Chapter 11

The Control of Gene Expression

To Clone or Not to Clone

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- Began in 1950’s- Dolly (1997)- proposed for endangered species- may create new problems

stop work on habitat preservationdoes not increase genetic diversityclones animals are less healthy

Gene regulation is important for the well-being of all organisms *How are genes regulated *applications *gene regulation and embryonic development

Gene Regulation Gene expression Early

understanding of gene control

Came from studies of the bacterium

Escherichia coli

In prokaryotes, genes for related enzymes

Are often controlled together in units operons

Lac Operon Can be turned on and off according to

the environmental circumstances: availability of lactose

Lac Operon

Lack of substrate inactivates the enzyme

Lac Operon

Substrate presence inactivates the repressor of the gene

Other Kinds of Operons trp operon similar to the lac operon, but functions

somewhat differently

Inactive repressor

TryptophanLactose

Active repressorPromoter Operator Genes Promoter Operator Genes

Lac Operon trp operon

Differentiation yields a variety of cell types, each

expressing a different combination of genes

multicellular eukaryotes cells become specialized as a zygote

develops into a mature organism

Muscle Pancreas Blood

Genetic Potential Differentiated cells may retain all of their

genetic potential Most differentiated cells retain a

complete set of genes

root cell cultured cell division plantlet adult plantin nutrient medium in culture

single cell

tissue

DNA Packing DNA packing in

eukaryotic chromosomes helps regulate gene expression

A chromosome contains DNA

Wound around clusters of histone proteins, forming a string of beadlike nucleosomes

Inactive Chromosomes In female mammals, one X chromosome is inactive in

each cell An extreme example of DNA packing in interphase cells X chromosome inactivation in the cells of female

mammalsEarly embryo cells of adult cat

X chromosome

Allele for orange fur

Allele for black fur

Cell division and randomInactivation of X chromosome

active

active

inactive

Control of Eukaryotic Transcription

A variety of regulatory proteins interact with DNA and with each other

Known as transcription factors “Default” state seems to be “off” They turn the transcription of genes

on or off Only a small percentage of the genes

Transcription Factors Assist in initiating eukaryotic transcription

Enhancers Promoter gene

Transcription Factors

Activatorprotein

OtherProteins

RNA polymerase

Bending of DNA

Transcription

Coordinating Eukaryotic Gene Expression

Coordinated gene expression in eukaryotes

Seems to depend on the association of enhancers with groups of genes

Eukaryotic RNA May be spliced in more than one way After transcription, alternative splicing May generate two or more types of mRNA from the

same transcript

DNA

EXONS

RNA transcript

mRNA

RNA splicing allows more than one type of polypeptide from a single gene

Introns are removed

Regulation Translation and later stages of

gene expression are also subject to regulation

After eukaryotic mRNA is fully processed and transported to the cytoplasm

There are additional opportunities for regulation

1. Breakdown of mRNA

lifetime of an mRNA molecule Short-lived mRNA: bacteria Long-lived mRNA: eukaryotes

Helps determine how much protein ismade

Example of Long-lived mRNA

Red blood cells of reptiles, amphibians and fish

Manufactures hemoglobin Last the same as the lifetime of

the RBC

2. Initiation of Translation

Control of the starting point of polypeptide synthesis

Example synthesis of Hemoglobin Iron in the heme group has to be

present

3. Protein Activation After translation is complete Polypeptides may require an alteration to become

functional

Inactive polypeptide folded polypeptide active form of insulin

SH

SH

HS

SHHS

SHSS

S S

SS

Folding of polypeptide

Formation of S-S linkage

Cleavage SS

SS

S S

Insulin

4. Protein Breakdown

Occurs in some of the proteins that trigger metabolic changes in cells

Are broken down within a few minutes or hours

Allows cell to adjust the kinds and amounts of proteins in response to the environment

Review

Multiple mechanisms regulate geneexpression in eukaryotes

Animal Cloning Nuclear transplantation can be used to

clone animals and/or therapeutic use

Removenucleus fromegg

Add somaticcell from adult

Grow in culture to producean early embryo (blastocyst)

Nucleus from donor

Surrogate mother clone of donorwith implantedblastocyst

Remove embryonic stem cells from blastocyst and growin culture

Induce stem cellsto form speciali-zed cells (therapeutic)

Connection: Reproductive Cloning Reproductive cloning has valuable

applications, but human reproductive cloning raises ethical issues

Reproductive cloning of nonhuman mammals is useful in research, agriculture, and medicine

Connection: Therapeutic Cloning can produce stem cells with great medical potential Like embryonic stem cells, adult stem cells can

perpetuate themselves in culture and give rise to differentiated cells

Adult stem cell in bonemarrow

Cultured embryonic stem cells

Different culture conditions

Different types ofDifferentiated cells

Blood cells

Nerve cells

Muscle cells

Genetic Control of Embryonic Development Cascades of gene expression and cell-to-cell

signaling direct the development of an animal Early understanding of the relationship

between gene expression and embryonic development

Came from studies of mutants of the fruit fly Drosophila melanogasterNormal Mutant

antenna

leg

D. melanogaster 1. egg protein signals follicle cells 2. follicle cells signal back to the

egg 3. egg responds localizing

specific mRNA which indicates the localization of the fly’s head

4. regulatory protein is produced 5. other proteins are also

produced and form a gradient 6. segmentation occurs 7. adult fly is the final product

1

2

3

4

5

6

7

Follicle cells

mRNA

Regulatory proteins

segmentation

ADULT

EGG

LARVA

PUPA

Signal Transduction Pathways Convert messages

received at the cell surface to responses within the cell

Signaling cell

Signaling molecule

Receptor protein

Target cell

12

3

4

5

6

Relay proteins

Transcription factor

Nucleus

DNA

Transcription

mRNA

Translation

New Protein

Key Developmental Genes are very ancient Homeotic genes contain

nucleotide sequences, called homeoboxes

That are very similar in many kinds of organisms

Fruit fly embryo (10h) mouse embryo (12d)

Adult fruit fly adult mouse

THE GENETIC BASIS OF CANCER

Cancer results from mutations in genesthat control cell division

divide uncontrollably Result from mutations in genes

whose protein products affect the cell cycle

Proto-Oncogenes A mutation can change a proto-

oncogene (a normal gene that promotes cell division) into an oncogene, which causes cells to divide excessively

Proto-oncogene DNA

Mutation within gene Normal Gene at other locus

oncogene

Hyperactive growth-stimulating protein innormal amount

Normal growth-stimulating protein in excess

New promoter

Onkos= tumor

Tumor-Suppressor Genes Mutations that inactivate tumor suppressor genes Have similar effects as oncogenes

Tumor-suppressor gene Mutated tumor-suppressor gene

Normal growth-inhibiting protein Defective, nonfunctional protein

Cell division under control Cell division not under control

Interference with Normal Signal Transduction Pathways Oncogene

proteins Can stimulate

signal transduction pathways

Growth factor

receptor

Target cell

Hyperactiverelay proteinIssues signals on

its own

Normal product

Relay proteins

Transcription factor(activated)

transcription

DNA

translation

Protein that stimulatesCell division

Interference… Faulty tumor-suppressor

proteins Can inhibit signal

transduction pathways

Growth inhibitingfactor

receptor

Relay proteinNonfunctional transcriptionFactor: cannot trigger transcription

Normal product ofP53 gene

Transcription factor(activated)

transcription

translation

Protein thatinhibits celldivision Lack of such

protein

Development of Cancer

Multiple genetic changes underlie thedevelopment of cancer

Cancers result from a series of genetic changes in a cell lineage

Colon cancer Develops in a stepwise fashion

Cellular changes Increased cell division growth of a polyp Growth of a malignant tumor (carcinoma)

DNA changes Oncogene activated Tumor-suppressor gene inactivated 2nd tumor-suppressor gene inactivated

1 2 3

Accumulation of mutations Can lead to

cancer

Chromosomes 1 mutation 2 mutations 3 mutations 4 mutations

Normal cell Malignant cell

Talking about Science Mary-Claire King discusses mutations

thatcause breast cancer

Researchers have gained insight into thegenetic basis of breast cancer

By studying families in which a diseasepredisposing

mutation is inherited

Avoiding carcinogens can reduce the risk of cancer Reducing exposure to carcinogens

(which induce cancer-causing mutations)

And making other lifestyle choices canhelp reduce cancer risk

Cancer in the U.S.

The End