EI sir ;Molecular biology

                                    What is gene expression?

Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as rRNA genes or tRNA genes, the product is a functional RNA

Ø      Biological processes, such as transcription, and in case of proteins, also translation, that yield a gene product.

Ø      A gene is expressed when its biological product is present and active.

Ø      Gene expression is regulated at multiple levels.

Regulation of Gene Expression in Eukaryotes

Gene expression occurs in two major stages -

The first is transcription. In this process, the gene is copied to produce an RNA molecule (a primary transcript). Most human genes are divided into exons and introns, and only the exons carry information required for protein synthesis. Most primary transcripts are therefore processed by splicing to remove intron sequences and generate a mature transcript or messenger RNA (mRNA) that only contains exons.

The second stage is protein synthesis. This stage is also known as translation.

—        Virtually every cell in your body contains a complete set of genes

—        But they are not all turned on in every tissue

—        Each cell in your body expresses only a small subset of genes at any time

—        During development different cells express different sets of genes in a precisely regulated fashion

—        Constitutive ( house keeping) genes:

—         - Are expressed at a fixed rate, irrespective to the cell condition.

—         - Their structure is simpler

Controllable genes:

—         - Are expressed only as needed. Their amount may increase or decrease with respect to their basal level in different condition.

—         - Their structure is relatively complicated with some response elements

Levels of Control of Gene Expression in Eukaryotes

Eukaryotes have more complex means to regulate gene expression, because they have compartments (e.g., nucleus) within cells, and often multicellular structures that require differentiation of cells.

Levels at which expression of protein-coding genes is regulated in eukaryotes

1. Transcription.
2. mRNA processing and transport.
3. Translation.
4. Degradation of mRNA.
5. Protein processing.

                 f.     Protein degradation.