Differentiation & Variation (Edexcel A Level Biology (A) SNAB): Flashcards
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Define stem cell.
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Define stem cell.
A stem cell is a cell that can divide an unlimited number of times, and can differentiate into specialised cell types.
Define potency.
Potency is the ability of a stem cell to differentiate into different specialised cell types.
Define totipotency.
Totipotency is the ability to differentiate into any cell type, including the extra-embryonic cells of the placenta and umbilical cord.
Define pluripotency.
Pluripotency is the ability to differentiate into any cell type in the embryo, but not the placenta or umbilical cord.
What is the key difference between totipotent and pluripotent cells?
Totipotent cells can form all cell types including extra-embryonic tissue.
Pluripotent cells can form all embryonic cell types but not extra-embryonic tissue.
Which cells in early human development are totipotent?
The zygote and the embryonic cells up to the 16-cell stage.
Define multipotency.
Multipotency is the ability of an adult stem cell to differentiate into only a limited range of cell types.
Give an example of a multipotent adult stem cell.
Bone marrow stem cells, which produce different types of blood cell.
By what process do stem cells become specialised?
Differentiation.
Adult stem cells, such as those in bone marrow, are described as because they form only a limited range of cell types.
Adult stem cells, such as those in bone marrow, are described as multipotent because they form only a limited range of cell types.
True or False?
A pluripotent cell can develop into the cells of the placenta.
False.
Only totipotent cells can form the placenta; pluripotent cells can form embryonic cells but not extra-embryonic tissue.
Where are stem cells found in plants?
In areas of growth such as the shoots and roots (meristems).
Why are stem cells valuable in medicine?
They can differentiate into specialised cell types.
This gives them the potential to replace damaged tissues and cells.
Name the two sources of human stem cells used in medicine.
Embryonic stem cells.
Adult stem cells.
Give an example of an established stem cell therapy.
Treatment of leukaemia with a bone marrow transplant, which replaces the patient's blood-forming stem cells.
What is an advantage of embryonic stem cells over adult stem cells?
Embryonic stem cells can differentiate into almost any cell type.
Adult stem cells can only form a limited range of cell types.
Why do many people have ethical objections to using embryonic stem cells?
The embryos used could develop into human beings.
Using them destroys a viable embryo.
Why is using adult stem cells less controversial than embryonic stem cells?
The donor can give consent.
No embryo is destroyed, so fewer ethical objections are raised.
Why might donated adult stem cells be rejected by a patient?
The patient's immune system may recognise the cells as foreign.
It then attacks them, unless the donor is a close antigen match.
Why is using a patient's own stem cells ideal for treatment?
There is a much lower chance of rejection.
The cells are not recognised as foreign by the immune system.
What is the role of regulatory authorities in stem cell research?
They review and license research and monitor research centres.
They provide guidelines and advice, weighing the benefits against the ethical issues.
Embryonic stem cells used in research are often spare embryos left over from treatment.
Embryonic stem cells used in research are often spare embryos left over from IVF treatment.
True or False?
The use of embryonic stem cells is regulated in the same way in every country.
False.
Rules vary: some countries ban their use, while others such as the UK allow it under tight regulation.
Give two conditions being researched for stem cell therapies.
Spinal cord injuries, by replacing damaged nerve tissue.
Heart disease, by replacing damaged heart tissue.
Define differential gene expression.
Differential gene expression is when only certain genes in a cell are switched on, so different cell types are produced from the same genome.
How can cells with the same genome become different from one another?
Different genes are expressed in different cells.
Only the active genes are transcribed and translated into proteins that specialise the cell.
Outline how a stem cell becomes specialised.
Certain genes are activated and others inactivated.
Active genes are transcribed to mRNA and translated into proteins that modify the cell.
Define transcription factor.
A transcription factor is a protein that binds to a specific region of DNA and controls the rate of transcription of a gene.
What is the difference between an activator and a repressor?
An activator increases transcription by helping RNA polymerase bind.
A repressor decreases transcription by preventing RNA polymerase binding.
What does the lac operon control?
The production of the enzyme lactase.
Lactase breaks down lactose so it can be used for energy.
Name the parts of the lac operon.
A promoter, an operator, and the structural genes (lacZ, lacY, lacA).
What happens at the lac operon when lactose is absent?
The repressor protein binds to the operator.
This blocks RNA polymerase, so the structural genes are not transcribed and no lactase is made.
What happens at the lac operon when lactose is present?
Lactose binds to the repressor, changing its shape so it can no longer bind the operator.
RNA polymerase binds the promoter, the genes are transcribed and lactase is made.
Why is it an advantage for bacteria to only make lactase when lactose is present?
It avoids wasting energy and materials.
Lactase is only produced when there is lactose to break down.
In the lac operon, the repressor protein binds to the region to prevent transcription of the structural genes.
In the lac operon, the repressor protein binds to the operator region to prevent transcription of the structural genes.
True or False?
Once a cell has differentiated, the process is usually irreversible.
True.
Once specialised, a cell normally remains in its specialised form.
Define epigenetics.
Epigenetics is the control of gene expression by factors other than the DNA sequence, switching genes on or off without changing the genetic code.
Name the two main types of epigenetic modification.
DNA methylation.
Histone modification (such as acetylation).
How does DNA methylation affect gene expression?
Methyl (-CH₃) groups are added to the DNA.
This inhibits transcription factors from binding, switching the gene off.
How does acetylation of histones affect gene expression?
Acetyl groups make the DNA wind less tightly around the histones.
RNA polymerase and transcription factors can bind more easily, so the gene is switched on.
How does how tightly DNA is wound around histones control transcription?
Tightly wound DNA hides the gene from transcription factors, switching it off.
Loosely wound DNA exposes the gene, allowing transcription.
Give three factors that can cause epigenetic changes.
Any three of:
Smoking
Stress
Diet or exercise
How is epigenetics different from a mutation?
A mutation changes the DNA base sequence itself.
An epigenetic change only affects how the code is read, not the code.
How can epigenetic changes be passed on?
The epigenome is heritable.
When a cell divides, its epigenetic tags can be passed to daughter cells, and sometimes to gametes.
Define epigenome.
The epigenome is the collection of all the epigenetic tags (such as methyl groups) on an organism's DNA and histones.
DNA methylation switches a gene by preventing transcription factors from binding.
DNA methylation switches a gene off by preventing transcription factors from binding.
True or False?
Epigenetic changes alter the DNA base sequence.
False.
Epigenetic changes affect how genes are expressed, without changing the DNA base sequence.
To which base is a methyl group commonly added in DNA methylation?
Cytosine.
Define phenotype.
The phenotype is the observable characteristics of an organism, resulting from its genotype and its environment.
What two factors interact to produce an organism's phenotype?
Its genotype and its environment.
Why is variation caused by the environment not inherited?
It does not change the DNA of the gametes.
Only changes to the genetic material can be passed on to offspring.
Give an example of phenotypic variation caused only by the environment.
Genetically identical plants grown in different conditions reach different heights.
Or plants grown in the dark become yellow (chlorosis).
Define discontinuous variation.
Discontinuous variation is qualitative variation that falls into distinct categories with no intermediates, such as ABO blood group.
Define continuous variation.
Continuous variation is quantitative variation with a range of values between two extremes, such as human height.
What type of gene control usually causes discontinuous variation?
Control by a single gene (monogenic).
What causes continuous variation?
The combined effect of many genes (polygenic inheritance).
The environment also contributes.
Define polygenic inheritance.
Polygenic inheritance is when a characteristic is controlled by many genes at different loci, giving continuous variation.
What is meant by the additive effect of polygenes?
Several genes each have a small effect on the same characteristic.
Their effects add together to determine the phenotype.
Characteristics controlled by many genes at many loci usually show variation.
Characteristics controlled by many genes at many loci usually show continuous variation.
True or False?
Human height is an example of discontinuous variation.
False.
Height shows continuous variation — it has a range of values rather than distinct categories.
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