Control over Genes

Chapter 15

15.1 Control Mechanisms

Which genes are expressed in a cell depends upon:

• 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

Control Mechanisms

• Negative control

– Regulatory proteins slow down or curtail gene activity

• Positive control

– Regulatory proteins promote or

enhance gene activities

Chemical Modifications

• Methylation of DNA can inactivate genes

• Acetylation of histones allows DNA unpacking and transcription

Figure 15.2Page 240

15.2 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

gene 1 gene 2 gene 3

lactose operon

regulatory gene

transcription,translation

operator

operator

promoter

repressor protein

Figure 15.3aPage 241

Low Lactose

• Repressor binds to operator

• Binding blocks promoter

• Transcription is blocked

Figure 15.3bPage 241

High Lactose

gene 1operator operatorpromoter

mRNA RNA polymerase

lactose

allolactose

Figure 15.3cPage 241

CAP Exerts Positive Control

• CAP is an activator protein

• Adheres to promoter only when in

complex with cAMP

• Level of cAMP depends on level of

glucose

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

15.3 Controls in Eukaryotic Cells

• Control of transcription

• Transcript processing controls

• Controls over translation

• Controls following translation

14.4 Types of Control Mechanisms

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

• The remaining genes are selectively expressed

Homeotic Genes

• Occur in all eukaryotes

• Master genes that control development of body parts

• Encode homeodomains (regulatory proteins)

• Homeobox sequence can bind to promoters and enhancers

X Chromosome Inactivation

• One X inactivated in

each cell of female

• Creates a “mosaic”

for X chromosomes

• Governed by XIST

gene

Figure 15.6Page 245

15.5 Signaling molecules

• Hormones

• Stimulate or inhibit activity in target cells

• Mechanism of action varies

– May bind to cell surface

– May enter cell and bind to regulatory proteins

– May bind with enhancers in DNA

Polytene Chromosomes

• Occur in salivary glands of midge larvae

• Consist of multiple DNA molecules

• Can produce multiple copies of transcripts

Figure 15.8Page 246

Chromosome Puff

• Portion of the 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

Vertebrate Hormones

• Some have widespread effects

– Somatotropin (growth hormone)

• Others signal only certain cells at

certain times

– Prolactin stimulates milk

production

Phytochrome

• Signaling molecule in plants

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

• Changes in phytochrome activity influence transcription of certain genes

Controlling the Cell Cycle

• Cycle has built-in checkpoints

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

• Proteins are products of checkpoint genes

• Kinases

• Growth factors

Oncogenes

• Have potential to induce cancer

• Mutated forms of normal genes

• Can form following insertions of viral

DNA into DNA or after carcinogens

change the DNA

Cancer Characteristics

• Plasma membrane and cytoplasm

altered

• Cells grow and divide abnormally

• Weakened capacity for adhesion

• Lethal unless eradicated

Apoptosis

• Programmed cell death

• Signals unleash molecular weapons of

self-destruction

• Cancer cells do not commit suicide on

cue