Using a specific example, can some one please tell me how gene regulation breaks down in eukaryotes?

Question

Using a specific example, can some one please tell me how gene regulation breaks down in eukaryotes?

Answer 1

Prokaryotes have two levels of metabolic control

Vary the numbers of specific enzymes made (regulation of gene expression)

Slow, but can have a dramatic effect on metabolic activity

Regulate enzymatic pathways (feedback inhibition, allosteric control)

Rapid and can be fine-tuned, but if the enzyme system does not have this level of control, then it is useless

Prokaryotes are "simple," single celled organisms, so they have "simple" systems

Genes are grouped together based on similar functions into functional units called operons

MANY GENES UNDER ONE CONTROL!!!

There is one single on/off switch for the genes

Function - to produce enzymes which break down lactose (milk sugar)

lactose is not a common sugar, so there is not a great need for these enzymes

when lactose is present, they turn on and produce enzymes

Two components - repressor genes and functional genes

Three functional genes:

lacZ produces B-galactosidase.  This enzyme hydrolyzes the bond between the two sugars, glucose and galactose

lacY produces permease.  This enzyme spans the cell membrane and brings lactose into the cell from the outside environment. The membrane is otherwise essentially impermeable to lactose.

lacA produces B-galactosidase transacetylase.  The function of this enzyme is still not known.

Promoter (P) - aids in RNA polymerase binding

Operator (O) - "on/off" switch - binding site for the repressor protein

Repressor (lacI) gene

Repressor gene (lacI) - produces repressor protein w/ two binding sites, one for the operator and one for lactose

The repressor protein is under allosteric control - when not bound to lactose, the repressor protein can bind to the operator

When lactose is present, an isomer of lactose, allolactose, will also be present in small amounts.  Allolactose binds to the allosteric site and changes the conformation of the repressor protein so that it is no longer capable of binding to the operator

It Gets More Complicated - the lac Operon Revisited

It is not enough for lactose to be present to induce the lac operon

Glucose is the sugar of choice of E. coli and if glucose is in supply, then the bacteria will preferentially break down glucose over lactose

If glucose is present, the lac operon will be repressed - how does this happen you ask?

RNA polymerase has a low affinity for the promter of the lac operon unless helped by a regulatory proten - cAMP receptor protein (CRP)

CRP only becomes activated if the concentration of cyclic AMP (cAMP) is high

Glucose inhibits the formation of cAMP

If the concentration of glucose is high, the concentration of cAMP is low

If the concentration of glucose is low, the concentration of cAMP is high

Therefore, if the concentrations of glucose and lactose are high, the concentration of cAMP will be low, CRP will not be activated, RNA polymerase will not be able to bind well to the promoter, and the operon will be operating at a very low level (i.e. almost off)

However, if the concentrations of glucose is low and lactose is high, the concentration of cAMP will be high, CRP will be activated and bind to the DNA which will promote RNA polymerase binding and initiate transcription

five genes (trpA, trpB, trpC, trpD, and trpE) involved in the production of the amino acid tryptophan

another gene (trpR) produces an inactive repressor protein

accumulation of the end product (tryptophan) represses synthesis of the enzymes

tryptophan binds to the inactive repressor protein at an allosteric site

the conformation changes and the repressor + tryptophan complex binds to the operator, repressing the operon

tryptophan can accumulate due to internal production or from external sorces

remember, E. coli is found in the intestines of humans so if you eat a tryptophan-rich meal, this will accumulate in the bacteria and turn off the operon

why waste resources when a supply of this amino acid is readily available?

Gene Control in Eukaryotes

Much more complex - take humans for example

Every cell (except gametes) have the same DNA, with the same information

This is known as genetic totipotency

Almost all eukaryotic genes must be shut off in order to allow for cell normal function (a liver cell cannot have genes for lung cells running, not can it?)

Usually, every gene has more than one gene regulator (all of which must be on for the gene to function)

here is a specific example... or w/e

Gene Regulation in Eukaryotes

The latest estimates are that a human cell, a eukaryotic cell, contains approximately 35,000 genes.

Some of these are expressed in all cells all the time. These so-called housekeeping genes are responsible for the routine metabolic functions (e.g. respiration) common to all cells.

Some are expressed as a cell enters a particular pathway of differentiation.

Some are expressed all the time in only those cells that have differentiated in a particular way. For example, a plasma cell expresses continuously the gene for the antibody it synthesizes.

Some are expressed only as conditions around and in the cell change. For example, the arrival of a hormone may turn on (or off) certain genes in that cell.

How is gene expression regulated?

There are several methods used by eukaryotes.

Transcription Control  

The most common type of genetic regulation

Turning on and off of mRNA formation

Post-Transcriptional Control

Regulation of the processing of a pre-mRNA into a mature mRNA

Translational Control

Regulation of the rate of Initiation

Post-Tranlational Control

Regulation of the modification of an immature or inactive protein to form an active protein

Answer 2

good lord, that's a very specific answer