DNA & Protein Synthesis (AQA A Level Biology)

State one difference between the genome and proteome of a cell.

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

Messenger RNA (mRNA) is used during translation to form polypeptides. Describe how mRNA is produced in the nucleus of a cell. 

Give two differences between the structure of mRNA and the structure of tRNA.

Describe the function of ribosomes in protein synthesis.

Explain the role of transfer RNA (tRNA) in the process of translation.

The amino acid sequence of the enzyme lactase has been determined. The first five amino acids of this sequence are shown in Table 1.

Table 1

Table 2 shows the genetic code (mRNA codons).

A student was asked to use Table 2 to determine an mRNA nucleotide sequence that would correspond to the first five amino acids of lactase, their results are shown in Table 1. 

Table 2

The student has not completed Table 1 correctly. Determine which amino acids have been incorrectly labelled by the student and complete the table using a ✓ or a X to indicate this.

Figure 1 below shows the exposed bases (anticodons) of two tRNA molecules involved in the synthesis of a protein.

Figure 1

Complete the boxes to show the sequence of bases found along the corresponding section of the coding DNA strand.

Describe how a gene codes for a polypeptide.

Figure 1 is the template strand of a DNA molecule that codes for six amino acids.

Figure 1

TAC GCG TCA ATG ATA CTG 

Identify the sequence of anticodons on the transfer RNAs that would synthesise the string of six amino acids?

Complete Table 1 to show the differences between DNA, mRNA and tRNA.

Table 1

Suggest which group of molecules in Figure 2 (A or B) best represents the proteome of a cell. Give a reason for your answer.

Figure 2

Contrast the chemical structure of DNA and RNA nucleotides.

Tobacco plants have been genetically modified to produce human haemoglobin. The first three triplets of the antisense strand of the human haemoglobin gene are:

ATG GTG CAT

Identify the anticodons of the tRNA that would be part of the second step in producing the haemoglobin protein.

Table 1 shows some of the events which take place in protein synthesis.

Table 1

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

________  ________  ________  ________  ________  ________  ________

Identify the steps in Table 1 that are involved in transcription.

Compare the structure of DNA with RNA.

Figure 1 shows the sequence of bases in a section of the mRNA strand used to synthesise papain.

Figure 1

G   U   U   A   A   A   G   U   U   U   C   A   A   C   G   A   A   A   A   A   C

Give the DNA sequence which would be complementary to the first six bases in this section of mRNA.

Using Figure 1, deduce how many different types of tRNA molecules would attach to the section of mRNA shown in the diagram?

A piece of mRNA is 972 nucleotides long but the DNA coding strand from which it was transcribed is 1215 nucleotides long.

i) Explain why there is a difference in the number of nucleotides.

ii) State the maximum number of amino acids in the polypeptide translated from this piece of mRNA and explain your answer.

In the early 1990s a company in the United States attempted to market a genetically engineered tomato (Solanum lycopersicum) that had an extended shelf life. The gene they inserted that interfered in the production of the enzyme Beta polygalacturonase was just one of the 31,760 genes that a tomato plant has. 

Explain what the 31,760 genes represent in a tomato. 

Describe the structure of transfer RNA (tRNA) which is involved in the process of translation.

The Flavr Savr tomato plant was genetically engineered to ripen and soften more slowly. The inserted gene prevented the enzyme Beta polygalacturonase from breaking down pectin which softened the tomatoes. 

The matching parts of the base sequences for the mRNA produced from the transcription of the softening gene (in the normal tomato plant) and the mRNA base sequence of the inserted gene are shown in the diagram below.

Softening gene mRNA                 …AAUCGGAAU…

Inserted gene mRNA                   …UUAGCCUUA…

Suggest how the inserted gene reduces the production of the softening enzyme.

Figure 1 shows the sequence of DNA bases coding for seven amino acids in the enzyme papain.

Figure 1

5’ C  A  A  T  T  T  C  A  A  A  G  T  T  G  C  T  T  T  T  T  G 3’  

Figure 2 shows the genetic code (mRNA codons).

Figure 2

Use Figure 2 to determine the sequence of amino acids in this part of the enzyme. Write your answer in the boxes below.

Use information in the passage below and your own knowledge to answer the questions below.

In 2018 the US Food and Drug Administration and the European Medicines Agency approved two drugs that targeted mRNA molecules. These drugs were designed to treat a rare genetic disease that causes a blood protein produced by the liver to form clumps (amyloid deposits) in the cardiovascular system, digestive system and around nerve fibres. This disease called hereditary transthyretin (hATTR) amyloidosis affects about 50,000 people in the world. It is an autosomal dominant inherited condition that causes the mutation of the blood protein called transthyretin. The two drugs were designed to bind with the mRNA molecules.

Suggest why the two drugs could be used as a treatment for hereditary transthyretin (hATTR) amyloidosis.

Distinguish between the genome and proteome of a cell.

Transcription occurs differently between eukaryotic and prokaryotic cells. 

Explain why there is a difference.

The KRT35 gene codes for an acidic protein which is a type I hair keratin. Along with type II keratin proteins, it forms the hair and nails on our body. This gene is found on chromosome 17.

Explain how a gene like KRT35 would code for a protein.

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 of ribosomes. As a result, the ribosome changes shape.

Ricin causes the death of cells and is very poisonous to many animals. Suggest how the action of ricin on ribosomes could cause the death of cells.   

Draw labelled diagrams comparing an mRNA and a tRNA molecule.

Table 1 below shows some mRNA codons and the amino acids for which they code.

Table 1

i) Give the DNA sense strand sequence for leucine.

ii) Name the amino acid carried by the tRNA with the anticodon CAU.

Cycloheximide binds to specific sites on the eukaryotic ribosomes. This is shown in Figure 1 below.

Figure 1

Explain how cycloheximide stops protein synthesis.

Scientists studied two Diptera animals Anopheles gambiae (mosquito) and Drosophila melanogaster (fruit fly) who diverged 250 million years ago. Their study was to determine how similar their genes and proteins were. It was known that D. melanogaster has 17,864 genes which code for 30,717 proteins whereas A. gambiae has 13,247 genes coding for 14,102 proteins. The study showed that almost half of their genes were similar and that they had diverged more rapidly than vertebrates.

Using examples from the passage to explain the difference between genome and proteome.

The skin is the organ that separates our body from the environment and therefore from foreign antigens, thus the skin not only acts as a physical barrier but it also acts as an immunological barrier. Keratinocytes are the primary cells found in the skin epidermis. These cells contain genes that code for cytokines and when the cells are triggered the genes become activated and mRNA transcripts are produced. 

Explain why the mRNA transcripts leaving the nucleus have fewer nucleotides than the RNA initially produced by DNA transcription.

Explain the role ATP has in translation.

Mitochondrial diseases (MD) are a group of genetic disorders where the body’s cells cannot aerobically respire properly. MD causes a wide range of symptoms especially in the organs that need the most energy (e.g. brain, heart and muscles) making them hard to diagnose. Multi-organ failure is a common sign that the person has a mitochondrial disorder. 

Mitochondrial DNA (mtDNA) is inherited from our mothers, although recent scientific studies suggest that people may inherit paternal mitochondria. Most mitochondrial diseases occur due to mutations of nuclear genes that are involved in the functioning of mitochondria, although some result from mutations of mitochondrial genes inside the mitochondria. One example of an MD is caused by the mutation of a mitochondrial gene that codes for a tRNA. The mutation changes the anticodon on the tRNA, the guanine base is substituted with adenine. This results in the formation of a non-functional protein in the mitochondrion.

Suggest how the change in the anticodon of a tRNA leads to an MD. 

Lactate dehydrogenase (LDH) has a key role in ensuring glycolysis continues. The enzyme catalyses the reversible conversion of pyruvate into lactate, oxidising the cofactor NAD+. LDN is a tetrapeptide containing two types of polypeptides: polypeptide M (muscle), which is coded for by the LDH-A gene and polypeptide H (heart), which is coded for by the LDH-B gene. These two types of polypeptides in various combinations make up five isoenzymes. Isoenzymes are enzymes that differ in amino acid sequence but catalyse the same chemical reaction. 

The base sequences of the LDH-A and LDH-B genes and the sequences of the amino acids encoded by these genes were determined. Figure 1 shows the first ten amino acids of polypeptides M and H and the corresponding base sequences of one of the DNA strands of each gene.

Figure 1 

Table 1 shows the genetic code (mRNA codons). 

Table 1

Compare and contrast both sequences shown in Figure 1, using the information given.

Read the following passage.

Herpes simplex virus (HSV) infects nerve cells in the face. HSV can remain inactive inside the body for years, but when it becomes active, it causes cold sores around the mouth. 

Human cells infected with a virus may undergo programmed cell death. While HSV is inactive inside the body, only one of its genes is transcribed. This gene is the latency-associated transcript (LAT) gene that prevents programmed cell death of an infected nerve cell. Scientists have found that transcription of the LAT gene produces a microRNA. This microRNA binds to some of the nerve cell’s own mRNA molecules. These mRNA molecules are involved in the programme cell death of nerve cells. 

Use information from the passage and your own knowledge to answer the following question.

Scientists have concluded that the production of microRNA allows HSV to remain in the body for years. Suggest how the presence of the microRNA allows HSV to remain inactive for years.

Scientists have genetically engineered maize plants to resist attack by insects known as stem borers. The scientists inserted a gene from Bacillus thuringiensis (Bt), a soil bacterium, that produces proteins that are highly toxic to the stem borers. These proteins bind to an embedded enzyme aminopeptidase (APN 1) on cell-surface membranes causing the membranes to become more permeable affecting the cells ability to maintain an optimal osmotic potential. 

In Kenya, a study was carried out to see which types of Bt genes and their protein products would be most efficient against three species of stem borer. The stem borers were allowed to feed on nine types of maize (A–I), modified with Bt genes. Figure 2 below shows the leaf areas damaged by the stem borers after feeding on maize leaves for five days.

Figure 2

Calculate the percentage difference in leaf area damaged by Busseola fusca between the control and maize type H. Show your working.

Recently, scientists have seen insects like Busseola fusca (maize stalk borer) developing resistance to genetically engineered maize plants. Cornell researchers have discovered that the insects with resistance have a different type of aminopeptidase (APN 6) which allows insects to digest their food and Bt without harm. 

Explain how two different aminopeptidases could evolve resulting in some insects developing resistance to the Bt toxin.