Manipulating genes

© Sir Ralph Riley

Ever since humans have been domesticating animals and raising cropsthey have been (unwittingly) manipulating genes

By cross pollination and cross breeding they have tried to introduce the beneficial characteristics of one variety into a different varietyof the same species*

For example, a bull born to a cow that has a good milk yield, might be mated with a cow from a low-yielding stock, in the hope that the offspring will inherit the characteristics which lead to a high milk yield

This has been done for thousands of years without any knowledgeof genes or the mechanism of inheritance

Cross breeding

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In the following (hypothetical) example, a variety of high yielding wheat which has poor resistance to disease…

…is crossed with a variety which has good disease resistance but gives a poor yield

The gene* for ‘high yield’ is represented by H

The gene for ‘low yield’ is represented by h

The gene for ‘good disease resistance’ is represented by R

The gene for ‘poor disease resistance’ is represented by r

Crossing

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HHrrhigh yieldlow resistance

pollengrain

ovule

hhRRlow yieldhigh resistance

The F1 consists ofplants with high yieldand good resistance

zygote

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Can you see any disadvantages in this method of manipulating genes ?

Try working out what would happen if you tried to breed from the F1

Work out the various gene combinations in the gametes

Put them into a 4x4 Punnett Square

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F1 cross HhRr x HhRr

Possible combinationof genes in gametes HR Hr hR hr

HR Hr hR hr

HR

Hr

hR

hr

HHRR HHRr HhRR HhRr

HHRr HHrr HhRr Hhrr

HhRR HhRr hhRR hhRr

HhRr Hhrr hhRr hhrr

The F1 does not breed true. Of the 16 possible combinations of genes, 7 do not have the combined beneficial genes

F1 cross 6

a b c d e

a x b = c c x d = e

Hybrid wheat (c) was crossed with wild wild grass (d) to give hybrid wheat (e) used for making flour andbread

Manipulating genesby cross breeding

Wheat variety (a)was crossed with wild grass (b) to givehybrid wheat (c)

wheat

© Sir Ralph Riley

7

Interbreeding transfers the complete genome of one variety toanother.

This means that many new and unpredictable gene combinationsmay be formed in addition to those intended

This method of genetic recombination can take place only betweenvarieties of the same or closely related species

Genetic engineering makes it possible to transfer single genes

The genes can also be transferred from one species to a totallydifferent species

Genetic engineering

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There are several ways in which genes from one organism can be inserted into a different organism

They can be coated on to microscopic gold particles and ‘fired’into the cells

They can be delivered by viruses

They can be transmitted by using structures, called plasmids, present in bacteria

For example, the human gene for making insulin can be transferred to bacteria, which are then allowed to reproduce in a culture mediumfrom which the insulin can be extracted

Plasmids

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in addition to a loop of DNA… …bacteria also contain numerous rings of DNA called plasmids

cell wall

cytoplasm

cell membrane the plasmids can be extracted and used forgenetic engineering

0.001mm

A bacterium 10

plasmid

restriction enzyme cutsplasmid

the samerestriction enzyme cutsthe insulin geneout of thehuman DNA

human DNAstrand

insulin gene

Inserting a gene

the insulin geneis inserted intothe plasmid

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The recombinant plastids are inserted into a bacterium *

the insulin gene makes thebacterium produce insulin

Recombinant plastids

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Only about 1 in 100,000 bacteria take up the recombined plasmids

There are techniques for identifying and isolating these bacteria

The bacteria with the insulin gene are then allowed to reproduce in a culture solution from which the insulin can be extracted*

Human growth hormone can be made in a similar way

Factor VIII, needed by haemophiliacs, (blood clotting disorders)can be produced from hamster cells containing plasmids with the factor VIII genes

Chymosin, used for clotting milk in cheese-making, can be produced from yeast cells with recombinant plasmid DNA

Applications

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As well as producing useful substances from genetically altered cells, whole organisms can be genetically modified.Some examples are ….

A bacterial gene which makes an insecticide can be introduced intocrop plants, e.g. maize and cotton, to make them resistant to attackby moth caterpillars

A gene which confers resistance to herbicides has been insertedinto crop plants so that spraying kills weeds but not the crop plants

A gene introduced to oilseed rape makes the oil more suitablefor commercial processes, e.g. detergent production

Genes which control the production of human enzymes have beeninserted into sheep so that the enzymes can be recovered from their milk

Applications

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Genetic engineering does not always have to involve gene transfer between unrelated organisms

Genes in a single organism can be modified to improve theircharacteristics or their products

A gene for the production of ß carotene (a precursor of Vitamin A)has been introduced to rice to benefit countries where rice is thestaple diet and Vitamin A deficiencies are common*

The next slide shows tomatoes which have been genetically modified to suppress production of an enzyme which causes the fruit to soften as it ripens. This improves the keeping qualities

Applications

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Genetically modifiedGenetically modified tomatoesControl tomatoes

After storage After storage

© AstraZenecaTomatoes

Opponents of genetic engineering stripped the bark off these poplarsin order to kill them.

A gene had been inserted which softened the cell walls so that fewer

environmentally damaging chemicals were needed in paper-making.

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When organisms reproduce asexually, all the offspring receive a full set of genes from the parent.

As a result they are identical to each other and to the parent

Examples are Bacteria and single-celled organisms

Plants with vegetative reproduction by bulbs, corms etc.

Fungi

Some of the lower invertebrates

A population of identical individuals arising from asexual reproduction is called a clone

Cloning 18

A clone of crocuses

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Clone of crocuses

Next slide

Bacterial clone

Vertebrates do not reproduce asexually but clones can be producedartificially

In some cases this is done by transferring the nucleus from a body cell to an egg cell (ovum) from which the nucleus has been removed

The following slide illustrates one of the first successfultechniques for cloning a mammal

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cells in sheep A’smammary gland

one cellisolated

diploidnucleus

egg cell (ovum)from sheep B

nucleus removedthe two cells

are fused together *

embryo implantedin uterus of sheep C

cloned lamb born

cell division producesearly embryo

Dolly

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If the process becomes cheap and reliable it means that beneficial genes will be present in all the offspring, thus eliminating the chances of their being lost during conventional breeding

Before the early embryo is implanted in the surrogate mother, it canbe broken up into its individual cells. Each of these can develop into a new embryo

Sheep, pigs, horses, cows and, by now, probably many more animalshave been cloned

So far, this is being done on an experimental basis

Hundreds of embryos have to be prepared and implanted to obtain one or two successful births

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fertilised frog egg

cell division to forman embryo

growth and development toproduce tadpole and frog

at the 8-cell stage, any one of thesecells can develop into a frog

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8-cell frog embryo

cells separated

each cell can develop into a frog

Clone of frogs 24

The cells from the 8-cell embryo are called embryonic stem cells….

…because each one can form all the cells and tissues toproduce a complete frog

After the 16-cell stage, the cells lose this ability and can only produce specialised cells such as blood, bone and nerve cells

Cells capable of dividing to produce specialised cells arecalled stem cells

Specialised cells normally lose the power to divide and may have a limited life span

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The tissues produced by specialised cells usually contain some stem cells which retain the power of division

section through skin

epidermis

dermis

basal layerhair

fat layer

basal cells(skin stem cells)

these stem cells keepdividing and pushingnew skin cells to the outside

cells dividing

cells worn away

2mm

Skin stem cells

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stem cell in red bone marrowproduces ……..

red cells

several types of white cell

platelets

Blood stem cells

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Skin stem cells can normally give rise only to skin epidermal cells

Bone marrow stem cells can normally give rise only to 6 types of blood cell

But embryonic stem cells can produce all the cells of the body

Human embryonic stem cells can be obtained from 10 day embryos*

These embryonic stem cells can be cultured in a special nutrient solution

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section through a 10-day human embryo

0.5 mm

these cells will contribute to the placenta

these cells will form the embryo (stem cells)

stem cells cultured (cloned)

nutrient medium*

stem cells transferred to culture dish

Human ESCs

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All the cells in the body have a full set of genes

When the cells become specialised, they lose their ability to divideand many of the genes are ‘switched off’

For example, the genes for producing hydrochloric acid in a stomachcell would not be functional in a skin cell

Even though tissues consist mainly of specialised cells, most of themalso contain their own stem cells

It may become possible to treat stem cells from specialised tissues with hormones and growth factors that cause them to produce a wider range of specialised cells*

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Applications of stem cells

Most applications of stem cells are in the experimental stage, are undergoing clinical trials or have been tried on very few patients

Possibilities are

Replacement of damaged tissues such as heart muscle, skin,bone and cartilage

Treatment of disease, e.g. diabetes by injecting islet cells into the pancreas; or Parkinson’s disease by injecting nervestem cells into the brain

If the stem cells can be derived from the patient’s own tissue, rejection by the immune system is avoided

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Question 1

What are the possible gene combinations in the gametesFrom genotypes AAbb and aaBB ?

(a) Ab

(b) AB

(c) ab

(d) aB

Question 2Which of the following statements is correct?

F1 hybrids from cross breeding or cross pollination…

(a) …may not be able to reproduce

(b) …can contain genes from unrelated species

(c) …may contain unwanted gene combinations

(d) …may not breed true

Question 3

Genetic engineering can

(a) Transfer genes only within a species

(b) Transfer single genes between species

(c) Create new species

(d) Modify a species

Question 4The bacterial components which can be used to transfer genes are

(a) mitochondria

(b) DNA

(c) plasmids

(d) proteins

Question 5

DNA which has been genetically engineered is called…

(a) Engineered DNA

(b) Hybrid DNA

(c) Modified DNA

(d) Recombinant DNA

Question 6

Which of the following can be made by genetically engineered bacteria ?

(a) Human insulin

(b) Human growth factor

(c) Blood-clotting Factor VIII

(d) Blood platelets

Question 7

Which of the following could be described as a clone ?

(a) A litter of kittens

(b) A clump of daffodils

(c) A bacterial culture

(d) An F1 hybrid

Question 8

A cell is removed from cow P. An ovum is obtained from cow Qand its nucleus is removed. The cell from P is fused with the enucleated ovum from Q. The combined cell starts to form anembryo which is transplanted into the uterus of Cow R and in due course a calf is born. Which of these cows is the biological parent of the calf?

(a) P

(b) Q

(c) R

(d) The calf does not have a biological parent

Question 9

Which of these statements is correct ?

(a) All cells can produce new tissue

(b) Only stem cells can produce new tissue

(c) Stem cells can divide

(d) All cells can divide

Question 10

Embryonic stem cells differ from other stem cells because …

(a) They can produce only one type of tissue

(b) They can produce a complete organism

(c) They can produce all kinds of cell

(d) They cannot be cloned

Answer

Correct

Answer

Incorrect