Presentation of Cell Differentiation

8/10/2019 Presentation of Cell Differentiation

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Cell Differentiation

Dr. Ritesh Vaidya

Senior Lecturer

Mehsana Urban bank Institute of

BiosciencesGanpat University

Kherva-382 711


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Introduction

It is known that the protoplasm of different

parts of the embryo is somewhat different. The

initial differences in the protoplasmic regions

may be supposed to affect the activity of

genes. The genes will then in turn affect the

protoplasm, which start a new series of

reciprocal reactions. In this way we can pictureto ourselves the gradual elaboration and

differentiation of the various regions of the

embryo.

-T. H. Morgan, 1934


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Introduction

Cell differentiation is

the process by which

stable differences

arises between cells.

All higher organisms

develop from a

single cell, thefertilized ovum,

which gives rise to

the various tissues

and organs.

Mature frog oocyte


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Events of Cell Differentiation

In most animal species females produce large

unfertilized eggs that contain most of the materials and

nutrients required to form an embryo. Development is

triggered by fertilization. The sperm contributes a small,condensed nucleus (male pronucleus), which rapidly

enlarges in the egg cytoplasm, fuses with female

pronucleus, and finally divides.

The fertilized egg then undergoes a series of vary rapid

cyclesconsisting of DNA synthesis followed by celldivisions.

These divisions are called cleavage, because, unlike

normal cell division, the cytolasm is partitioned without

growth.


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Events of Cell Differentiation

Blastula- The cells form ahollow sphere (blastula) inwhich tissues are not yetevident.

Gastrulation- Some of thecells then invaginate in aseries of cell movementknown as gastrulation, andthe first sigs ofmorphological differentiationappear.

These complex changestake place in acomparatively short time.

Xenopus laevis - aswimming tadpolecontaining most

differentiated tissues suchas blood, nerve, eye, muscle


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Variable Gene Activity

Cell specialization involves the preferential

synthesis of some specific proteins such ashemoglobin in erythrocytes, antibodies in

plasma cells and ovalbumin in oviduct.

Each eukaryotic cell expresses only a small

percentage of the genes it contains, and cellsof different tissues express different sets of

genes.

Necessary to understand the mechanisms of

gene regulation.


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Whether all genes, present in a cell are tissue specific?

The hou sekeeping genes Expected to be active in all types

of cells.

Required for building membranes, ribosomes, mitochondria andglycolytic enzymes, which are components common to all types

of cells.

Luxury funct ions The genes that are expressed

differentially, such as globin, ovalbumin and immunoglobulins.

Better knowledge of cytoplasmic functions is also important forclarifying how the initial differences between cells are

established in early embryos.

Cytop lasm ic determ inants It is believed that the cytoplasm

of most eggs contains cytoplasmic determinants of

development which at some point become unequally distributedamong the cells of an embryo and subsequently change the


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Characteristics of Cell Differentiation

The differentiated state is StableOnce the differentiatedstate is established, it is very stable and can persist

throughout many cell generations.

e.g. Nerve Cells (Neuron), skin cell

These persistent changes are very different from the type of

regulation involved in enzyme induct ion and repression in

bacteria, which is specially designed to respond rapidly to

changes in the environment.

Cell differentiation is induced by various stimulus The

cell differentiation is induced in the organism by variousstimuli, but once it has been established, it can persist even in

the absence of the stimuli.


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Determination can precede Morphological Differentiation

Before it is possible to recognize morphologically that a cellhas differentiated, there is a period during which the cell is

already committed to a particular change.

After this determination has been made, the cell will

differentiate along a specific pathway even if several cell

generations intervene before overt morphologicaldifferentiation.

e.g. The imaginal discs of Drosophi la

The discs are groups of cells that are present in the larva in

an undifferentiated form, but that upon metamorphosis willgive rise to legs, wings, antennae and so forth.

Disc transplantation experiment by Hadorn.


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Cell Differentiation Results from stepwise

decisions that are genetically controlled. Understanding the complicated patterns of cell arrangements can be

made simple by the use of genetics, which allows the analysis of the

effect of single gene o development. T.H. Morgan discovered famous white-eyed fly.

Morgan realized that development could not be understood before

understanding how genes worked, and so he began studying genes

by obtaining and analyzing mutants.

The most interesting mutations are those that can, by inactivating asingle gene, change one segment of the body into a different one.

These are called homeot ic mutat ions.


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Adult flies have a head

(made of six segments),

three thorasic segments

(pro, meso and metathorax)

and eight abdominal

segments.

Each thorasic segments has

a pair of legs, and the

second thorasic segment

also has a pair f wings. Thethird thorasic segment does

not have wings in flies but

instead a pair of halters,

which are small drumstick-

shaped stumps used fore uilibrium durin fli ht.


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A mutant four winged fly

generated by a mutation in

the so-called bi thoraxlocus.

A single mutation changes a

haltere into a wing. The

bithorax locus is a complex of

many genes. Some of these

genes suppress the formation

of lags in the abdominal

segments, which suggeststhat insects evolved from

ancestors such as milipedes

with a pair of legs in each

segment.

Another homeotic mutation

called antennapedia


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Local izat ion o f Cytoplasm ic Determinants in

Eggs


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Experimental Embryology 100 years Ago

O. Hertwig in 1875 showed that the sea urchin sperm

donated a nucleus , which later fused with the female one.

Thus it seemed that the nucleus would contain the hereditary

material.

Weissmannproposed his theory of heredity in the year 1883

and proposed that it is the germ cells that carry heredity and

that the body or soma is a mere offshoot of the germ cellswhose main function is to carry the germ cells.

Although Weissmanns views on heredity were essentially

correct, his theory on development turned out to be wrong. He

proposed that the fertilized egg had all the information to

make an individual, but that with each successive division part

of the information was lost.

Weissmann thought that in the end some cells retain only

genetic material to make a specific type of tissue, and

lose all other genetic material.


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The Genome Remains Constant during Early Development-

Nuclear Transplantation

Experimental embryology began in 1888 when Roux killedone of the cells of the frog embryo at the two-cell stage with a

hot needle. He observed that the other cell gave rise to a half-

embryo, usually a right or a left half.

In 1892 Driesch separated the two cells of a sea urchin

embryo and found that both could give rise to complete,although smaller, embryos.

Roux experiment leaving the dead cell in situ interfered with

the invagination of cells during gastrulation.

J. B. Gurdon has carried out nuclear transplantationexperiments in the frog egg.

Xenopus laevis unfertilized eggs can be irradiated with

ultraviolet light to destroy the endogenous nuclei and can then

be injected with a singleXenopusdiploid nucleus.


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Cytoplasmic Localizations May Determine the Initial

Differences between Embryonic Cells If the information contained in the cleavage nuclei is identical, then

the initial differences between cells should reside in the cytoplasmthey inherit.

In some eggs the segregation of cytoplasmic components, which are

thought to be able to affect the activity of genes, is particularly clear.

In Dentalium, the cytoplasm of the vegetal pole is extended

transiently during the first and second cleavage. This polar lobe

lacks a nucleus and after cell division is incorporated into one of the

blastomeres.

The cell that inherits the polar lobe cytoplasm eventually gives rise

to the mesoderm.

E. B. Wilson (1904) removed the polar lobe at the two-cell stage by

sucking the eggs up and down a thin pipette. He found that the

lobeless embryos lacked mouth, shell gland, and foot, as well

as other mesodermal tissues, and he concluded that the polar

lobe cytoplasm contains mesodermal determinants.


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Germ Cell Determinants

Best example of determinants in development is provided by the

germ plasm.

Amphibians and other eggs contain in their vegetal pole a

specialized region of cytoplasm that can be recognized

morphologically by the presence of special granules.

Drosophila eggs have an equivalent region located in the posterior

pole of the egg which is therefore called the poleplasm. This

cytoplasm has the property of inducing germ cell formation i.e. those

cells that contain the germ plasm will eventually become the germ

cells of the new organisms.

When the posterior poles of eggs are irradiated with ultraviolet light,

sterile animals were obtained. If UV-treated eggs are injected with

pole-plasm of normal eggs, fertile flies are obtained.

If cytoplasm containing the germ cell determinants is injected into

the anterior part of a Drosophila egg, germ cells develop in an

anterior position.


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Egg Stockpile Materials for Use During Early Development

Eggs are in general very large cells that stockpile many of the

molecules required for early development. Xenopus egg contains about 1,00,000 times more RNA

polymerases, histones, mitochondria and ribosome than a

normal adultXenopus somatic cell.

The reason for accumulating these ready-made materialsduring oogenesis, rather than making them de novo

during early embryogenesis is the extraordinary rapid

rate of cell division during cleavage.

This rapid pace allows little time for new RNA and protein

synthesis, but it is during this period that the first differencesbetween cells are established.

Presumably most of the developmentally important

substances are made during oogenesis and stored in the egg.


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The Mid-Blastula Transition The first cleavage of the Xenopus egg takes place 90 minutes after

fertilization.

The 11 subsequent cleavages take place synchronously every 35minutes. This compares with a doubling time of 24 hours for an adult

frog somatic cell.

In order to achieve this extraordinary short cell cycle, cleaving

embryos increase the number of origins of replication in their DNA

which shortens the S phase and omit the G1 and G2 phases of thecell cycle, so that the end of DNA synthesis is immediately

followed by mitosis.

During this initial phase of development there is no transcription of the

DNA.

After 12th

cleavage, or 4000-cell stage, the cell cycles become longerand asynchronous, the cells become motile and RNA synthesis starts.

The turning point in development is called mid -blastula transi t ion.

several types of RNA start to be expressed simultaneously at the mid-

blastula transition.


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The Mid-Blastula Transition

Why they are not expressed earlier?

There is a critical ratio of nuclei to egg cytoplasm beforetranscription can start.

Polyspermic fertilization or the injection of large amounts

of extra DNA will induce the mid-blastula transition

earlier. It seems thatXenopuseggs may have a substance that

binds to chromatin and turns it off transcriptionally.

The amount of this substance would be sufficient to

block up to 4000 nuclei, but then become exhausted andexpression of the genome would start.


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Cell Interactions Become Increasingly Important as

Development Advances

Cytoplasmic determinants laid down in the egg during

oogenesis are undoubtedly very important in establishing

early differences between cells, they cannot entirely explain

development.

No evidence of cytoplasmic localization in mammalian eggs.

As the development advances, cell interactions becomeincreasingly important.

At gastrulat ion, extensive cell movements and migrations

occur, and different types of cells interact with each other in

the phenomenon known as embryon ic induct ion.


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Cell Interactions Become Increasingly Important as

Development Advances

The influence of neighbouring cells on cell differentiation can

sometimes be quite dramatic.

Tetratomasare tumors of the germ cells, which are the most

frequent tumors of human testis or ovary. When they are proliferating rapidly, they remain

undifferentiated and are highly malignant, but sometimes they

have patches of several tissues, such as teeth, hair, muscle

and nerves.

In some ways tetratomas are like disorganized embryos.


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Nucleocytoplasmic

Interactions


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Introduction Nucleocytoplasmic Interaction

Nucleus and cytoplasm are interdependent; one cannot survuve

without other.

The cytoplasm provides most of the energy for the cell through

oxidative phosphorylation and anaerobic glycolysis, and the

cytoplasmic ribosomes contain most of the machinery for

protein synthesis. The nucleus provides mRNA and also supplies

the other important RNA molecules.

Nucleus is necessary for cell survival- Waller (1852).

Nerve cells containing the nucleus survived, while the axons

degenerated.

In Protozoa, enucleated cells sustain most cellular activities.However, these cells generally survive for only a limited

time and cannot multiply.

Example of extreme case of survival is Acetabularia.


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Red Blood Cell Nuclei Can Be Reactivated by Cell Fusion

Cells can be fused through the use of inactivated Sendaivirus (a member of the parainfluenza viruses) and other

agents that affect membrane structure, such as

polyethyleneglycol and lysolecithin. Through this

techniques a nucleus can be placed in a differentcytoplasmic environment.

Heterokaryon- A single containing nuclei of two types.

Synkaryon- Both nuclei enter in mitosis synchronously,

divide and produce a hybrid cell line. The cells of ahybrid cell line have a single nucleus containing

chromosomes from both parental nuclei.


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H. Harris (1965)- Chick erythrocyte nuclei are reactivated when

fused to He La cells. These heterokaryones are of interest because the erythrocyte

nucleus does not normally synthesize RNA or DNA.

Chick erythrocytes are terminally differentiated cells that have a

highly condensed nucleus and are destined to die.

When fused to He La cells, the chick erythrocyte nucleusincreases 20 times in volume, disperse its chromatin, resumes

RNA synthesis, develops a nucleolus, and eventually replicates

its DNA.

The process is accompanied by the uptake of large amounts of

human nuclear proteins which are thought to reactivate theerythrocyte nucleus.

These experiment clearly show that the synthesis of

macromolecules in a nucleus is controlled by the cytoplasmic

environment. Even though the erythrocytes are terminally

differentiated cells, they can resume RNA and DNA synthesis.


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Only the cell cytoplasm is required to reactivate the chickerythrocyte nucleus. This was established by fusing erythrocytes to

enucleated He La cells, which were still able to reactivate the nuclei.

Populations of enucleated cultures cells can be obtained by

centrifugation after treatment with cytochalasin B, a drug that

inhibits actin microfilaments. The detached nucleus in the pellet is surrounded by the cell

membrane and a small amount of cytoplasm and is called a

Karyoplast. The enucleated cytoplasm is called a cytoplast.

Cells enucleated by the cytochalasin method are viable for at least

two days after enucleation and perform many cell functions such ascellmovement, pinocytosisand contact inhibition.

Thus it is evident that many cytoplasmic functions are

independent of the cell nucleus.


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Red Blood Cell Nuclei Can Be Reactivated by Cell

Fusion

Because karyoplasts are still surrounded by the cell

membrane, it is possible to fuse them to a different cytoplasmby using Sendai virus.

Reconstituted cells, which arise from the fusion of a

karyoplast and a cytoplast can survive longer than the

enucleated cytoplasm, can synthesize RNA, and in some

cases can undergo cell division.

It is possible to activate latent genes coding for specific

proteins by fusion of differentiated cells.

Rat Hepatoma cells (Secrets albumin) fused with Mouse

lymphocytes (do not produce albumin). Clones of this hybrid are able to produce both rat and mouse

albumin.


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Cell Fusion Yields Pure Antibodies of Medical

Importance

When animals are injected with a macromolecule of a shape

that is recognized as foreign to that individual, antibodiesappear in the serum several days later.

Many different antibodies appear in the serum of an

immunized animal, each one recognizing a different part of

the antigensshape.

Furthermore, individuals of the same species have differentimmunological responses, so that two antisera directed

against the same antigen can in fact be very different.

This is a major problem in medicine because, success or

failure of an organ transplantation depends on whether thedonor and the recipient patient have the correct

histocompatibilityantigens on the cell surface.

Antibodies that can be used throughout the world as

standardized diagnostic reagents are highly desirable.


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Cell Fusion Yields Pure Antibodies of Medical Importance

Antibodies are produced by the lymphocytes. Each antibody-

producing cell can synthesize only one type of antibody.

Lymphocytes do not multiply in culture, but G. kohler and C.

Milstein developed a technique whereby a single antibody-

producing cell can be propagated indefinitely in culture by

hybridization with a tumor cell.

Using Sendai virus, the lymphocytes from the spleen of the

mouse are fused to a mouse cell line derived from plasma cell

tumors.

The fusion with the tumor cell immortalizes the spleen

lymphocyte, which can be grown indefinitely in culture orinjected into the mice, where the cells produce secreting

tumors that can be maintained by serial transplantation.

Because all of the cells of one clone are derived from a single

lymphocyte, amonoc lona l ant ibody

of high purity isproduced.


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Gene Expression by Somatic Nuclei Is Reprogrammed in

nopus

Oocytes

Frog oocytes are growing egg cells, obtained from the abdominal

cavity of frogs, which due to their large size (1.2 mm) and tolerance

for micromanipulation have been used in numerous microinjection

experiments.

Oocytes are active in RNA synthesis but do not synthesize DNA, ad

if somatic cell nuclei are injected into them, the transplanted nuclei

also show this type of synthetic activity.

The oocyte cytoplasm not only affects the pattern of

macromolecular synthesis but is also able to reprogram the

expression of individual genes in transplanted nuclei.

Nuclei of He La cells were injected intoXenopusoocyte.

The injected nuclei resemble the oocytes own nucleus

morphologically. The oocyte cytoplasm reprograms the gene

expression of the injected nuclei in such a way that only those genes

that are normally active in oocytes are expressed.


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Gene Expression by Somatic Nuclei Is Reprogrammed in

nopus

Oocytes

In one such experiment, Xenopus cultured kidney cell

nuclei were injected into oocytes of a different amphibian

species the Salamander (Pleurodeles). Those genes that are normally expressed in kidney cells

but not in oocytes became inactive after injection into

Pleurodelesoocytes.

More importantly, some oocyte-active genes that were notexpressed by the kidney cell nuclei were activated by the

oocyte cytoplasm.

The work with cell fusion and Xenopus oocytes

suggests that the cytoplasm of all cells contains

components that determine the state of activity of

nuclear genes. If these components were distributed

asymmetrically among daughter cells, they could play

a crucial role in the establishment of cell

differentiation.


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Molecu lar Mechanisms of Cel lDif ferentiat ion


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There is not a single eukaryotic gene for which we understand

for certain how it is expressed in some tissues and not inothers.

More we known about eukaryotic gene regulation, more it

becomes clear that eukaryotic genes are controlled at multiple

levels.

Two main mechanisms may be envisaged.

(1) Those in which genes are differentially activated at the

Transcriptional, Post-transcriptional or Translational level

or

(2) Those in which the genes themselves are altered byAmplification, DNA rearrangements, and Methylation.

These mechanisms are known to operate in different cell

systems.

Control at the Level of Transcription


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The nuclear transplantation experiments showed that in most cases

genes are not irreversibly lost during cell differentiation. Therefore

the differences between specialized cells must be explained in

terms of variable gene activity.

Transcriptional control is probably the most important mechanism

and the clearest example is provided by polytene chromosomes,

in which transcription can be directly visualized in the form of

puffs.

Transcriptional control has been clearly proven also for those genes

that code for abundant specialized proteins such as globin,

ovalbumin and silk fibrin.

To make hybridization probes for these genes the mRNA is copied

into DNA with reverse transcriptase (producing complementary DNA

or cDNA) and then the cDNA is cloned into plasmid.

With these hybridization probes, transcripts from the corresponding

gene can be detected only in those cells that produce the protein

product.


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Enormous amounts of protein can be produced from the

transcriptional induction of a single protein-coding gene because

stable mRNA molecules can be translated many times.

A fully induced chicken ovalbumin gene makes 17 mRNA molecules

per minute (24,500 molecules a day).

A single silkworm fibroin gene makes 1010 protein molecules in the

course of a few days; roughly 105

times each. Chromatin structure is one possible level at which transcription

may be controlled.

DNA methylation has also been considered a likely candidate to

control transcription. However, there are some species (e.g.

Drosophi la) in which the DNA is never methylated, and thesignificance of DNA methylation is still under debate.

The post transcriptional processing of the transcripts is also of

considerable regulatory importance.

Translational Control


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Translational Control

One possibility that has been considered is that cells have

mechanisms by which some mRNAs can be translated in a given

cell type but not in others. Microinjection studies, however, have

shown that living cells can translate wide variety of mRNAs. MicroinjectedXenopusoocytes can translate SV 40viral mRNAs.

In fact, the frog oocytes can also efficiently transcribe and process

injected DNAs, copying them into mature mRNAs, provided that

the DNA is injected into the nucleus, which contain RNA

polymerases and other factors required for transcription.

When the living oocytes are used solely as a test tube for translation

of mRNA, the mRNAs are introduced into the cytoplasm, which

contains the protein synthesis machinery.

Many mRNAs of animal and plant origin are efficiently translated in

oocytes such as those coding for globin, immunoglobulins,

thyroglobulin, interferon, collagen, tobacco mosaic virus coat

proteins and many others.

This finding suggests that oocytes do not have a mechanism that

excludes the translation of certain injected mRNAs.

Gene Amplification- A Rare Event


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Gene Amplification A Rare Event

One way of obtaining differential gene activity would be to increase

the number of copies of a specific gene.

There are three circumstances in which gene amplification is known

to occur.

(1) Ribosomal DNA amplification observed in the oocytes of

amphibians and insects.Xenopusoocytes selectively replicate their

rDNA genes; a mature oocyte has 2 million copies of them

(compared to 900 for a diploid somatic nucleus) and 1000 nucleoli in

order to produce the vast number of ribodsomes (1012) contained in

a single egg.

(2) DNA replication observed in certain puffs of the dipterian

Rhynchosciara salivary gland polytene chromosomes. DNA

amplification can be visualized by an increase in the amount of DNA

in the particular polytene band when the puff collapses after the

phase of active RNA synthesis is over.

DNA puffs in Rhynchosciaracode for salivary proteins.


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(3) The third case is that of the egg shell (chorion) proteins of

Drosophi la. The egg chorion is formed by the folicle cells during a

period of five hours. It has been found that the folicle cells replicatemore of a 90-kilobase segment of DNA containing the chorion

protein genes, so that finally it is 16 times more abundant than the

rest of the DNA.

Although it is clear that selective gene amplification does occur, in

all known cases the specially amplified DNA is not passed on tofuture cell generations. The larval polytene salivary glands die at

metamorphosis, the follicle cells die when the egg is laid, and the

amplified oocyte rDNA is not inherited by the frog embryo.

Nucleic Acid Hybridization experiments have shown that there is

only one copy per haploid genome of globin or chicken ovalbumingenes in all tissues, regardless of whether the gene is preferentially

expressed. In other words, a single globin gene, when fully

activated, can give rise to all the globin required by a red blood cell.

Gene amplification does not seem to be a widespread

phenomenon that can explain most cases of cell differentiation.


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Drug Resistance Can Induce Gene Amplification

Cells in culture can be forces to amplify genes for certain

proteins by selective pressure. Methotrexate is an analogue of folic acid that inhibits the

enzyme dihydrofolate reductase (DHFR).

The product of DHFR action (tetrahydrofolate) is required for

purineand thymidinebiosynthesis.

Methotrexate is used for the clinical treatment of rapidly

dividing tumor cellsand it has been known for a long time

that tumor cells eventually become resistant to the drug.

Although the amplification of genes can clearly be forced

by drugs or other selective agents, there is no evidence that itplays a role in any developmental process leading to normal

cell differentiation.

Transposable Genes in Yeast and Trypanosomes


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a sposab e Ge es east a d ypa oso es

DNA rearrangements cause stable changes in patterns of gene

expression.

e.g. The mating-type switch in yeast.

Yeast generally propagates as haploid cells, but at a low frequency twocells may fuse and produce diploids that are able to produce spores if

life becomes hard. The cells can fuse, however, only if they are of

opposite sex or mating type.

The mating type are a and .

When haploid strains of one mating type are cultured, some cells aretransformed into cells of the opposite sex with a certain frequency. If the

latter are cultured again, they can switch again. These changes are

stable and persist over several cell generations.

The genetic information for the or a mating types is stored in silent

placeson yeast chromosomes. These sto rage sites are cal led si lentcasset tes, and their expression is activated only when these genes are

transported into the expressed position on the MATmating locus.

Another example of transposition of silent genes into an expression site

is provided by the surface antigens of Trypanosom a brucei.

Control of Immunoglobulin Secretion by Differential RNA


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Control of Immunoglobulin Secretion by Differential RNA

Processing

After a B lymphocyte has undergone DNA rearrangement, it expresses

IgM (and usually IgD as well) on its membrane.

This cell will stay quiescent until the binding of specific antigen to itssurface induces it to proliferate vigorously and to differentiate further.

Proliferation will produce clones of cells producing the same type of

antibody. The immune system works by the clonal select ion (by

proliferation) of those B cells that have an antibody on their surface that

has a good fit with the shape of the antigen. After a cell has been stimulated by antigen, it starts secreting IgM. This

switch from membrane-bound to secreted IgM is controlled at the

level of RNA processing.

After stimulation by antigen, the transcriptional unit is polyadenylated

earlier, eliminating the membrane exons and thus the hydrophobic

COOH region. The resulting IgM protein will now be secreted into the

surrounding medium.

Thus RNA processing is an important level of control in the

expression of immunoglobulin genes.

Cell Differentiation in Adult Tissues


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Cell Differentiation in Adult Tissues

Clearly gene expression can be regulated at many levels. We still do

not know why a globin gene (or any other gene) is active in red blood

cells and inactive in other cells.

It may turn out that the principles involved in the initial establishment ofthe differentiated state will be very different from those involved in the

maintenance of differential gene expression.

In differentiation of adult tissues it is frequently observed that only

one of the daughter cells becomes specialized; the other one

remains as a stem cell, which is able to divide again. During nerve cell differentiation in grasshoppers, some cell divisions

result in the formation of a neuron (ganglion cell) and a stem cell

(neuroblast), which are always in the same position and

morphologically recognizable. By introducing a needle at mitosis, it is

possible to rotate the spindle and chromosomes 180 degrees; but

despite this rotation, the resulting daughter cells still have the

neuron and stem cell in the normal position.

Thus, the ability to become a neuron does not depend on a

particular chromosome set but rather on the type of cytoplasm

inherited by the daughter cell.

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