Tagged: Cloning

Cloning in Mammals: A* understanding for iGCSE Biology 5.19 5.20

The most famous sheep in the world currently resides in a display cabinet at the National Museum of Scotland.  Dolly became internationally famous in 1996 as the first animal cloned using a nucleus from an adult animal.  Cloning experiments had been going on since the 1950s but previous to Dolly only embryonic cells could be used as the source of the donor DNA.

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So how was Dolly the sheep made?  She was a clone of an adult sheep and because the cell used to provide the nucleus from this adult donor was from an udder cell, the researchers decided to call her Dolly after the American singer Dolly Parton.  (No further explanation required but the photo of Dolly Parton below might give you a clue….)

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The technique used to make Dolly was given the catchy title somatic cell nuclear transfer.  A nucleus was taken from an adult cell in the udder (somatic cells are any cells whose DNA will not be inherited to the next generation) and this was fused with an enucleated egg cell (an egg cell which has had its nucleus removed).  This cell contains the nuclear DNA from the adult donor but will divide and develop into an embryo.  The embryo can then be implanted into the uterus of a surrogate sheep for it to complete its development.

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You can see that three sheep (all female in this instance) were used to produce Dolly.  The tissue cell donor provided the udder cell and all the nuclear DNA for the cloned sheep.  A second sheep provided the egg cell which was then enucleated. And finally a third sheep acted as a surrogate mother and so provided the uterus in which Dolly would develop to birth.  The researchers cleverly used three breeds of sheep so they could check that the various processes all worked correctly.

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Potential Applications of Cloned transgenic animals

If researchers combine this “Dolly technology” with the ability to genetically modify the embryonic cells produced, then it will be possible in the future to produce cloned transgenic animals.  A transgenic organism is one that contains DNA from more than one species.

One potential application is that cloned animals could be genetically-modified to produce human antibodies.  These polyclonal antibodies are useful in treating various medical conditions (see the New Scientist article below)

https://www.newscientist.com/article/dn2658-cloned-cows-produce-human-antibodies/

A second potential application could be to use cloned animals that have been genetically modified as potential donors for medical organ transplantation in humans.  You all know that in most developed countries there is a shortage of organs for transplants to patients who need them.  Added to this is the fact that the recipient’s immune system will recognise the donated organ as ‘foreign’ and so mount an immune response against it.  This rejection can be combatted with immunosuppressant drugs but these have nasty side effects.  Imagine if a replacement organ could be grown in a cloned animal for you, and this organ could express your own surface protein markers on its cell membranes.  No problem with shortage of donors, no problems of rejection!  Pigs are being used in research at the moment as pig abdominal organs are very similar in size to humans….  Here is a newspaper article looking at another possible way cloning technology could be applied to organ transplantation.

http://www.telegraph.co.uk/news/science/science-news/10774097/Breakthrough-in-human-cloning-offers-new-transplant-hope.html

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Micropropagation in Plants: A* understanding for iGCSE Biology 5.17 5.18

Cloning is the process of producing many genetically identical organisms.  I am going to write two posts on cloning: this one on cloning plants and then later, a second post on cloning animals.  As this blog grows to cover the entire iGCSE specification, it does mean that I will have to blog on some of the less exciting (to me at least) topics in Biology.  Micropropagation of plants does not get to me in the same way as other topics, so I apolgise in advance if this post is rather dull……

Cloning in Plants

Plants are relatively straightforward to clone, at least compared to animals.  Their bodies are made of fewer tissues and the genetic developmental program in plants is simple enough that many cells retain their totipotency into maturity.  The process of micropropagation, also known as tissue culture, is the main mechanism for cloning a plant.  Small samples from an adult plant are cut out.  These tiny explants can be sterilised in dilute bleach to kill surface bacteria and fungi, then grown in a suitable culture medium.  The explants divide by mitosis and grow but if the chemicals in the growth medium are correct, they will also start to differentiate into roots, shoots and leaves.

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The experiment that you will have done on this in school involves cloning cauliflower plants.  A cauliflower can be divided up into many thousands of explants and these are cultured on Murashiga and Skoog growth medium.  This M&S medium contains the minerals, sugars and amino acids needed to grow but it also contains plant growth substances such as auxins and cytokinins.  These growth substances can switch on root and leaf development and so rather than just producing a larger ball of cells called a callus, the explants will develop roots and shoots so they can be planted into compost.  Seeing as the only cell division in this whole process is mitosis, the plantlets will all form a clone of the original parent plant.

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In practice what tends to happen in these experiments is that the surface sterilisation does not work fully and so students end up growing a boiling tube full of bacteria and fungi.

Micropropagation does have commercial applications as it can produce large quantities of genetically identical plants.  This allows the plant breeder to produce plants all year round without needing pollinating insects.  The breeder can also guarantee the exact genetic make up of every plant as they are all a clone.  Sexual reproduction of course introduces genetic variation into the offspring and although this might be advantageous to plants in the wild, for a plant breeder it is unwelcome as many of the offspring may not grow as well or be as tasty to eat as their parent.  So if she starts with a plant with desirable features (good tasting fruit/disease resistance/easy to grow etc.) then every single plant in the clone will have the same desirable characteristics. Banana plants can be produced commercially by micropropagation.

If you find micropropagation interesting, you are a better biologist than me.  But I hope this post proves useful if not entertaining….

Feedback on Zondle Biology revision challenge part 2

The questions in this revision test were more challenging than last time.  I hope that players found them interesting and useful.

The plant transport questions at the start were well answered overall.  Osmosis is the only way water can ever cross a cell membrane and although active transport does occur in the root hair cells (pumping mineral ions such as nitrates into the cell against the concentration gradient), water cannot be directly pumped against its concentration gradient using energy from respiration.

The cloning questions were difficult but I think the low scores here were perhaps more to do with problems with my school wifi than with your abilities to answer them!  Micropropagation is the way that you learned when a small part of a plant is cut out, sterilised, washed and then added to a culture medium that triggers cell differentiation.  You probably did this experiment with explants from a cauliflower.  The aim was to produce whole new plants from these small explants.  This technique could not work with animals simply because animal’s bodies contain many more types of tissue and have a more complex internal architecture that requires a much more sophisticated genetic programme of development.

I want to talk about a few questions in the latter stages of the test that were not well answered.  I am sure there is plenty you can all learn from these.

The first was the one that asked you what was meant by a “diploid cell”.  More than half of you thought that  diploid meant having 46 chromosomes.  This is almost a trick question because of course in humans, diploid cells will have 46 chromosomes.  But diploid can be applied to any cell that has chromosomes found in homologous pairs.  The number 23 is only important to humans as for our species it is the number of homologous pairs of chromosomes found in our diploid cells.  Different species have differing numbers of pairs of chromosomes, some less than the number in humans but in many species they have more.

The second big idea question was the true or false question on whether energy is recycled in the ecosystem like carbon atoms.  It is vital you understand that there is absolutely no recycling of energy ever in an ecosystem.  Energy enters in the form of light energy being trapped by plants in photosynthesis and all this energy ultimately ends up as heat energy in the atmosphere.  To find out the details of how it gets there, please read the relevant sections on my blog.  Try the tag energy from the Tag cloud on the right of the screen.

There was one question in the quiz which not a single player answered correctly and it is the one about which type of cells produce antibodies.  Antibodies are made from a cell called a plasma cell.  Plasma cells secrete antibodies in large numbers to combat an infection.  Plasma cells are descended from B lymphocytes that have been activated by the presence of antigen.  This clonal selection theory is one of the most complicated bits in iGCSE Biology so make sure you have looked carefully at it.  The final question was about active v passive immunity.  This is not specifically mentioned in the specification so perhaps is a bit mean to include but if you can understand it properly, you understand how immunity works.  Passive immunity is the name for when antibodies are transferred, perhaps across the placenta for a foetus or in an injection as an adult.  Antibodies are made of protein and so do not exist for long in the blood – after a month or two they will all have been broken down and cleared from the blood.  So passive immunity cannot give long-lasting protection.  Active immunity is when memory cells are produced via a clonal selection response.  These memory cells can survive for an entire lifetime and so do provide long lasting protection.

By far the biggest thing you can learn from this quiz however was about virus structure.  I asked you whether “viruses are made from a different kind of cell not found in animals or plants – true or false.”  Almost everyone went for false but remember this can’t be correct:  viruses are definitely not made of cells!  They are much simpler than even the simplest cell and just consist of a protein coat with some genetic material (DNA or RNA) inside.  No cell membrane, no cytoplasm, no metabolism – just two chemicals associated into one simple particle.

Anyway I hope you enjoyed the quiz – look out for the next one on my Twitter feed and please use the comment facility on this blog to get in touch if you have any questions or want more explanations.

Feedback on Zondle Biology revision challenge part 2

The questions in this revision test were more challenging than last time.  I hope that players found them interesting and useful.

The plant transport questions at the start were well answered overall.  Osmosis is the only way water can ever cross a cell membrane and although active transport does occur in the root hair cells (pumping mineral ions such as nitrates into the cell against the concentration gradient), water cannot be directly pumped against its concentration gradient using energy from respiration.

The cloning questions were difficult but I think the low scores here were perhaps more to do with problems with my school wifi than with your abilities to answer them!  Micropropagation is the way that you learned when a small part of a plant is cut out, sterilised, washed and then added to a culture medium that triggers cell differentiation.  You probably did this experiment with explants from a cauliflower.  The aim was to produce whole new plants from these small explants.  This technique could not work with animals simply because animal’s bodies contain many more types of tissue and have a more complex internal architecture that requires a much more sophisticated genetic programme of development.

I want to talk about a few questions in the latter stages of the test that were not well answered.  I am sure there is plenty you can all learn from these.

The first was the one that asked you what was meant by a “diploid cell”.  More than half of you thought that  diploid meant having 46 chromosomes.  This is almost a trick question because of course in humans, diploid cells will have 46 chromosomes.  But diploid can be applied to any cell that has chromosomes found in homologous pairs.  The number 23 is only important to humans as for our species it is the number of homologous pairs of chromosomes found in our diploid cells.  Different species have differing numbers of pairs of chromosomes, some less than the number in humans but in many species they have more.

The second big idea question was the true or false question on whether energy is recycled in the ecosystem like carbon atoms.  It is vital you understand that there is absolutely no recycling of energy ever in an ecosystem.  Energy enters in the form of light energy being trapped by plants in photosynthesis and all this energy ultimately ends up as heat energy in the atmosphere.  To find out the details of how it gets there, please read the relevant sections on my blog.  Try the tag energy from the Tag cloud on the right of the screen.

There was one question in the quiz which not a single player answered correctly and it is the one about which type of cells produce antibodies.  Antibodies are made from a cell called a plasma cell.  Plasma cells secrete antibodies in large numbers to combat an infection.  Plasma cells are descended from B lymphocytes that have been activated by the presence of antigen.  This clonal selection theory is one of the most complicated bits in iGCSE Biology so make sure you have looked carefully at it.  The final question was about active v passive immunity.  This is not specifically mentioned in the specification so perhaps is a bit mean to include but if you can understand it properly, you understand how immunity works.  Passive immunity is the name for when antibodies are transferred, perhaps across the placenta for a foetus or in an injection as an adult.  Antibodies are made of protein and so do not exist for long in the blood – after a month or two they will all have been broken down and cleared from the blood.  So passive immunity cannot give long-lasting protection.  Active immunity is when memory cells are produced via a clonal selection response.  These memory cells can survive for an entire lifetime and so do provide long lasting protection.

By far the biggest thing you can learn from this quiz however was about virus structure.  I asked you whether “viruses are made from a different kind of cell not found in animals or plants – true or false.”  Almost everyone went for false but remember this can’t be correct:  viruses are definitely not made of cells!  They are much simpler than even the simplest cell and just consist of a protein coat with some genetic material (DNA or RNA) inside.  No cell membrane, no cytoplasm, no metabolism – just two chemicals associated into one simple particle.

Anyway I hope you enjoyed the quiz – look out for the next one on my Twitter feed and please use the comment facility on this blog to get in touch if you have any questions or want more explanations.