Controls Over Genes

Chapter 14

Gene Control

Which genes are being expressed in a cell depends upon:

• The type of cell• Internal chemical conditions

• External signals

• Built-in control systems

Mechanisms of Gene Control

Controls related to transcription

Transcript-processing controls

Controls over translation

Post-translation controls

Regulatory Proteins

Can exert control over gene

expression through interactions with:

– DNA

– RNA

– New polypeptide chains

– Final proteins

Activator and Repressor Proteins

• Positive Control System

– Activator protein enhances

some activity

• Negative Control System

– Repressor protein inhibits

some activity

Some Controls Affect Noncoding DNA

• Promoter signals beginning

of gene

• Enhancer is a binding site for

activator protein

Chemical Modifications

• Methylation of DNA can inactivate genes

• Acetylation of histones allows DNA unpacking and transcription

Gene Control in Prokaryotes

• No nucleus separates DNA from

ribosomes in cytoplasm

• When nutrient supply is high,

transcription is fast

• Translation occurs even before

mRNA transcripts are finished

The Lactose Operon

Negative Control – Low Lactose

b In the absence of lactose, the repressor binds to two operators in DNA. It makes the DNA loop out in a way that blocks operon gene transcription; it stops RNA polymerase from binding to its promoter.

Negative Control – High Lactose

lactose

c When lactose is present, some is converted to a form that binds to the repressor and alters its shape. The altered repressor can’t bind to operators, so RNA polymerase is free to transcribe the operon genes.

allolactosetranslation into polypeptidechains for the three enzymes

mRNA

RNA polymerase

operator operator gene 1promoter

Positive Control of Lactose Operon

• CAP is an activator protein

• CAP affects promoter

• CAP will adhere to promoter only

when in complex with cAMP; when

glucose levels are high

Positive Control – High Glucose

• There is little cAMP

• CAP cannot be activated

• The promoter is not good at binding

RNA polymerase

• The lactose-metabolizing genes are

not transcribed very much

Positive Control – Low Glucose

• cAMP accumulates

• CAP-cAMP complex forms

• Complex binds to promoter

• RNA polymerase can now bind

• The lactose-metabolizing genes are

transcribed rapidly

Cell Differentiation

• All cells in a multicellular organism

inherited the same genes

• Some of those genes are used in all cell

types

• During development, cells activate and

suppress other genes in selective ways

Controls of Eukaryotic Gene Expression

DNApre-mRNAtranscript

mRNA

NUCLEUS

mRNA

CTYOPLASMtranslational

controlproteinproduct

proteinproductcontrol

inactivatedprotein

inactivatedmRNA

mRNAdegradation

control

mRNAtransportcontrol

transportprocessing

control

transcriptioncontrol

Most Genes Are Turned Off

• Cells of multicelled organisms rarely use more than 5-10 percent of their genes at any given time

• The remaining genes are selectively expressed

Polytene Chromosomes

• Occur in salivary glands of midge larvae

• Consist of multiple DNA molecules

• Can produce multiple copies of transcripts

Chromosome Puff

• Portion of the polytene chromosome in

which the DNA has loosened up to

allow transcription

• Appears in response to ecdysone

• Translation of transcripts from puffed

region produces protein components

of saliva

Controls Over Gene Expression

• Work at certain stage before, during, and after transcription and translation

• Most controls over gene expression occur at translation

Selective Gene Expression

• All differentiated cells in a complex, multicelled body use most of their genes in much the same way

• Each type also uses a fraction of those genes in a unique, selective way

Homeotic Genes

• A class of master genes in most eukaryotic organisms

• Transcribed in specific locations in the developing embryo – products form in local tissue regions

• Guide formation of organsand limbs

X Chromosome Inactivation

• Mammalian females have two X

chromosomes per cell

• One X is inactivated in each cell

• Inactivation is random

• Female is a “mosaic”

Barr Body

• Condensed X chromosome

• Visible in micrographs

• May be either the maternal or the paternal X chromosome

Clues to Gene Controls

• Research using Drosophila melanogaster has led to remarkable discoveries about how embryos develop