Regulation and Control of Gene Expression

15.1 Gene Expression in Eukaryotic Cells

Gene controls govern the kinds and amounts of substances in a cell at any given interval

Various control processes regulate all steps between gene and gene product

Which Genes Get Tapped?

Differentiation• The process by which cells become specialized• In multicelled organisms, most cells differentiate

when they start expressing a unique subset of their genes

• Which genes are expressed depends on the type of organism, its stage of development, and environmental conditions

Control of Transcription

Transcription factors• Regulatory proteins that affect the rate of

transcription by binding to special nucleotide sequences in DNA

• Activators speed up transcription when bound to a promoter; or may bind to distant enhancers

• Repressors slow or stop transcription

Control of Transcription

Chemical modifications and chromosome duplications affect RNA polymerase’s access to genes• Interactions between DNA and histone proteins

(methylation) prevent transcription• Polytene chromosomes (many copies) increase

transcription rates in some organisms

Controls of mRNA Transcripts

mRNA processing• DNA splicing controls products of translation

mRNA transport controls delivery of transcripts• Passage through nuclear pores• Delivery within cytoplasm (mRNA localization)

Translational Controls

Controls over molecules involved in translation

Controls over mRNA stability• Depends on base sequence, length of poly-A tail,

and which proteins are attached to it

RNA interference• Expression of a microRNA complementary to a

gene inhibits expression of the gene

Post-Translational Modification

Post-translational modification can inhibit, activate, or stabilize many molecules, including enzymes that participate in transcription and translocation

Points of Control over Eukaryotic Gene Expression

DNA NUCLEUSA TranscriptionBinding of transcription factors to special sequences in DNA slows or speeds transcription. Chemical modifications and chromosome duplications affect RNA polymerase’s physical access to genes.

new RNAtranscript

B mRNA ProcessingNew mRNA cannot leave the nucleus before being modified, so controls over mRNA processing affect the timing of transcription. Controls over alternative splicing influence the final form of the protein.

mRNA

Fig. 15-2, p. 230

Stepped Art

mRNA

CYTOPLASM

C mRNA TransportRNA cannot pass through a nuclear pore unless bound to certain proteins. Transport protein binding affects where the transcript will be delivered in the cell.

polypeptidechain

D TranslationAn mRNA’s stability influences how long it is translated. Proteins that attach to ribosomes or initiation factors can inhibit translation. Double-stranded RNA triggers degradation of complementary mRNA.

active protein

E Protein ProcessingA new protein molecule may become activated or disabled by enzyme-mediated modifications, such as phosphorylation or cleavage. Controls over these enzymes influence many other cell activities.

15.1 Key Concepts: Overview of Controls Over Gene Expression

A variety of molecules and processes alter gene expression in response to changing conditions both inside and outside the cell

Selective gene expression also results in cell differentiation, by which different cell lineages become specialized

15.2 A Few Outcomes of Eukaryotic Gene Controls

Selective gene expression can give rise to visible traits

X Chromosome Inactivation

X chromosome inactivation• In cells of female mammals, either the maternal

or paternal X chromosome is randomly condensed (Barr body) and is inactive

• Occurs in an early embryonic stage, so that all descendents of that particular cell have the same inactive X chromosome, resulting in “mosaic” gene expression

X Chromosome Inactivation

Calico: Mosaic Gene Expression in a Female Mammal

Prokaryotic Gene Control

In prokaryotes, genes that are used together often occur together on chromosomes

Operon• A promoter and one or more operators that

collectively control transcription of multiple genes

Operators• DNA regions that are binding sites for a repressor

The Lactose Operon

E. coli digest lactose in guts of mammals using a set of three enzymes controlled by two operators and a single promoter (the lac operon)• When lactose is not present, repressors bind to

the operators and inactivate the promoter; transcription does not proceed

• When lactose is present, allolactose binds to the repressors; repressors don’t bind to operators to inactivate the promoter; transcription proceeds

The Lactose Operon Repressor

Lactose Intolerance

Human infants and other mammals produce the enzyme lactase, which digests the lactose in milk – adults tend to lose the ability to produce lactase, and become lactose intolerant