Protein Synthesis (CIE A Level Biology)

The sequence below shows the DNA bases coding for seven amino acids in the enzyme papain. Note that the sequence shown is from the non-template strand.

C  A  A  T  T  T  C  A  A  A  G  T  T  G  C  T  T  T  T  T  G

Fig. 1 shows the genetic code and the amino acids coded for.

Fig. 1

Use the information in Fig. 1 to identify the sequence of amino acids in this part of the enzyme.

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

Table 1

Identify the DNA non-template strand sequence for leucine.

[1]

Identify the amino acid carried by the tRNA with the anticodon CAU.

[1]

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.

Cycloheximide is a fungicide produced by the bacterium Streptomyces griseus, which binds to specific sites on the eukaryotic ribosomes. This is shown in Fig. 2 below.

Fig. 2

Explain how cycloheximide stops protein synthesis.

Explain the role of hydrogen bonds in the process of transcription.

mRNA is a single-stranded molecule involved with the process of protein synthesis.

Explain the importance of mRNA being a single-stranded molecule.

The genome refers to the complete set of genes in a cell while the proteome is the full range of proteins that a cell can produce.

Suggest why the proteome of different cells in an organism may vary despite having the same genome.

Hereditary transthyretin (hATTR) amyloidosis is an inherited condition that causes a blood protein called transthyretin to form clumps in the cardiovascular system, digestive system and around nerve fibres. Drugs designed to bind to the mRNA molecules are used as a form of treatment for this condition.

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

One mechanism for regulating gene expression in cells uses an enzyme, called an RNase, to break down mRNA as it travels from the nucleus to the ribosome. 

Explain how this would be an effective way to regulate gene expression. 

The RNase enzymes involved in this process must be able to target specific sequences of mRNA. 

The enzymes can achieve this by binding to single stranded RNA. 

mRNA can also be used to produce vaccines against certain diseases, such as for Covid-19. 

mRNA vaccines involve injecting mRNA stands that code for viral proteins into the blood, which are then taken in by human immune cells and expressed. 

Explain what happens in the target cell after it takes in the mRNA. 

mRNA vaccines are a relatively recent scientific discovery and have been praised by the scientific community for many reasons. 

One such reason is that the risk of mutation to human DNA is very low. 

Suggest one reason why mRNA vaccines pose a very low risk to the structure of human DNA. 

Tay-Sachs disease is caused by a mutation to a gene called HEXA on chromosome 15. The HEXA gene codes for a lysosomal enzyme that breaks down toxins in the brain and spinal cord. The mutation causes this enzyme to be non-functional, resulting in a build of up toxins in the brain and spinal cord, which over time destroys the nerve cells in these areas. This disease is usually fatal in babies and young children. 

There are more than 100 different mutations that can lead to this disease. 

A very common form of this mutation is caused by a DNA change in the 11th exon out of 14 in the HEXA gene. 

The template strand of the mutated DNA is as follows:

Describe the type of mutation that has occurred. 

Fig. 1 shows which mRNA codons code for each amino acid during translation. 

Fig. 1

Using the table in part (a) as well as the resource in Fig. 1, explain how the resulting protein would be changed by the mutation in the HEXA gene. 

Tay-Sachs disease can be caused by over 100 types of mutations. 

Suggest how it is possible for over 100 different changes in the DNA to result in the same disease. 

Tay-Sachs is an autosomal recessive disease. 

Explain whether a person with Tay-Sachs disease would need to inherit two of the same type of mutation to develop the condition, or whether it could be two different ones. 

Describe the role of the ribosome in translation.

Fig. 1 shows the base sequence of a gene that codes for a short polypeptide, the RNA codons and the primary structure of this polypeptide.

Fig.1

Describe the role of DNA as indicated in Fig.1.

Fig. 2 illustrates some changes that are made to the RNA that is produced during transcription.

Fig. 2

Contrast the main differences between R and S in Fig.2.

Fig.1 is a diagram of a section of mRNA showing the sequence of five of the codons.

The triplets of bases in a DNA molecule that codes for some of the amino acids are listed in Table 1.

Fig.1

Table 1

Identify the amino acid sequence on this section of the mRNA molecule, using the information in Fig.1 and Table 1.

The five codons in Fig.1 are near the start of the sequence coding for a polypeptide.

Explain the consequence of a mutation which deletes one of the bases from codon 3.

If the guanine (G) in codon 4 changed to adenine (A), describe and explain the effect this would have on the amino acid sequence of this section of mRNA shown in Fig.1.

Explain why substitution mutations are less likely to have a dramatic effect on polypeptides than frameshift mutations.

The DNA in the nucleus is known as nuclear DNA.

In the cells of the grasshopper, Chorthippus brunneus, 20% of the nucleotides in nuclear DNA contain thymine.

Calculate the percentage of nucleotides in the nuclear DNA of C. brunneus that contain guanine and explain your answer in terms of the structure of DNA.

State another location, other than the nucleus, where DNA occurs in cells of C. brunneus.

Fig. 1 is a diagram of a molecule of tRNA.

The region labelled R shows detail of part of the tRNA molecule.

Fig. 1

Complete Fig.1 by writing the sequence of bases in the region labelled R.

State the name of region Q and explain the role of region Q in translation.

State the function of region P.

State the number of types of amino acid carrying tRNA molecules.  

Fig. 1 shows a section of a polypeptide synthesised during translation.

Fig. 1

Complete the diagram to show the missing section in the area shaded grey. 

Fig.1 shows the amino acids alanine and serine. 

Describe the sequence of events that lead to the correct amino acids being joined to the polypeptide strand. 

When the mRNA is read by the ribosome, it is read in a 5' to 3' direction. 

Explain what is meant by mRNA having a 5' and 3' end.