biology manipulating genes
TRANSCRIPT
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
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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
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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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