Control of Genetic Expression

Question 1

Why do cells only have the ability to produce particular things but have the genes to produce everything that the body needs?

Answer 1

Only certain genes are expressed (switched on) in any one cell at any one time.

Question 2

What types of genes are expressed in all cells?

Answer 2

E.g. genes that code for essential chemicals, such as the enzymes involved in respiration, are expressed in all cells.

Question 3

What types of genes are permanently switched off?

Answer 3

E.g. the genes for insulin in cells lining the small intestine.

Question 4

How do differentiated cells differ from one another?

Answer 4

Mainly because they each produce different proteins. The proteins that a cell produces are coded for by the genes that are expressed.

Question 5

What are totiponent cells?

Answer 5

Can give rise to all cell types. E.g. a fertilised egg.
The early cells that are derived from the fertilised egg are also totipotent. These later differentiate and become specialised for a particular function.

Question 6

How do cells differentiate?

Answer 6

During the process of cell specialisation, only some genes are expressed. This means that only part of the DNA of a cell are translated into proteins. The cell therefore only makes those proteins that it requires to carry out its specialised function.

Question 7

How are genes prevented from expressing themselves?

Answer 7

• Preventing transcription and hence preventing the production of mRNA.
• Breaking down mRNA before its genetic code can be translated.

Question 8

WHat happens to xylem cells and red blood cells once they mature?

Answer 8

They lose their nucleus so can’t further specialise into other cells.

Question 9

Why is specialisation irreversible in most animal cells?

Answer 9

Once cells have matured and specialised they lose their totipotency.

Question 10

What are a type of totipotent cell that exist in mature animals?

Answer 10

Adult stem cells.

Question 11

What are stem cells?

Answer 11

Undifferentiated dividing cells that occur in adult animal tissues and need to be constantly replaced.
Under certain conditions stem cells can develop into any type of cells.

Question 12

Where are stem cells found?

Answer 12

inner lining of the small intestine, in the skin and in the bone marrow (produces red and white blood cells)`.

Question 13

What can stem cells be used to do?

Answer 13

Used to treat a variety of genetic disorders, such as blood diseases thalassaemia and sickle cell anaemia.

Question 14

In addition to adult stem cells , where else do stem cells occur?

Answer 14

Earliest stages of the development of an embryo, before the cells have differentiated. Embryonic stem cells.

Question 15

Why are plants different to animals in terms of stem cells?

Answer 15

Mature plants have many totipotent cells. Under the right conditions, many plant cells can develop into any other cell.
For example if you took a cell from the root of a carrot, place it in a suitable nutrient medium and give it certain chemical stimuli at the right time, we can develop a complete new carrot plant.
Growing cells outside of a living organism this way is called in vitro development.
New carrot plant is genetically identical so therefore a clone.

Question 16

What are plant growth factors?

Answer 16

Chemically involved in the growth and development of plant tissues.

Question 17

What are the features of plant growth factors?

Answer 17

• Wide range of effects on plant tissues.
• The effects on a particular tissue depend upon the concentration of the growth factor.
• The same concentration affects different tissues in different ways.
• The effect of one growth factor can be modified be the presence of another.

Question 18

What stem cells have the greatest potential to treat human disease?

Answer 18

human embryonic stem cells. These cells can be grown in vitro and then induced to develop into a wide range of different human tissues.

Question 19

What are the uses of embryonic stem cells?

Answer 19

These cells can be grown in vitro and then induced to develop into a wide range of different human tissues.
Uses:
-Used to regrow human tissues that have been damaged in some way,wither by accident or as a result of disease (neurodegenerative, such as Parkinson’s)
-Heart muscle cells (heart disease)
-Skeletal muscle cells (muscular distophy)
-B cells in pancreas (Type 1 diabetes)
-Nerve cells (parkinson’s, multiple sclerosis, strokes, Alzheimer’s, paralysis)
-Blood cells (Leukemia, inherited blood diseases)
-Skin cells (Burns and wounds)
-Bone cells (Osteoporosis)
-Cartilage cells (Oseoarthiritis)
-Retina cells of the eye (macular degeneration)

Question 20

What conditions are there for embryonic stem cell research in the UK?

Answer 20

Only allowed in UK under licence and specific conditions.

-Use as a means of increasing knowledge about embryo development and serious diseases, including their treatment.

Question 21

Where so embryos for research come from?

Answer 21

in vitro fertilisation

Question 22

What are the ethical issues surrounding embryonic stem cell research?

Answer 22

• Whether a human embryo less than 14 days old should be afforded the same respect as a fetus or an adult person. undermines respect for human life and could progress to use of fetuses and even newborn babies.
• Further move to reproductive cloning.
• Embryo at such an early stage of development is just a ball of identical, undifferentiated cells, bearing no resemblance to a human being.
• Laws against cloning are sufficient.
• Wrong to allow human suffering to continue when there is a possibility of alleviating it.
• better than destroying superfluous embryos.

Question 23

What are the general principles involved in preventing the expression of a gene by preventing transcription?

Answer 23

• For transcription to begin the gene needs to be stimulated by specific molecules that move from the cytoplasm into the nucleus. These molecules are called transcriptional factors.
• Each transcriptional factor has a site that binds to a specific region of the DNA in the nucleus.
• When it binds, it stimulates this region of DNA to begin the process of transcription.
• Messenger RNA is produced and the genetic code it carries is then translated into a polypeptide.
• When a gene is not being expressed, the site on the transcriptional factor that binds to DNA is blocked by an inhibitor molecule.
• This inhibitor molecule prevents the transcriptional factor binding to DNA and so prevents transcription and polypeptide synthesis.

Question 24

How do hormones like oestrogen switch on a gene and thus start transctription?

Answer 24

• Oestrogen is a lipid-soluble molecule and therefore diffuses easily through the phospholipid portion of cell-surface membranes.
• Once inside the cytoplasm of a cell, oestrogen combines with a site on a receptor molecule of the transcriptional factor. The shape of this site and the shape of the oestrogen molecule complement one another.
• By combining with the site, the oestrogen changes the shape of the receptor molecule. This change of shape releases the inhibitor molecule from the DNA binding site on the transcriptional factor.
• The transcriptional factor can now enter the nucleus through a nuclear pore and combine with DNA.
• The combination of the transcriptional factor with DNA stimulates transcription of the gene that makes up the portion of DNA.

Question 25

TRANSCRIPTIONAL FACTOR

Answer 25

For transcription to begin the gene need to be stimulated by specific molecules that move from the cytoplasm into the nucleus- transcriptional factors.

Question 26

How can gene expression be prevented?

Answer 26

BY breaking down mRNA before its genetic cod can be translated into a polypeptide.
Essential to this process are small double-stranded sections of RNA called small interfering RNA (siRNA).

Question 27

Describe how gene expression is prevented by the breaking down of mRNA .

Answer 27

• An enzyme cuts large double-stranded molecules of RNA into smaller sections called small interfering RNA (siRNA).
• One of the two siRNA strands combines with an enzyme.
• The siRNA molecule guides the enzyme to a messenger RNA (mRNA) molecule by pairing up its bases with the complementary ones on a section of the mRNA molecule.
• Once in position the enzyme cuts the mRNA into smaller sections.
• The mRNA is no longer capable of being translated into a polypeptide.
• This means that the gene has not been expressed, that is, it has been blocked.

Question 28

What are the scientific and medical uses of siRNA?

Answer 28

• It could be used to identify the role of genes in a biological pathway. Some siRNA that blocks a particular gene could be added to cells. By observing the effects (or lack of them) we could determine what the role of the blocked gene is.
• As some diseases are caused by genes, it may be possible to use siRNA to block these genes and so prevent the disease.

Question 29

What is the “two hit” cancer hypothesis?

Answer 29

Tumours can develop as a result of a mutation of proto-oncogenes that causes cells to divide more rapidly than normal. Tumours can also develop by a mutation of tumour suppressor genes that prevents them from inhibiting cell division. It only takes a single mutated allele to activate proto-oncogenes but it takes a mutant of both alleles to inactivate tumour suppressor genes (two-hits). As natural mutation rates are slow, it takes a considerable time for both tumour suppressor alleles to mutate. This explains why the risk of cancers increase as one gets older. Thought that some people are born with a mutated allele. These people are at greater risk of cancer as they need only one further mutation, rather than two, to develop the disease.

Question 30

Where do transcription factors move from?

Answer 30

The cytoplasm to the nucleus.

Question 31

What are the two types of transcription factors?

Answer 31

ACTIVATORS- increase the rate of transcription- e.g. they help RNA polymerase bind to the start of the target gene and activate transcription.
REPRESSORS- decrease the rate of transcription- e.g. they bind to the start of the target gene, preventing RNA polymerase from binding, stopping transcription.

Question 32

How do stem cells become specialised?

Answer 32

• Stem cells all contain the same genes- but during development not all of them are transcribed and translated (expressed).
• Under the right conditions, some genes are expressed and others are switched off.
• mRNA is only transcribed from specific genes.
• The mRNA from these genes is translated into proteins.
• These proteins modify the cell- they determine the cell structure and control cell processes (incl. the expression of more genes).
• Changes to the cell produced by these proteins cause the cell to become specialised. These changes are difficult to reverse, so once a cell has specialised it stays specialised.