7.3 Translation

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 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.

Explain the role of ATP in translation.

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.

The image below shows the structure of ricin.

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.

One mark is available for clarity of communication throughout this question.

Discuss the importance of hydrogen bonds in the process of translation.

Outline the uses of bioinformatics in scientific research.

Draw labelled diagrams contrasting the structure of an mRNA and tRNA molecule.

The table below shows some of the events which take place in protein synthesis.

Identify the correct order of letters to show the sequence of events during protein synthesis, starting with the earliest.

Haemoglobin is a protein made of alpha and beta polypeptides. Each alpha polypeptide has 141 amino acids and each beta polypeptide has 146 amino acids.

Deduce the total number of peptide bonds present in one alpha polypeptide and one beta polypeptide.

Haemoglobin is a quaternary protein.

Describe the structures of haemoglobin that make it a quaternary protein.

State the types of bonding present in the different levels of protein structure.

Describe the function of ribosomes in protein synthesis.

Within a cell ribosomes can be found free or bound to structures.

Contrast free ribosomes with bound ribosomes.

The image below shows the structure of a ribosome. Ribosomes contain an A site, an E site, and a P site.

Label the A site, the E site, and the P site on the image above.

Ribosomes are made of ribosomal RNA (rRNA). Messenger RNA (mRNA), transfer RNA (tRNA) and DNA are all involved in the synthesis of proteins.

Complete the table to show the differences between DNA, mRNA and tRNA.

Enzymes play an important role during transcription and translation.

Discuss the importance of enzyme-substrate specificity in the activation of tRNA molecules.

The tRNA-activating enzyme relies on phosphorylation.

Outline the role of phosphorylation during translation.

Enzymes, such as the tRNA-activating enzyme, are proteins.

State, with named examples, two functions of proteins.

One mark is available for clarity of communication throughout this question. 

Describe how the process of translation leads to the production of a polypeptide.

Explain why cellular enzymes are made predominantly from protein.

Contrast protein synthesis in eukaryotes with protein synthesis in prokaryotes.