The five codons in the diagram at part a) are near the start of the sequence coding for a polypeptide. A mutation led to the deletion of one of the bases from codon 3.
Explain the possible consequences of this mutation.
Duchenne muscular dystrophy (DMD) is a genetic disorder that leads to the degeneration of muscle tissue over time due to changes in a protein called dystrophin.
Dystrophin is a rod-shaped protein that acts as a link connecting actin filaments in muscle fibres to the extracellular matrix by attaching to a protein complex (DAPC) located in the sarcolemma.
Dystrophin is coded for by the DMD gene, and the complete protein consists of four domains (N-terminal, Rod, Cys-domain and C-terminal), as shown in the diagram below.
The following diagram shows the regions of the DMD gene that codes for the different domains of dystrophin.
One of the causes of Duchenne muscular dystrophy is a substitution mutation that leads to the formation of a stop codon in the rod domain of the DMD gene.
Explain the impact this mutation would have on the resulting dystrophin protein by using the information in the diagrams.
After transcription of the DMD gene, the pre-mRNA measures about 2.1 megabases (Mb) while the mature mRNA consists of about 14 kilobases (kb). Note that 1 Mb = 103 kb.
Calculate the percentage decrease in size of the mRNA molecule after modification. Show your working and give your answer to three significant figures.
Dystrophin contains many hydrophobic regions that plays an important role in maintaining its structure. Some of the mutations leading to DMD replaces amino acids within the hydrophobic regions with ones containing polar or charged R-groups.
Suggest the effect that this would have on the structure of dystrophin.
Hereditary transthyretin (hATTR) amyloidosis is an inherited condition that is caused by a mutation of a gene that codes for the blood protein transthyretin.
This mutation results in the protein forming clumps in different areas of the body, such as the cardiovascular system, digestive system and around nerve fibres.
Certain drugs that are designed to bind to mRNA molecules are used as treatment for this condition.
Suggest why these drugs could be used as a treatment for hATTR.
The gene that codes for transthyretin is known as the TTR gene. The following diagram shows a section of this gene in a normal individual and someone suffering from hATTR.
The table below shows the genetic code and the amino acids that it codes for.
Use the information in the diagram and table to describe the effect the mutation would have on transthyretin.
Mitochondrial diseases (MD) are a group of genetic disorders where body cells cannot aerobically respire properly.
One example of an MD is caused by the mutation of a mitochondrial gene that codes for a tRNA molecule. The mutation leads to the replacement of a guanine base with adenine in the anticodon of the tRNA molecule. This results in the formation of a non-functional protein in the mitochondrion.
Suggest how the change in the anticodon of a tRNA molecule leads to an MD.
Ricin is a protein produced by castor beans. In animal cells, ricin acts as an enzyme which removes the adenine base from one of the nucleotides in the RNA of ribosomes. As a result, the ribosome changes shape. Ricin causes the death of cells and is very poisonous to certain animals.
Suggest how the action of ricin on ribosomes could cause the death of cells.
Image courtesy of Aza Toth. Licensed under Creative Commons Attribution 3.0 Unported license. Reused and distributed under conditions found at: https://creativecommons.org/licenses/by/3.0/deed.en
Discuss the level(s) of protein structure visible in the diagram.
The Flavr Savr tomato plant was genetically engineered to ripen and soften more slowly than a normal tomato. The inserted gene prevents the enzyme Beta polygalacturonase from breaking down pectin which softens the tomatoes.
The diagram below shows the matching parts of the base sequences for the mRNA produced from the transcription of the softening gene in a normal tomato and that of the inserted gene.
Suggest how the inserted gene reduces the production of the softening enzyme.
