GENE EXPRESSION

Prokaryotic and Eukaryotic Gene Regulation

VOCABULARY Regulatory gene Transcriptional

control Posttranscriptional Translational control Posttranslational

control Operon Promoter

(regulatory sequence)

Lac operon E.coli RNA polymerase Positive regulation Negative Regulation Lac repressor Β-galactosidase Inducer Operator Transcription

factors

GENE EXPRESSION the process by which information from a

gene is used in the synthesis of a functional gene product. These products are often proteins or functional RNA.

is used by all known life - eukaryotes (including multicellular organisms) and prokaryotes (bacteria and archaea)

Prokaryotes, since they only have one chromosome (and are unicellular) transcribe almost all of their genes into RNA.

Eukaryotes, on the other hand, do not make all of their proteins at the same time.

Eukaryotes, most of which are multicellular, have many different specialized types

A human consists of about 2000 types of cells. Almost every one of these cells contain the same set of about 100,000 genes.

Yet each kind of cell expresses only a fraction of these

EUKARYOTIC CELLS

embryo

bone

liver

muscle

sperm

GENE EXPRESSION IN EUKARYOTIC CELLS A typical eukaryotic cell transcribes only

about 20% of its DNA into RNA. In a human skin cell, for example,

transcribes membrane proteins, ribosomal proteins, and metabolic enzymes, collagen (elastic protein that make skin tough)

CELLS CONTROL GENE EXPRESSION Cells can control how and when their

genes are expressed in many ways. The most common and efficient way is

(1)transcriptional control. Cells increase and decrease the amount

of mRNA that is transcribed from the DNA.

This occurs in the nucleus of the cell

CELLS CONTROL GENE EXPRESSION Cells can control gene expression by

changing the amount of RNA that is released into the cytoplasm from the nucleus, (2) postranscriptional control

Or a cell can vary the speed at which it translates individual types of mRNAs (3) translational control

CELLS CONTROL GENE EXPRESSION A cell can also change the structure of

the protein (4) posttranslational control.

And example would be the addition of phosphate groups to proteins (turning enzymes on and off)

GENE EXPRESSION – PROKARYOTIC CELL Scientists use Bacteria to study gene

regulation because of the simplicity of the bacterial cell’s chromosome structure.

Eschericia coli (E.coli) is used extensively in the lab.

common inhabitant of the human colon and helps us to digest the proteins lipids and carbohydrates in our food.

circular chromosome that contains about five million DNA base pairs, only 1/600th the haploid amount of DNA in a human cell.

Bacteria have the ability to make the right enzymes at the right time to survive in their environment.

For example, if a bacterium’s environment contains a lot of amino acids it will not waste energy making more amino acids and will devote its energy to division.

For example, the synthesis of histidine from carbon and nitrogen compounds requires 7 different enzymes.

The genes for these enzymes are found right next to each other on the bacteria’s DNA.

HISTIDINE OPERON

Scientists were surprised to find that the bacterial cell transcribes the 7 genes into a huge mRNA instead of 7 different mRNA molecules.

This ensures that all enzymes needed to make the final product (histidine) are turned on and off at the same time.

A stretch of DNA that includes several genes under coordinated control is called an operon

E.COLI AND THE LAC OPERON The best studied operon in

prokaryotes controls the use of lactose by E.coli.

Enables E.coli to produce enzymes only when needed.

When glucose is available in the intestines, E.coli will not digest lactose.

E.COLI AND THE LAC OPERON When glucose is absent in the

intestines, and the host (you) drinks milk, then the E. coli begins synthesizing 3 proteins that allow it to digest lactose

Β-galactosidase is one of the enzymes (it splits lactose into galactose and glucose)

The 3 genes that code for the 3 proteins are found next to each other on the E.coli DNA and are part of the lactose operon.

E.COLI AND THE LAC OPERON

LAC OPERON The lac operon regulates the use of

lactose by E.coli. It sits below the promoter on the DNA.

The promoter is a sequence that signals the start of the gene or genes in this case.

When glucose is present in the environment the lac repressor binds to the operator on the DNA. The operator is a regulatory site. This process prevents RNA polymerase from binding to the promoter and transcribing the three genes on the operon.

LAC OPERON When there is no glucose in the

environment and you drink milk. The lactose acts as an inducer binds to the lac repressor such that it cannot bind to the operator and block transcription.

This is called negative regulation of gene expression.

POSITIVE REGULATION Describe an example of positive

regulation in the prokaryotic cell.

EUKARYOTES Eukaryotic DNA has a regulatory

sequence called the TATA box. Sequence above the gene that allows the RNA polymerase to grab on and express the gene.

A regulatory protein “TATA box binding protein” grabs on to the TATA box