Control over GenesControl over Genes

Chapter 15Chapter 15

Skin CancerSkin Cancer

Basal Cell CarcinomaBasal Cell Carcinoma

Squamous Cell CarcinomaSquamous Cell Carcinoma

Malignant MelanomaMalignant Melanoma

Changes in DNA Changes in DNA Trigger CancerTrigger Cancer

Ultraviolet Ultraviolet

radiation can radiation can

cause breakscause breaks

In-text figurePage 238

Gene ControlGene Control

Which genes are expressed in a Which genes are expressed in a cell depends upon:cell depends upon:

• • Type of cellType of cell

• • Internal chemical Internal chemical conditionsconditions

• • External signalsExternal signals

• • Built-in control systemsBuilt-in control systems

Mechanisms of Gene ControlMechanisms of Gene Control

Controls related to transcriptionControls related to transcription

Transcript-processing controlsTranscript-processing controls

Controls over translationControls over translation

Post-translation controlsPost-translation controls

Regulatory ProteinsRegulatory Proteins

Can exert control over gene Can exert control over gene

expression through interactions with:expression through interactions with:

–DNADNA

–RNARNA

–New polypeptide chainsNew polypeptide chains

–Final proteinsFinal proteins

Control MechanismsControl Mechanisms

Negative controlNegative control– Regulatory proteins slow Regulatory proteins slow

down or curtail gene activity down or curtail gene activity

Positive controlPositive control

– Regulatory proteins promote Regulatory proteins promote

or enhance gene activitiesor enhance gene activities

Chemical ModificationsChemical Modifications

Methylation of Methylation of DNA can DNA can inactivate genesinactivate genes

Acetylation of Acetylation of histones allows histones allows DNA unpacking DNA unpacking and transcriptionand transcription

Figure 15.2Figure 15.2Page 240Page 240

Gene Control in ProkaryotesGene Control in Prokaryotes

No nucleus separates DNA No nucleus separates DNA

from ribosomes in cytoplasmfrom ribosomes in cytoplasm

When nutrient supply is high, When nutrient supply is high,

transcription is fasttranscription is fast

Translation occurs even before Translation occurs even before

mRNA transcripts are finishedmRNA transcripts are finished

The Lactose OperonThe Lactose Operon

gene 1 gene 2 gene 3

lactose operon

regulatory gene

transcription,translation

operator

operator

promoter

repressor protein

Figure 15.3aFigure 15.3aPage 241Page 241

Low LactoseLow Lactose

Repressor binds to Repressor binds to

operatoroperator

Binding blocks promoterBinding blocks promoter

Transcription is blockedTranscription is blocked

Figure 15.3bFigure 15.3bPage 241Page 241

High LactoseHigh Lactose

gene 1operator operatorpromoter

mRNA RNA polymerase

lactose

allolactose

Figure 15.3cFigure 15.3cPage 241Page 241

Lac-Operon AnalogyLac-Operon AnalogyLike an elephant that likes to sit on a Like an elephant that likes to sit on a railroad track blocking the train.railroad track blocking the train.The only thing that the elephant likes The only thing that the elephant likes better than to sit on his spot on the track better than to sit on his spot on the track is peanuts.is peanuts.If peanuts are available, the elephant If peanuts are available, the elephant leaves the track and eats the peanuts. leaves the track and eats the peanuts. While he is gone the train can proceed While he is gone the train can proceed down the track until the elephant goes down the track until the elephant goes back to sit on the track after eating all the back to sit on the track after eating all the peanuts.peanuts.

Controls in Eukaryotic CellsControls in Eukaryotic Cells

Control of transcriptionControl of transcription

Transcript processing controlsTranscript processing controls

Controls over translationControls over translation

Controls following translationControls following translation

Most Genes Are Turned OffMost Genes Are Turned Off

Cells of a multicelled organism Cells of a multicelled organism rarely use more than 5-10 percent rarely use more than 5-10 percent of their genes at any given timeof their genes at any given time

The remaining genes are The remaining genes are selectively expressedselectively expressed

Homeotic GenesHomeotic Genes

Occur in all eukaryotesOccur in all eukaryotes

Master genes that control Master genes that control development of body partsdevelopment of body parts

Encode homeodomains (regulatory Encode homeodomains (regulatory proteins)proteins)

Homeobox sequence can bind to Homeobox sequence can bind to promoters and enhancerspromoters and enhancers

X Chromosome X Chromosome InactivationInactivation

One X inactivated in One X inactivated in

each cell of femaleeach cell of female

Creates a “mosaic” Creates a “mosaic”

for X chromosomesfor X chromosomes

Governed by Governed by XISTXIST

gene gene

Figure 15.6Figure 15.6Page 245Page 245

HormonesHormones

Signaling moleculesSignaling molecules

Stimulate or inhibit activity in target Stimulate or inhibit activity in target cellscells

Mechanism of action variesMechanism of action varies

– May bind to cell surfaceMay bind to cell surface

– May enter cell and bind to regulatory May enter cell and bind to regulatory proteinsproteins

– May bind with enhancers in DNAMay bind with enhancers in DNA

Vertebrate HormonesVertebrate Hormones

Some have widespread effectsSome have widespread effects

– Somatotropin (growth Somatotropin (growth

hormone)hormone)

Others signal only certain cells Others signal only certain cells

at certain timesat certain times

– Prolactin stimulates milk Prolactin stimulates milk

production production

PhytochromePhytochrome

Signaling molecule in plantsSignaling molecule in plants

Activated by red wavelengths, Activated by red wavelengths, inactivated by far-red wavelengthsinactivated by far-red wavelengths

Changes in phytochrome activity Changes in phytochrome activity influence transcription of certain influence transcription of certain genesgenes

Controlling the Cell CycleControlling the Cell Cycle

Cycle has built-in checkpointsCycle has built-in checkpoints

Proteins monitor chromosome Proteins monitor chromosome structure, whether conditions favor structure, whether conditions favor division, etc.division, etc.

Proteins are products of checkpoint Proteins are products of checkpoint genesgenes

KinasesKinases

Growth factors Growth factors

OncogenesOncogenes

Have potential to induce cancerHave potential to induce cancer

Mutated forms of normal genesMutated forms of normal genes

Can form following insertions of Can form following insertions of

viral DNA into DNA or after viral DNA into DNA or after

carcinogens change the DNAcarcinogens change the DNA

Cancer CharacteristicsCancer Characteristics

Plasma membrane and Plasma membrane and

cytoplasm alteredcytoplasm altered

Cells grow and divide abnormallyCells grow and divide abnormally

Weakened capacity for adhesionWeakened capacity for adhesion

Lethal unless eradicatedLethal unless eradicated

ApoptosisApoptosis

Programmed cell deathProgrammed cell death

Signals unleash molecular weapons Signals unleash molecular weapons

of self-destructionof self-destruction

Cancer cells do not commit suicide Cancer cells do not commit suicide

on cue on cue