copyright © 2003 pearson education, inc. publishing as benjamin cummings most differentiated...

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

• Most differentiated (specialized) cells retain a complete set of genes

– In general, all somatic cells of a multicellular organism have the same genes whether it is a liver cell, heart cell, muscle cell etc.

11.3 Differentiated cells may retain all of their genetic potential


Table 11.2


• In multicellular eukaryotes, cells become specialized as a zygote develops into a mature organism

– Different types of cells make different kinds of proteins

– Different combinations of genes are active in each type

11.2 Differentiation yields a variety of cell types, each expressing a different combination of genes

CELLULAR DIFFERENTIATION AND THE CLONING OF EUKARYOTES


• A chromosome contains a DNA double helix wound around clusters of histone proteins

• DNA packing tends to block gene expression

11.6 DNA packing in eukaryotic chromosomes helps regulate gene expression

GENE REGULATION IN EUKARYOTES


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)


• An extreme example of DNA packing in interphase cells is X chromosome inactivation

11.7 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


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

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

11.8 Complex assemblies of proteins control eukaryotic transcription

Enhancers

DNA

Activatorproteins

Otherproteins

Transcriptionfactors

RNA polymerase

Bendingof DNA

Transcription

Promoter

Gene

Figure 11.8


Exons

DNA

RNA splicing or

RNAtranscript

mRNA

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

11.9 Eukaryotic RNA may be spliced in more than one way

Figure 11.9


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

– The protein may need to be activated in some way

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

Figure 11.10

Folding of polypeptide andformation of S–S linkages

Initial polypeptide(inactive)

Folded polypeptide(inactive)

Cleavage

Active formof insulin


• Each stage of eukaryotic expression offers an opportunity for regulation

– The process can be turned on or off, speeded up, or slowed down

• The most important control point is usually the start of transcription

11.11 Review: Multiple mechanisms regulate gene expression in eukaryotes


Chromosome

GENE

RNA transcript

mRNA in nucleus

mRNA in cytoplasm

Polypeptide

ACTIVE PROTEIN

GENEExon

Intron

TailCap

NUCLEUS

Flowthroughnuclear 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


– So a carrot plant can be grown from a single carrot cell

Figure 11.3A

Root ofcarrot plant

Adult plantRoot cells cultured in nutrient medium

Cell divisionin culture

Single cell

Plantlet


• Early experiments in animal nuclear transplantation were performed on frogs

– The cloning of tadpoles showed that the nuclei of differentiated animal cells retain their full genetic potential

Figure 11.3B

Tadpole (frog larva)

Intestinal cell

Frog egg cell

Nucleus

Nucleus

UV

Nucleusdestroyed

Transplantationof nucleus

Eight-cellembryo

Tadpole


• In reproductive cloning, the embryo is implanted in a surrogate mother

• In therapeutic cloning, the idea is to produce a source of embryonic stem cells

– Stem cells can help patients with damaged tissues


Removenucleusfrom eggcell

Donorcell

Add somaticcell fromadult donor

Grow in culture to producean early embryo (blastocyst)

Nucleus fromdonor cell

Implant blastocystin surrogate mother

Remove embryonic stem cells from blastocyst andgrow in culture

Clone of donoris born(REPRODUCTIVEcloning)

Induce stemcells to formspecialized cellsfor THERAPEUTICuse


• The first mammalian clone, a sheep named Dolly, was produced in 1997

– Dolly provided further evidence for the developmental potential of cell nuclei

Figure 11.3C


• Scientists clone farm animals with specific sets of desirable traits

• Piglet clones might someday provide a source of organs for human transplant

11.4 Connection: Reproductive cloning of nonhuman mammals has applications in basic research, agriculture, and medicine

Figure 11.4


• Adult stem cells can also perpetuate themselves in culture and give rise to differentiated cells

– But they are harder to culture than embryonic stem cells

– They generally give rise to only a limited range of cell types, in contrast with embryonic stem cells

11.5 Connection: Because stem cells can both perpetuate themselves and give rise to differentiated cells, they have great therapeutic potential


• Differentiation of embryonic stem cells in culture

Figure 11.5

Culturedembryonicstem cells

Different cultureconditions

Different types ofdifferentiated cells

Heart muscle cells

Nerve cells

Liver cells


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

– Our earliest understanding of gene control came from the bacterium E. coli

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

GENE REGULATION IN PROKARYOTES

Figure 11.1A


• In prokaryotes, genes for related enzymes are often controlled together by being grouped into regulatory units called operons

• Regulatory proteins bind to control sequences in the DNA and turn operons on or off in response to environmental changes


• The lac operon produces enzymes that break down lactose only when lactose is present

Figure 11.1B

DNA

mRNA

Protein

Regulatorygene

Promoter Operator Lactose-utilization genes

OPERON

RNA polymerasecannot attach topromoter

Activerepressor

OPERON TURNED OFF (lactose absent)

DNA

mRNA

Protein

OPERON TURNED ON (lactose inactivates repressor)

LactoseInactiverepressor

RNA polymerasebound to promoter

Enzymes for lactose utilization


• Two types of repressor-controlled operons

Figure 11.1C

Tryptophan

DNA

Promoter Operator Genes

Activerepressor

Activerepressor

Inactiverepressor

Inactiverepressor

lac OPERON trp OPERON

Lactose

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