Biology Unit 5 Chapter 15 - Control of Gene Expression
Explain the advantage of growing papaya plants from tissue culture rather than from seeds. (1)
All plants will have desired characteristics
(Refer to exam q) Plant tissue culture is a method used to propagate plants. The flow diagram shows one method of plant tissue culture. Name the type of reproduction involved in plant tissue culture. (1)
Asexual reproduction
Transcriptional factors are important in the synthesis of particular proteins. Describe how. (2)
Bind to DNA; Stimulate transcription
Preventing the expression of a gene can be done by preventing transcription. Outline how this occurs. (2)
Binding site on transcriptional factor that binds to DNA is blocked by an inhibitor molecule; Prevents transcriptional factor binding to DNA so prevents transcription and protein synthesis
The carrier molecule on its own may be able to prevent the infection of cells by HIV. Explain how. (2)
Carrier binds to protein on HIV; Prevents HIV binding to receptor on human cell
What are totipotent cells? (1)
Cells which can develop into any other cell of the organism
(Refer to June 2013 paper) Human immunodeficiency virus (HIV) particles have a specific protein on their surface. This protein binds to a receptor on the plasma membrane of a human cell and allows HIV to enter. This HIV protein is found on the surface of human cells after they have become infected with HIV. Scientists made siRNA to inhibit expression of a specific HIV gene inside a human cell. They attached this siRNA to a carrier molecule. The flow chart shows what happens when this carrier molecule reaches a human cell infected with HIV. When siRNA binds to mRNA, name the complementary base pairs holding the siRNA and mRNA together. One of the bases is named for you. (1)
Cytosine and Guanine, Adenine and Uracil
Name the process by which totipotent cells of plant tissue culture change in appearance and develop into shoot or root cells. (1)
Differentiation
Describe how siRNA affects gene expression. (5)
Enzyme cuts large double-stranded RNA into smaller sections called siRNA; One siRNA strand combines with an enzyme; siRNA molecule guides enzyme to an mRNA molecule and pairs up its unpaired bases with mRNA's complementary bases; Enzyme cuts mRNA into smaller sections; mRNA unable of being translated into a polypeptide
(Refer to exam q) A child was treated with genetically engineered stem cells. The graph shows the number of functioning white blood cells in the child during the year following treatment. Children who do not suffer from SCID have between 5000 and 8000 white blood cells per mm3 of blood. Describe and explain these results. (3)
Few modified cells to start with; Modified stem cells replace non-functional cells; Levels off because the number of white cells reaches the normal level
Suggest one ethical argument for and one against stem cell research. (2)
For: It is wrong to allow human suffering when there is a possibility of alleviating it by obtaining information from research; Against: Embryos should be given same respect as an adult person
The promoter DNA from sheep (Stage 3) causes transcription of genes coding for proteins found in sheep milk. Suggest the advantage of using this promoter DNA. (2)
Gene only expressed in mammary glands; Do not need to kill sheep to obtain Factor IX
(Refer to June 2011 paper) Plant physiologists attempted to produce papaya plants using tissue culture. They investigated the effects of different concentrations of two plant growth factors on small pieces of the stem tip from a papaya plant. Their results are shown in the table. Callus is a mass of undifferentiated plant cells. Plantlets are small plants. Explain the evidence from the table that cells from the stem tip are totipotent. (2)
Gives rise to new plants; So must be able to develop into different tissues
(Refer to June 2011 paper) The flowchart shows how small interfering RNA (siRNA) affects the expression of a particular target gene. The siRNA-protein complex attaches to an mRNA molecule coding for a particular protein (step 2). Explain what causes the siRNA to attach only to one sort of mRNA molecule. (1)
Has complementary base sequence
How does the distribution of totipotent cells in animals differ from that in plants? (3)
In animals only a few cells are totipotent; Adult stem cells found in bone marrow, skin etc and embryonic stem cells; In plants many of the cells are totipotent
All cells possess the same genes and yet a skin cell can produce the protein keratin but not the protein myosin, while a muscle cell can produce myosin but not keratin. Explain why. (4)
In skin cells the gene coding for keratin is expressed but not gene for myosin; Genetic code for keratin translated into protein; In muscle cells the gene coding for myosin is expressed but not gene for keratin; Genetic code for myosin translated into protein
Papaya plants reproduce sexually by means of seeds. Papaya plants grown from seeds are very variable in their yield. Explain why. (2)
Independent assortment; Random fertilisation
Describe and explain how expression of the target gene is affected by siRNA. (2)
No longer able to make specific protein; Because mRNA has been cut into pieces
Describe how oestrogen affects gene expression. (5)
Oestrogen combines with a complementary site on a receptor molecule of the transcriptional factor; Oestrogen changes the shape of the receptor molecule; Inhibitor molecule is released from DNA binding site; Transcriptional factor can now bind with DNA; Stimulates transcription of the gene and protein synthesis
This siRNA would only affect gene expression in cells infected with HIV. Suggest two reasons why. (4)
Only infected cells have HIV protein on surface; So carrier only attaches to these cells; siRNA base sequence complementary to one mRNA; Only infected cells contain mRNA of HIV
Oestrogen only affects target cells. Explain why oestrogen does not affect other cells in the body. (1)
Other cells do not have the oestrogen receptors
How can genes be prevented from expressing themselves? (2)
Preventing transcription and hence the production of mRNA; Breaking down mRNA before its genetic code can be translated
(Refer to June 2010 paper) Some breast tumours are stimulated to grow by oestrogen. Tamoxifen is used to treat these breast tumours. In the liver, tamoxifen is converted into an active substance called endoxifen. Figure 2 shows a molecule of oestrogen and a molecule of endoxifen. Use Figure 2 to suggest how endoxifen reduces the growth rate of these breast tumours. (2)
Similar shape to oestrogen; Prevents oestrogen binding
Scientists have suggested that siRNA may be useful in treating some diseases. Suggest why siRNA may be useful in treating disease. (2)
Some diseases are genetic; siRNA will stop product of this gene
Oestrogen is a hormone that affects transcription. It forms a complex with a receptor in the cytoplasm of target cells. Explain how an activated oestrogen receptor affects the target cell. (2)
Stimulates RNA polymerase; Increases transcription
(Refer to exam q) Children with severe combined immunodeficiency disorder (SCID) cannot produce the many types of white blood cells that fight infections. This is because they do not have the functional gene to make the enzyme ADA. Some children with SCID have been treated with stem cells. Stem cells can divide and develop into any type of blood cell, including white blood cells. Using the information given, suggest and explain two reasons why stem cells were used in this treatment. (4)
They can divide to form new blood cells; So can replace existing faulty cells; Child's own cells; So little risk of rejection
Give three possible uses of stem cells which have been grown in vitro and then induced to develop into a wide range of different human tissues. (3)
Used to regrow tissues that have been damaged such as: heart muscle cells which have been damaged due to heart attack; skin cells which have been damaged by burns or wounds; can be used to produce B cells of the islets of Langerhans in people with Type I diabetes
SCID is a severe inherited disease. People who are affected have no immunity. Doctors carried out a trial using gene therapy to treat children with SCID. The doctors who carried out the trial obtained stem cells from each child's umbilical cord. Give two characteristic features of stem cells. (2)
Will replace themselves; Can develop into other cells
Give two characteristic features of stem cells. (2)
Will replace themselves; Undifferentiated
Callus tissue develops into either shoots or roots depending on the relative concentration of the plant growth regulators used. Use your knowledge of genes to suggest how these plant growth regulators determine the type of plant tissue formed. (1)
express different genes
Give two advantages of producing plants using this method rather than from seeds. (2)
favourable characteristic can be maintained in all plants; higher success rate
Why is a viral infection more likely to destroy a complete batch of plants grown by plant tissue culture than a batch of plants grown from seeds? (1)
no genetic variation
