Transcription

     DNA contains the information required for the synthesis of cells specific proteins. DNA is located in the nucleoid (prokaryotes) or nucleus (eukaryotes) and protein synthesis occurs in the cytoplasm. DNA does not move to the site of protein synthesis (ribosomes) to directly guide the process. Instead, it transfers its information to mRNA molecules which move to the ribosomes to direct protein synthesis. The process of the formation of RNA from the DNA template is called transcription. It involves rewriting the genetic message coded in DNA into an RNA molecule.

RNA Types

     The three different types of RNA, namely, messenger RNA (mRNA), ribosomal RNA (rRNA) and transfer RNA (tRNA) are transcribed from different regions of the DNA molecule. Three different RNA polymerases: I, II and III catalyses the transcription of rRNA, mRNA and tRNA respectively in eukaryotes. In prokaryotes, a single RNA polymerase composed of different subunits does this work. Transcription of RNA also occurs in the 5-3 direction like the replication of DNA.

Translation-Synthesis of Proteins

     Translation is the mechanism by which the triplet base sequence of an mRNA guides the linking of a specific sequence of amino acids to form a polypeptide (protein) on ribosomes. All the proteins a cell needs are synthesized by the cell within itself.

Mechanism of Protein Synthesis

     The events in protein synthesis are better known in bacteria than in eukaryotes. Although these are thought to be similar in the two groups there are some differences. The following description refers mainly to protein synthesis in bacteria on the 70S ribosome.

Gene Expression in Eukaryotes

     In higher eukaryotes the genome is highly complex. Drosophila for example, has any where between 5,000 and 10,000 genes. Human haploid genome is known to have any where between 30,000 and 1,00,000 genes. As a result gene expression in eukaryotes is highly complex. The eukaryotic gene has sequences of nitrogen bases called introns which do not code for amino acids. The introns occur in between stretches of coding sequences called exons. The mRNA transcribed hence has several unwanted regions. These regions get removed by nucleases and the coding sequences are joined together by a process called splicing. The processed mRNA is used for translation.

Genetic Code

     The complex process by which the information in RNA is decoded into a polypeptide is one of the exciting discoveries the genetic code.

Summary

     The molecular mechanism by which a gene shows to potential in the phenotype of an organism, is called gene expression. Every gene contains the blue print required for the function of a given polypeptide, in the form of a specific sequence of nucleotides.