GENE REGULATION

Virtually every cell in your body contains a complete set of genesBut they are not all turned on in every tissueEach cell in your body expresses only a small subset of genes at any timeDuring development different cells express different sets of genes in a precisely regulated fashion

GENE REGULATION

Gene regulation occurs at the level of transcription or production of mRNA

A given cell transcribes only a specific set of genes and not others

Insulin is made by pancreatic cells

CENTRAL DOGMA

Genetic information always goes from DNA to RNA to protein

Gene regulation has been well studied in E. coli

When a bacterial cell encounters a potential food source it will manufacture the enzymes necessary to metabolize that food

Gene Regulation

In addition to sugars like glucose and lactose E. coli cells also require amino acidsOne essential aa is tryptophan.

When E. coli is swimming in tryptophan (milk & poultry) it will absorb the amino acids from the mediaWhen tryptophan is not present in the media then the cell must manufacture its’ own amino acids

Trp OperonE. coli uses several proteins encoded by a cluster of 5 genes to manufacture the amino acid tryptophan

All 5 genes are transcribed together as a unit called an operon, which produces a single long piece of mRNA for all the genes

RNA polymerase binds to a promoter located at the beginning of the first gene and proceeds down the DNA transcribing the genes in sequence

Fig. 16.6

GENE REGULATION

In addition to amino acids, E. coli cells also metabolize sugars in their environment

In 1959 Jacques Monod and Fracois Jacob looked at the ability of E. coli cells to digest the sugar lactose

GENE REGULATION

In the presence of the sugar lactose, E. coli makes an enzyme called beta galactosidase

Beta galactosidase breaks down the sugar lactose so the E. coli can digest it for food

It is the LAC Z gene in E coli that codes for the enzyme beta galactosidase

Lac Z Gene

The tryptophane gene is turned on when there is no tryptophan in the mediaThat is when the cell wants to make its’ own tryptophanE. coli cells can not make the sugar lactoseThey can only have lactose when it is present in their environmentThen they turn on genes to beak down lactose

GENE REGULATION

The E. coli bacteria only needs beta galactosidase if there is lactose in the environment to digestThere is no point in making the enzyme if there is no lactose sugar to break downIt is the combination of the promoter and the DNA that regulate when a gene will be transcribed

GENE REGULATION

This combination of a promoter and a gene is called an OPERON

Operon is a cluster of genes encoding related enzymes that are regulated together

GENE REGULATION

Operon consists of A promoter site where RNA polyerase binds and begins transcribing the message

A region that makes a repressor

Repressor sits on the DNA at a spot between the promoter and the gene to be transcribed

This site is called the operator

LAC Z GENE

E. coli regulate the production of Beta Galactocidase by using a regulatory protein called a repressorThe repressor binds to the lac Z gene at a site between the promotor and the start of the coding sequenceThe site the repressor binds to is called the operator

LAC Z GENE

Normally the repressor sits on the operator repressing transcription of the lac Z gene

In the presence of lactose the repressor binds to the sugar and this allows the polymerase to move down the lac Z gene

LAC Z GENE

This results in the production of beta galactosidase which breaks down the sugar

When there is no sugar left the repressor will return to its spot on the chromosome and stop the transcription of the lac Z gene

GENE REGULATION

In eukaryotic organisms like ourselves there are several methods of regulating protein productionMost regulatory sequences are found upstream from the promoterGenes are controlled by regulatory elements in the promoter region that act like one/off switches or dimmer switches

GENE REGULATION

Specific transcription factors bind to these regulatory elements and regulate transcriptionRegulatory elements may be tissue specific and will activate their gene only in one kind of tissueSometimes the expression of a gene requires the function of two or more different regulatory elements

INTRONS AND EXONS

Eukaryotic DNA differs from prokaryotic DNA it that the coding sequences along the gene are interspersed with noncoding sequencesThe coding sequences are called

EXONS

The non coding sequences are called

INTRONS

INTRONS AND EXONS

After the initial transcript is produced the introns are spliced out to form the completed message ready for translation

Introns can be very large and numerous, so some genes are much bigger than the final processed mRNA

INTRONS AND EXONS

Muscular dystrophy

DMD gene is about 2.5 million base pairs longHas more than 70 intronsThe final mRNA is only about 17,000 base pairs long

RNA Splicing

Provides a point where the expression of a gene can be controlled

Exons can be spliced together in different ways

This allows a variety of different polypeptides to be assembled from the same gene

Alternate splicing is common in insects and vertebrates, where 2 or 3 different proteins are produced from one gene