Ricin is a protein produced by castor beans. In animal cells, ricin acts as an enzyme. This enzyme removes the adenine molecule from one of the nucleotides in the RNA that makes up the structure of ribosomes. As a result, the ribosome changes shape. Ricin causes the death of cells and is highly toxic to many animals.
Suggest how the effect of ricin on ribosomes could cause the death of cells.
Transcription factors are proteins that influence the process of transcription. One mechanism by which transcription factors affect transcription is illustrated and described below.
The transcription factor binds to region X at the start of a gene, also known as a promoter region.
This causes enzyme Y to bind to the DNA.
Transcription is initiated and enzyme Y moves along the DNA in the direction shown.
The transcription factor shown in part a) is a protein.
Suggest, with a reason, how a mutation in the gene that codes for the transcription factor protein might affect the expression of the gene shown in part a).
The transcription factor shown in part a) is an example of a type of transcription factor known as an activator. This means that it initiates transcription or increases the rate at which transcription takes place.
Use the illustration in part a) to suggest how a transcription factor might have the opposite effect and function as a repressor.
Scientists have modified the DNA of maize plants to enable them to resist attack by insects known as stem borers. The scientists transferred a gene from Bacillus thuringiensis (Bt), a soil bacterium, into the maize plants. The gene codes for proteins that are highly toxic to the stem borer insects. The toxic proteins bind to the cell-surface membranes of the insects, increasing the passage of ions through the membrane and into the insect cells.
Suggest how the Bt toxin causes the death of insect cells.
A study was carried out to investigate which of several Bt toxin gene variants was most effective against three species of stem borer insect. The stem borers were allowed to feed on nine maize varieties (A–I), each modified with a different variant of the Bt toxin gene. The graph below shows the leaf area damaged by the stem borers after feeding on maize leaves for five days.
Calculate the percentage difference in leaf area damaged by Busseola fusca between the control and maize type H.
A farmer read the results of the study in part b) and concluded that they should buy maize variety B to achieve maximum resistance against stem borer damage in their maize crop.
Another example of a genetically altered organism is the 'Flavr Savr' tomato. This tomato variety is genetically engineered to ripen and soften more slowly in order to increase its shelf-life.
The new gene is inserted into the tomato DNA alongside the normal gene that causes softening. The inserted gene prevents production of the softening enzyme beta polygalacturonase, which is coded for by the softening gene.
Parts of the base sequences for the mRNA produced during transcription of the softening gene and the inserted gene are shown below.
Softening gene mRNA …AAUCGGAAU…
Inserted gene mRNA …UUAGCCUUA…
Suggest how the inserted gene reduces the production of the softening enzyme.
Until the development of genetic modification technology to produce human insulin on a large scale, diabetic patients had to use bovine-derived (from cattle) or porcine-derived (from pigs) insulin to help control their blood sugar levels. This insulin is extracted from the pancreas left over from animal slaughter in commercial abattoirs.
Outline two drawbacks of using porcine-derived insulin for an insulin-dependent diabetic patient.
The strain of Escherichia coli (E. coli) used to produce human insulin has to be weakened in some way before it can be used to produce large quantities of insulin in industrial fermenters. This weakening step is only applicable to strains of E. coli designed for this process.
