Gene expression

#Introduction

Gene expression is the process by which the information contained in the genes of a DNA sequence is transcribed into RNA and, subsequently, translated into functional proteins, providing a detailed view of the molecular behaviour of genes under different conditions and biological contexts.

A DNA molecule is not just a long chain of nucleotides. In reality, it is divided into functional units called genes.

Each gene provides the instructions to form a functional product, that is, a molecule needed to carry out a job in the cell. In many cases, the functional product is a protein.

For example, in Mendel’s experiment, the gene for flower colour contains the instructions to make a protein that helps produce coloured molecules (pigments) in the petals of the flowers.

In this diagram, we see how the flower colour gene that Mendel studied is composed of a strip of DNA found on a chromosome.

The gene’s DNA specifies the production of a protein that helps form pigments. When the protein is present and functional, pigments are produced and the plant’s flowers have a lilac colour.

The functional product of most genes are proteins, or to be more precise, polypeptides.

The term polypeptide is just a word to designate a chain of amino acids. Although many proteins are made up of a single polypeptide, some are made of several polypeptides. Genes that specify polypeptides are known as protein-coding genes.

Not all genes code for proteins. By contrast, some provide instructions to produce functional RNA molecules, such as transfer RNAs and ribosomal RNAs that play roles in translation.

#How can the DNA sequence of a gene determine which protein is formed?

Many genes provide instructions to produce polypeptides. How exactly does DNA direct the construction of a polypeptide?

This process consists of two steps: transcription and translation.

• In transcription, the DNA sequence of a gene is copied to obtain an RNA molecule. This process is called transcription because it involves rewriting, or transcribing, the DNA sequence into a similar RNA “alphabet”. In eukaryotes, the RNA molecule must undergo processing to become a mature messenger RNA (mRNA).

• In translation, the mRNA sequence is decoded to specify the amino acid sequence of a polypeptide. The name translation reflects that the nucleotide sequence of the mRNA must be translated into the completely different language of amino acids.

For example, suppose the two DNA strands have the following sequences:

            5'-ATGATCTCGTAA-3' 
            3'-TACTAGAGCATT-5'

The transcription of one of the DNA strands produces an mRNA with a sequence almost identical to the other DNA strand. Because of the biochemical differences between DNA and RNA, the Ts in the DNA are replaced with Us in the mRNA.

The mRNA sequence is:

            5'-AUGAUCUCGUAA-5'

Once transcription is complete, translation involves reading the mRNA nucleotides in groups of three, each of which specifies an amino acid (or provides a termination signal indicating that translation has finished).

            3'-AUG AUC UCG UAA-5'

AUG → metionina
AUC → isoleucina
UCG → serina
UAA → "stop"