Gene Expression

1.0Introduction

For a cell to function properly, necessary proteins must be synthesized at the proper time. All cells control or regulate the synthesis of proteins from information encoded in their DNA. The process of “turning on” a gene to produce mRNA and protein is called gene expression. 

2.0Gene

A gene is a distinct sequence of DNA (or in some cases RNA) that serves as a unit of heredity. Genes are the basic physical and functional units of heredity passed from parents to offspring. They contain the instructions necessary for building, maintaining, and regulating an organism's cells and systems. 

Genes determine specific traits or characteristics in living organisms by providing the instructions for synthesizing proteins or functional RNA molecules. These instructions are encoded in the sequence of nucleotides within the gene and are responsible for the development and functioning of an organism. Genes are inherited and passed down through generations, contributing to genetic variation and the diversity of species.

Prokaryotic genes typically consist of continuous sequences of nucleotide pairs that directly encode the sequences of amino acids in resulting polypeptides. However, a groundbreaking discovery from molecular analyses revealed that unlike prokaryotes, eukaryotic genes contain both coding sequences and noncoding segments known as introns, interspersed within them. These introns, or "intervening sequences," were found within both untranslated and translated regions of specific genes. The segments that persist in mature mRNA, comprising the coding and noncoding sequences, were termed exons, or "expressed sequences." This finding revolutionized our understanding of gene structure in eukaryotic organisms.

3.0Gene expression

Gene expression involves the processes by which the information encoded in a gene is used to produce functional gene products, such as proteins or functional RNA molecules. The regulation of gene expression occurs at various levels. Both prokaryotes and eukaryotes cells control expression of genes. 

1. Gene expression in prokaryotes : 

In prokaryotic cells, the regulation of gene expression primarily occurs at the level of transcription. The processes of transcription (DNA to RNA) and translation (RNA to protein) often happen simultaneously. This allows for rapid protein synthesis. When a particular protein is no longer needed, transcription halts, reducing the production of that specific protein. Regulating transcription serves as the primary method to control which types of proteins are produced and in what quantities within a prokaryotic cell. 

2. Gene expression in eukaryotes : 

In eukaryotic cells, genetic material resides within a nucleus, separated from the rest of the cell by a nuclear membrane. Here, the DNA is transcribed into RNA. Once the RNA is produced, it exits the nucleus and moves into the cytoplasm, where it serves as a blueprint for protein synthesis by ribosomes. The steps of transcription and translation are physically segregated by the nuclear membrane: transcription occurs exclusively within the nucleus, and translation takes place outside the nucleus within the cytoplasm. The regulation of gene expression in eukaryotic cells is a multi-layered process that can occur at various stages:

1. Transcriptional level (formation of primary transcript) 

2. Post - transcriptional level (regulation of splicing) 

3. Transport of mRNA (from nucleus to the cytoplasm) 

4. Translational level