5. Control of gene expression

Question 1

What are totipotent stem cells?
Where are they found in animals?
Where are they found in plants?

Answer 1

Cells which can mature into any body cell
Only present in the early life of an embryo
Found in growing areas of the plant

Question 2

Two key properties of stem cells

Answer 2

Unspecialised cels that can develop into other types of cells
Dividing

Question 3

Describe the process by which a cell becomes specialised

Answer 3

Stem cells all contain the same genes - but during development not all of them are trasncribed and translated
Under the right condition, some genes are expressed and others are switched off
mRNA is only transcribed from specific genes
The mRNA from theses genes is translated into proteins
These proteins modify the cell - they determine cell structure and control cell processes
Changes to the cell produced by these proteins cause the cell be become specialised. They are difficult to reverse

Question 4

Describe how and why a tissue culture can be used to grow plants from a single totipotent cell

Answer 4

A single totipotent cell is taken from a growing area on a plant
The cell is placed in some growth medium, that contains nutrients and growth factors. It is sterile so microorganisms can’t grow and compete with the plant cells
The stem cell will grow and divide into a mass of unspecialised cells, and then the cells will mature into specialised cells
The cells grow and specialise to form a plant organ or an entire plant, depending on the growth factors

Question 5

Describe two potential sources of stem cells

Answer 5

Adult stem cells:
obtained from the body tissues if an adult
they can be obtained through a simple operation
they are not as flexible as embryonic stem cells - they can only specialise into a limited range of cells
Embryonic stem cells:
obtained from embryos at an early stage of development
created in a lab using IVF
at 4-5 days old, stem cells are removed and embryo destroyed
these stem called can develop into all types of specialised cells

Question 6

What is a transcription factor?

Answer 6

Molecules that move from the cytoplasm to the nucleus and allow transcription if a specific gene to take place

Question 7

Describe the principles involved in preventing gene expression by preventing transcription

Answer 7

• Each transcriptional factor has a site that bids to a specific region of the DNA
• When it binds, it stimulates this region of DNA to begin the process of transcription
• mRNA is produced and then translated
• When a gene is not being expressed, the site on the TF that binds to DNA is blocked by an inhibitor
• This inhibitor prevents the TF binding to the DNA and so prevents transcription

Question 8

Describe how oestrogen regulates gene expression

Answer 8

• oestrogen is lipid soluble, so diffuse easily through cell membrane
• in cytoplasm, oestrogen binds with a site on a receptor molecule of the transcription factors
• The receptor molecule changes shape, this released the inhibitor from DNA binding site
• The TF can now enter the nucleus and combine with DNA
• The combination of the TF with DNA stimulates transcription of the gene

Question 9

Describe how siRNA regulates gene expression

Answer 9

• An enzyme cuts large double stranded molecules of RNA into smaller section called small-interfering RNA
• One of the two siRNA strands combines with an enzyme
• The siren stad pairs with complementary bases on an mRNA strand
  The enzyme cuts the mRNA into smaller sections
• mRNA can no longer be translate
• This mean in gene has not been expressed

Question 10

Potential uses of siRNA

Answer 10

Used to identify the role of genes in biological pathway. Some siRNA that blocks a particular gene could be added to cells. By observing the effects 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