Gene Structure and Regulation

Gene Expression The expression of genetic information is one of the fundamental activities of all cells. Instruction stored in DNA are transcribed and processed into RNA molecules These RNA molecules have specific roles in how the information is translated and expressed as a gene product.

All cellular life forms have about 60 proteins in common! Most of these proteins are involved in translation as ribosomal proteins and tRNA enzymes. A couple are involved in transcription A few are involved in DNA replication and repair.

An overview of Gene Structure 3’ 5’ Regulatory region 3’ 5’ StartStop Coding Region Promoter regionTerminator Region

One gene can code for more than one protein! The human genome has about 30,000 genes but our proteome (the total number of different proteins) is much larger. How can this occur? Many genes can produce more than one protein because the mRNA transcript contains different combination of exons. This process is called alternative splicing.

The Coding Region in Eukaryotes Contain Introns Prokaryotes do not have introns Exon Exon Exon DNA Template Strand Intron Pre-mRNA transcript of DNA Exon Exon Exon Spliceosome Introns are spliced out by spliceosomes leaving only the sequences that will be expressed. This is an example of RNA processing. The result is a mature mRNA that will leave the nucleus.

Alternative splicing Exon 1 Exon 2 Exon 3 Exon 4 Possible mRNAs using different combination of exons Exon 1 Exon 2 Exon 3 Exon 4 Exon 1 Exon 2 Exon 4 Exon 2 Exon 3 Exon 4 When each mRNA is translated, a different protein is produced.

Some genes are expressed continuously These are called ‘housekeeping’ genes because they are required for basic functions of cells such as provision of energy, passage of molecules across the cell membrane, repair and cell division.

Gene Regulation Each somatic cell contains and entire organism’s genome. However, even though the cells in your eye have the gene form producing fingernail protein (keratin) this gene is not expressed. How do genes get switched on and off.

Types of Genes Structural genes - these genes express structural and/or functional proteins. Regulatory genes - these genes are short nucleotide sequences that express proteins that control the activity of structural genes by feedback mechanisms. They are normally found ‘upstream’ from the functional gene.

An overview of Gene Structure 3’ 5’ Regulatory region 3’ 5’ StartStop Coding Region Promoter regionTerminator Region

Why have cells evolved complex mechanisms to regulate their genes? Cells conserve energy and materials by blocking unneeded gene expression. If a substrate is absent in the environment why produce the enzyme for that substrate! Repressor molecules keep the cell from wasting energy by making mRNA and enzyme molecules that have no use.

Gene Regulation in Prokaryotes Bacteria have groups of genes that are controlled together and are turned on/off as required. When lactose is added to its growth medium, E.coli switches on a gene and make the enzyme B – galactosidase. This enzyme splits sugar lactose to produce the sugars glucose and galactosidase. Lactose sugar is rarely encountered by bacteria, so the enzyme is not usually produced. When lactose is absent a protein attaches to DNA and blocks the synthesis of mRNA for B – galactosidase. When lactose is present it binds to the repressor. This releases the repressor from the DNA so that the gene can be transcribed.

Lac I P B - Galactosidase gene Promoter region Repressor protein expressed by Lac regulatory gene

Repressor protein occupies the promoter region. Lac I P B - Galactosidase gene

When no lactose is present Transcription can’t occur Lac I P B - Galactosidase gene RNA polymerase

When lactose is present: Lactose Lac I P B - Galactosidase gene

Lactose removes repressor protein Transcription can occur Lac I P B - Galactosidase gene

B-galactosidase Transcription into mRNA Translation into an enzyme protein Lac I P B - Galactosidase gene

The gene is off or on depending on the nutrients available to the cell. The enzyme is not produced when there is no need for it (no lactose substrate)