Nucleic Acids and Protein Synthesis

In step 2 in Fig. 3.1, the single-stranded primary transcript loops and forms a section where dsRNA is present.

Explain how it is possible for the double-stranded section of RNA to be held together in step 2 and maintained this way in steps 3, 4 and 5.

After step 3, the shorter dsRNA produced by the action of Drosha is transported to the cytoplasm, where it is cleaved further by Dicer to produce ds siRNA.

Suggest why two different enzymes, Drosha and Dicer, are needed to cut dsRNA into shorter lengths.

From 2018, siRNA has been used as a therapeutic drug to treat a number of diseases.

The presence of molecules of siRNA in the cytoplasm can result in the cleavage of messenger RNA (mRNA) molecules coding for a protein involved in the disease. This prevents the synthesis of the protein.

Describe the differences between a molecule of mRNA and a molecule of siRNA, such as the siRNA shown in step 5 in Fig. 3.1.

With reference to Fig. 3.2, state why the passenger strand needs to be separated and released from the RISC.

The aim of siRNA therapy is to prevent or decrease the synthesis of a protein involved in the disease being treated.

A target mRNA molecule can be cleaved in a different location by a RISC with a different siRNA.

Suggest how cleaving mRNA in different locations will have different effects on protein synthesis and explain how these different effects can result in a lack of functioning protein.

Describe three ways in which the structure of messenger RNA (mRNA) differs from the structure of DNA.

In each of your answers, include information about the structure of mRNA and the structure of DNA.

1

2

3

State the letters that represent the two purine synthetic bases in Fig. 6.1.

State and explain which DNA base pair is most similar to the synthetic base pairs in Fig. 6.1.

Explain why Fig. 3.1 does not include any phosphodiester bonds.

Identify and describe the DNA-RNA nucleotide pair shown in Fig. 3.1.

You may add labels and annotations to Fig. 3.1 if you wish.

State the name of the type of base that includes U, T and C.

State why it is not correct to say that all nucleic acids have five bases.

Fig. 3.2 shows a stage in the replication of DNA. The circled part is enlarged in Fig. 3.3 to show the elongation of the DNA strand that is being synthesised.

Describe the sequence of events that occurs at the stage shown in Fig. 3.3 to extend the synthesised strand.

The two strands in a molecule of DNA are described as antiparallel.

With reference to Fig. 3.2 and Fig. 3.3, state what is meant by antiparallel, and explain how the antiparallel arrangement of the strands determines how new strands are synthesised.

The names of the bases present in RNA and DNA nucleotides can be abbreviated using a single letter. These are shown in Table 5.1.

Complete Table. 5.1 by stating:

• the name of each base
• whether the base is a purine or pyrimidine
• whether the base is present
  • only in an RNA molecule (write RNA in the table)
  • only in a DNA molecule (write DNA in the table)
  • in RNA and in DNA molecules (write the word both in the table).

Table 5.1

With reference to Fig. 5.1, explain whether carbovir triphosphate will replace a purine or a pyrimidine nucleotide in the elongating polynucleotide chain.

With reference to Fig. 5.1 and the action of DNA polymerase, suggest why the conversion of abacavir to carbovir triphosphate increases the chance of the analogue being added to the viral polynucleotide chain.

Suggest and explain how carbovir triphosphate interferes with the action of DNA polymerase and how this may prevent the synthesis of viral DNA.

Identify structure E and structure F in Fig. 6.1.

E ______

F ______

On Fig. 6.1 draw a circle around one nucleotide.

State the name of the covalent bond that links two nucleotides together.

Fig. 6.2 shows the RNA base sequence of a short length of primary transcript.

Complete Fig. 6.2 by writing the DNA base sequence of the template strand used to form the primary transcript.

In eukaryotic cells, the primary transcript is modified to form mRNA.

Explain how the primary transcript is modified to form mRNA.

The mRNA strand is translated at the ribosome to form a polypeptide.

Describe how the process of translation results in the formation of a polypeptide.

Name the stage of protein synthesis that is shown in Fig. 1.1.

Identify A, B and C in Fig. 1.1.

A ______

B ______

C ______

State the base sequences at D and E.

D ______

E ______

Mutagenesis is a process that leads to a change in the amino acid sequences of proteins. Scientists carry out mutagenesis to investigate the importance of particular amino acids in protein structure and function.

Outline how changing one amino acid in the $\beta$-globin polypeptide of haemoglobin may change the structure and function of a molecule of haemoglobin.

Name the purine bases shown in Fig. 6.1.

State the name given to the group of three bases found at J on the tRNA molecule.

Identify the three bases at J.

State how the three bases at J on tRNA interact with the bases on mRNA.

Name the bond labelled P.

bond P = ______

Use Fig. 4.1 to describe the role of tRNA in protein synthesis.

You may annotate Fig. 4.1 to help your answer.

tRNA molecules are synthesised inside the nucleus of eukaryotic cells.

Outline the process by which tRNA molecules are synthesised in the nucleus.

Name the bases P to S.

P ______

Q ______

R ______

S ______

Describe the role of the mRNA molecule shown in Fig. 6.1.