Gene regulation in prokaryotes and eukaryotes

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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Gene regulation in prokaryotes and eukaryotes

• How do organisms respond correctly to their environment?

• How do they control their growth and development?

• How do they control which genes are expressed?


• The process by which genetic information flows from genes to proteins is called gene expression

Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes

Figure 11.1A

Prokaryotes

codes for repressor protein

regulator gene

i gene p o z gene y gene a gene DNA

lac operon

promotoroperator

binding siteof RNApolymerase

codes for permease enzymethat transportslactose into cells

codes for-galactosidase,which clipslactose molecules

genes for related enzymes are often controlled in groups called operons


repressorprotein

repressor protein blocks binding of RNA polymerase

RNA polymerase

no transcription

Regulatory proteins bind to control sequences in DNA and turn operons on or off

regulator gene

lac operon

promotor

operator


repressor

lactose

galactose glucose

-galactosidase

mRNA transcript

transcription proceeds3

cell m

embran

e

lactose

RNA polymerasebinds to promoter

2

lactose the (inducer)inactivates the repressorso that it cannot bind to the operator

1

permease


• Two types of repressor-controlled operons

Figure 11.1C

Tryptophan

DNA

Promoter Operator Genes

Activerepressor

Activerepressor

Inactiverepressor

Inactiverepressor

lac OPERON trp OPERON

Lactose


Eukaryotic gene control

• Transcription is a major control point

• Individual genes are regulated, rather than operons

• Activation is more important than repression


– These interactions turn the transcription of eukaryotic genes on or off

Activators bind to enhancer regions of DNA and to other transcription factors

Enhancers

DNAActivatorproteins

Otherproteins

Transcriptionfactors

RNA polymerase

Bendingof DNA

Transcription

Promoter

Gene

Figure 11.8


Control of transcription

• Control access of enzymes to DNA:

1. Methylation of DNA - inhibits

2. Acetylation of histones - promotes


How do eukaryotes control genes of a single metabolic pathway?

• genes for individual enzymes of one pathway are often located far apart in the genome

• same enhancer region(s) common to all genes of a pathway.


Exons

DNA

RNA splicing or

RNAtranscript

mRNA

• After transcription, alternative splicing may generate two or more types of mRNA from the same transcript

Eukaryotic RNA may be spliced in more than one way

Figure 11.9


Translation and later stages of gene expression are also subject to regulation

• The lifetime of an mRNA molecule helps determine how much protein is made

• Initiation of translation


• The protein may need to be activated in some way

• Rate of protein degradation

Figure 11.10

Folding of polypeptide andformation of S–S linkages

Initial polypeptide(inactive)

Folded polypeptide(inactive)

Cleavage

Active formof insulin


Figure 11.6

DNAdoublehelix(2-nmdiameter)

Metaphase chromosome

700nm

Tight helical fiber(30-nm diameter)

Nucleosome(10-nm diameter)

Histones“Beads ona string”

Supercoil(200-nm diameter) •DNA packing

tends to block gene expression

multiple levels of DNA packing


In female mammals, one X chromosome is inactive in each cell

Figure 11.7

EARLY EMBRYO

Cell divisionand

X chromosomeinactivation

X chromosomes

Allele fororange fur

Allele forblack fur

TWO CELL POPULATIONSIN ADULT

Active X

Inactive X

Orange fur

Inactive X

Active X Black fur


Chromosome

GENE

RNA transcript

mRNA in nucleus

mRNA in cytoplasm

Polypeptide

ACTIVE PROTEIN

GENEExon

Intron

TailCap

NUCLEUSFlowthroughnuclear envelope

CYTOPLASM

Breakdown of mRNA

Translation Broken-down mRNA

Broken-down protein

Cleavage/modification/activation

Breakdownof protein

DNA unpackingOther changes to DNA

TRANSCRIPTION

Addition of cap and tail

Splicing

Figure 11.11


• Each step of gene expression can be turned on or off, speeded up, or slowed down

• most important control is usually the start of transcription

• DNA can be mobile; position will affect transcription. Transposons

Review: Multiple mechanisms regulate gene expression in eukaryotes

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What is a gene?

• First version: one gene, one polypeptide

• Now: DNA segment that is transcribed to RNA

• Proteins and RNA function in cells

Nature vs nurture

• Do environments or genes determine phenotype?

• Himalayan rabbits and fur color• Twin studies - identical cp. fraternal

Complex human traits

• Language - FoxP2• Depression• Social bonds

Do 460 bp determine our life partner?

Genome imprinting

• Same gene is expressed differently, depending on whether it was inherited from the male or female parent

• Ex. Corn kernel color• Ex. Xsome 15 deletion• Gene is “reset” during gamete formation

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Gene regulation in prokaryotes and eukaryotes